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PHOTOM — A Photometry Package

1. There are conflicting interests at work here On the one hand it is desirable to use a large background aperture to get good statistics but on the other hand this increases the probability of introducing extra contamination or of sampling areas which are uncharacteristic of the background near the object Of course if there is contamination in the background aperture then there will almost certainly be some present in the object aperture The effects of this on the measurement depend on the density of objects and the ratio of the sizes of the object and sky apertures If the sky aperture is larger than the object aperture and the objects are randomly distributed then there is a greater chance of having more contaminating objects in the sky aperture than in the object aperture If the density of objects is large then the proportion in each aperture will be similar but when the number of objects is small then the chances of the larger aperture being disproportionately endowed will increase This is best seen by considering the limiting case of one contaminating object and its most probable location In astronomical situations the smaller populations tend to be the brighter objects and thus have an even greater effect on the results An ideal filter would therefore include the many faint objects which inhabit each aperture equally but exclude the rare bright objects which are more likely to occur in the larger aperture Unfortunately in reality the
2. or more wayward outliers can affect the size of the standard deviation the mean and standard deviation are recalculated after each stage of clipping up to a maximum of three times The mode is superficially calculated from the empirical relation mode 3 x median 2 x mean but because 9 SUN 45 16 Performing interactive analyses this can be fooled by excessive skewness in the histogram there are rejection and averaging schemes in the algorithm to ensure stability The final option is to supply a constant for the sky which is used for all subsequent measurements This value is used until either a new value is chosen or one of the other methods of estimation is selected The sky variance is also requested so that if the errors are calculated from the sky variance command P then a realistic error can be assigned Both the sky value and variance should be given in data units When using a concentric background aperture it is recommended that the mode or mean with 20 rejection is used as these offer protection against contamination from other objects in the sky aperture For optimal extraction it is currently recommended that you use the modal sky estimate 5 1 12 V values This summarizes the current settings of the significant parameters on the terminal 5 1 13 X eXtraction This is a toggle switch which alters the way in which the photometry is carried out There are two methods available aperture extraction is the default 5 2 D
3. 5 2 2 maxshift maxiter search and toler 9 5 2 3 _exsource and ete 10 5 24 usemaskl bia a I ite a E E ee BOD ow BPO 10 6 Automated photometry 11 7 Altering program parameters 12 8 Using different image formats 12 9 Acknowledgements 13 10 Acknowledging this software 13 A Full routine descriptions 14 AUTOPHOTOM i 15 PHOTGREY i 5 a ar a sr Rw E 21 P AOTOMI gt 5 14 3134 ia a add a 22 B Techniques of aperture photometry 30 C Explaining optimal photometry 31 SUN 45 16 Contents D Calculation of the errors E PHOTOPT examining PHOTOM s performance PEO Fk og ote ie Reel iv 1 SUN 45 16 Running the photom software 1 Introduction PHOTOM is a package for performing photometry of digitized images It has two basic modes of operation using an interactive display to specify the positions for the measurements or obtaining those positions from a file In both modes of operation PHOTOM allows you to perform photometry using either the traditional aperture method or via optimal extraction When using the traditional aperture extraction method the target aperture can be circular or elliptical and its size and shape can be varied interactively on the display or by entering values from the keyboard Both methods allow the background sky level to be either sampled interactively by the manual positioning of an aperture or automatically from an annulus surrounding the target object PHOTOM is also used b
4. aperture minus the number due to the sky Object signal So P Py PADU D Ds ao as The error on the object signal is the quadratic sum of the errors on the individual measurements Using e to signify the error So Dy Ds 40 45 Assuming the errors are solely from photon statistics then the error on the signal is e S e P e P 40 45 The error from photon counting is the square root of the number of photons ab vP and e P VP 33 SUN 45 16 Calculation of the errors Therefore So Py Ps a2 a2 PADU x D D a2 a2 BIASLE xa 1 a a5 D The second method of calculating the errors assumes that the variance in the sky aperture corresponds to the photon noise This allows the photon errors to be calculated without knowing BIASLE One additional definition has to be given Standard deviation in sky aperture per pixel in data units If the photon error yP is equated to the standard deviation PADU 0 then the total number of photons in the sky aperture is given by P a x PADU x o The offset in the sky aperture can now be calculated BIASLE D as PADU 0 Substituting this into the calculation of the error gives e So PADU Do Dg 40 45 PADU 02 ao 1 ao as or So Sy PADU x 02 x ao 1 ao as 2 The third method of calculating the errors sums the data variances from t
5. background level in the sky aperture There are four possible choices selected by the integers 1 to 4 which have the following bindings 1 Mean All pixels in the sky aperture are averaged 2 Mean with 2 sigma rejection All pixels with data values within 2 standard deviations of the mean are averaged 3 Mode The peak of the histogram of pixel values the most likely value in the sky aperture is estimated This is the recommended choice when using optimal extraction 4 A constant A single value to be used for all measurements A sky variance standard deviation is also requested so that a realistic error can be assigned to the measurements if the error is calculated from the variance in the sky aperture SKYMAG _REAL Read The magnitude assigned to the sky level when calculating the magnitude of the object using the relation OBJMAG SKYMAG 2 5 x LOG10 SIGNAL where SIGNAL is the brightness of the object minus sky in photons SKYSIG _REAL Read A constant value for the sky variance This is an estimate of the standard deviation in the sky level in data units and is used when SKYEST is 4 TOLER _REAL Read The required positional accuracy in pixels to terminate the centroiding iterations USEMAGS _LOGICAL Read If TRUE then the output values are converted into magnitudes If FALSE the output values MAG and MAGERR are modified to be a mean photon count and the error in this count the other values remain the same i e
6. errors from variations in the sky aperture SUN 45 16 Performing interactive analyses 8 The first works out the errors from photon statistics in the sky and signal apertures This requires you to know and set up the parameters PADU and BIASLE which convert the data values to numbers of photons The message Errors from photon statistics will signify that this has been chosen The second method of calculating the errors is from the measured variance in the sky aperture This method assumes that the measured variance is due to photon statistics and scales the measurement in the object aperture accordingly This method still requires the parameter PADU to be known but does not need the BIASLE parameter to be known The message Errors from sky variance will signify that this has been chosen If neither the PADU or BIASLE parameters are known then it is best to use this method to indicate the reliability of the measurements but not to take the quoted error values as absolute since this method will be wrong by a factor VPADU where PADU is the unknown conversion factor The third method of calculating the errors is from the data errors that are stored with the image one per pixel This method of calculating the errors also requires the parameter PADU to be known The message Errors from data variance will signify that this has been chosen A variance component may not always be present in data file along with the data array ind
7. finally mouse button 3 or key 0 completes defining the aperture shape and returns you to the COMMAND prompt The parameters only have a limited number of preset values which can be cycled through by repeated presses of the middle mouse button key 2 If the range or interval of the values associated with a parameter aren t suitable then you can override these by setting a specific value using the N menu option When a parameter value is changed the aperture displayed at the cursor position is updated to reflect this The aperture is also redisplayed whenever the first mouse button is pressed or key 1 As this only changes the current parameter rather than any of the actual values you can use this function to reposition the aperture to inspect its suitability on more than one object but remember to return to the aperture parameter that you want to change before attempting to alter a value The initial values of the three parameters are just the current ones The semi major axis can be changed from a maximum of double to a minimum of a half of the current value with steps of a tenth or fifth of this interval The possible values of the eccentricity and orientation are limited to a number of preset values The value initially displayed is taken to be the member of the preset table which is closest to but lower than the current value of that parameter 5 17 M measure This performs interactive measurements of objects individually sel
8. format that PHOTOM programs will work with The native format that these use is the Starlink NDF see SUN 33 Files that contain an NDF are identified by the extension sdf and are accessed by PHOTOM programs when you just give the filename without an extension So for instance if you have a file image sdf that contains an NDF then you just supply the response image when prompted for an input image In addition to the NDF PHOTOM programs can also be made to use images in other formats such as disk FITS old style FIGARO and IRAF using the on the fly conversion abilities of the NDF library To make this work for you you need to setup the CONVERT package and then pass images to PHOTOM programs together with their file extensions So if you wanted to work on the IRAF image pix imh you d use a command sequence like 4 convert kappa xmake xwindows overlay ovcolour green display in pix imh device xw photom in pix imh device xov enter interactive command loop 13 SUN 45 16 Acknowledging this software In this case the file extension tells PHOTOM and the KAPPA program DISPLAY that they have an IRAF image You should look at the CONVERT document SUN 55 to see the list of formats that can be processed Thanks go to Nial Tanvir for his helpful comments on the sky background estimation section in Appendix C and a number of people for suggestions for improvements to PHOTOMover the years T
9. ptions This allows some of the defaulted parameters to be changed from within the program P hoton statistics This selects the method for calculating the errors either from photon statistics or from the measured variance in the sky aperture or from the variance component of the data array S ky This selects between the different methods of estimating the background level in the sky aperture V alues This summarizes the current settings of the programs parameters e X traction Toggle between doing optimal and aperture photometry CONCEN _LOGICAL Read Find the sky automatically from a concentric aperture or select the sky regions interactively If this is TRUE the sky level is estimated from an aperture which is concentric about the object aperture The shape and orientation of the sky aperture is the same as the object aperture and the size of the annular aperture is defined by the INNER and OUTER parameters This mode is used if the measurement positions are being supplied from a file This is the recommended mode for carrying out optimal extraction If this is FALSE the sky level is estimated from an aperture equal in size and shape to the object aperture which is positioned manually on the image display In this mode several consecutive sky measurements can be made around the object of interest and these are averaged to give the final sky estimate DEVICE DEVICE Read The name of the device to be used for int
10. your images so a statistical approach with some measurements in error is a necessity 2 You just want to use PHOTOM as an engine for other interactive or script based tools The second case is covered by the program AUTOPHOTOM which is used by the photometry mode in the GAIA SUN 214 image display tool The first case by the PHOTOM program in a special mode You can use PHOTOM in batch mode either interactively or by controlling it from an ICL command procedure or from a C shell script If you use a script of some kind then you ll also need another file that contains the commands you would have used interactively It may be necessary to run the program by hand first to verify the order of the prompts An example input file photom in say could contain the following commands FRAME Toonz ooo POSITIONS DAT E In this example the image data is assumed to be in a file FRAME in the default directory The size and shape of the aperture is set using the non interactive command N and the command F instructs the program to take the initial positions from the file POSITIONS DAT The E command ends the program The program could then be run in the background with the command photom lt photom in gt photom out SUN 45 16 Using different image formats 12 Using PET requires that a file with a specified format is created the details of which are described in Basically this describes each aperture and also allows inform
11. 20 rejection also exclude contaminating pixels but using the mask along with the mean estimator allows this to be done in a controlled way If the USEMASK flag is TRUE then a file containing a list of positions is requested MASKFILE The format of the file is the same as for inputting a list of positions to measure command F namely an index number followed by an x and y position The given coordinates define the centres of circles and any pixel with its centre within a circle will be excluded from the sky estimation The radius of the masking circle is defined by another parameter MASKRAD 11 SUN 45 16 Automated photometry The mask only affects pixels in the background aperture it does not exclude any pixels from the measurement aperture This means that identical lists can be used to create the mask and to provide a source for measurement The output from an automatic object finding package could be used in this way It is important not to confuse this mask with the point spread function masks discussed as part of the optimal extraction algorithm Other ways of masking out pixels are to use the KAPPA SUN 95 facilities ARDGEN and ARDMASK or the ARD and PATCH toolboxes of the GAIA SUN 214 image display tool 6 Automated photometry PHOTOM provides two methods to perform the photometry of objects in a non interactive fashion There are typically two reasons why you would want to do this 1 There are far too many objects on
12. EN ANGLE The n added to the SKY identifier indicates the number of the sky region being defined and is optional The other fields are the same as for an object aperture It is VERY heavily recommended that annuli are used for sky measurement when using the optimal extraction algorithm unless there are obvious reasons for not doing SO e The format of the object file must be correct 21 PHOTGREY SUN 45 16 Full routine descriptions PHOTGREY Displays a grey scale image Description Plots an image as a greyscale on a suitable device Usage PHOTGREY Parameters IMAGE IMAGE Read Name of the image XSTART _REAL Read The first X pixel of the image to be displayed XEND _REAL Read The last X pixel of the image to be displayed YSTART _REAL Read The first Y pixel of the image to be displayed YEND _REAL Read The last Y pixel of the image to be displayed LOW _REAL Read The data value corresponding to black on the display HIGH _REAL Read The data value corresponding to white on the display DEVICE DEVICE Read The image display device SUN 45 16 Full routine descriptions 22 PHOTOM PHOTOM Perform aperture photometry Description PHOTOM performs photometry It has two basic modes of operation using an interactive display to specify the positions for the measurements or obtaining those positions from a file In both modes the user may perform photometry using either the traditiona
13. ODE CLIP SEE POSITIONS Where the fields have the following meaning INDEX For PSF star this MUST be 0 XPOS Xcoordinate of object YPOS Y coordinate of object FWHM1 FWHM of the PSF in the X direction FWHM2 FWHM of the PSF in the Y direction ROT Rotation of the FWHM from strict X Y orientation CODE current object status CLIP clipping radius SEE estimate of the seeing in pixels POSITIONS how the sky regions are determined Values that are unknown initially eg FWHM1 FWHM2 ROT should be set to 0 0 the derived values will be used to replace these fields on exit The CODE field should be set to OK initially The POSITIONS field should have one the values annulus or regions to indicate how the sky regions are determined this is ignored if SKYEST is 4 Aperture must be circular for optimal extraction so no SHAPE field is provided Further stars should be entered with the following information INDEX XPOS YPOS MAG MAGERR SKY SIGNAL CODE POSITIONS SUN 45 16 Full routine descriptions 20 AUTOPHOTOM Pitfalls Where the fields have the following meaning INDEX unique integer identifying this object XPOS Xcoordinate of object YPOS Y coordinate of object MAG current magnitude mean count of object MAGERR current error in magnitude mean of object SKY current estimate of sky value for object SIGNAL current estimate of the total count in object CODE current object s
14. On line help is available for all the PHOTOM programs using the command photomhelp from the C shell or by using the command ICL gt help photom when running ICL Help can also be found by specifying or as the reply to any program prompt As an alternative to these approaches this document may also be viewed on line using a hypertext browser To do this use the command showme sun45 If you come across any bugs or problems when using PHOTOM then e mail a description to starlink jiscmail ac uk 5 Performing interactive analyses Interactive measurement is performed in PHOTOM using a program which is also called PHOTOM Alternatively if it is available on your system you can use the photometry toolbox which is part of the GAIA SUN 214 display tool In the following discussion it is assumed that you are using the PHOTOM program The first thing you need to do is display your image Although PHOTOM can work using a display that does not have an overlay it is best to have one This is so that any line graphics which PHOTOM draws can be erased Without this ability the display becomes quickly confused particularly when setting the size and orientation of the aperture You can create a GWM X windows display with an overlay using the command xmake xwindows overlay ovcolour green and can display an image in this window either using the KAPPA SUN 95 DISPLAY program or if this isn t available the PHOTGREY program Y
15. SUN 45 16 Starlink Project Starlink User Note 45 16 Nicholas Eaton Peter W Draper amp Alasdair Allan 27th November 2009 PHOTOM A Photometry Package Version 1 12 0 User s Manual SUN 45 16 Abstract ii PHOTOM is a package for measuring the sky corrected magnitudes and fluxes of astronomical objects within circular and elliptical apertures using either the aperture or optimal extraction algorithms pae ii SUN 45 16 Contents Contents 1_ Introduction 1 2 Newtoaperture photometry 1 3 New to optimal photometry 1 4 Running the photom software 1 LL SEIN Elle p Rae Ee ei 2 5 Performing interactive analyses 2 5 1 The PHOTON menu options gt lt LUE NUS es Ra PR presi 3 5 1 1 A amnnulus 44 44 44 2 3 5 1 2 C centroid 44 44 44 44 4 513 exitl ri sa 4 4 4 50 4 a ee ee dd ea da du nt nus de ee 4 5 1 4 F file of positions a ire RR ea d 4 5 1 5 sas Be SR ie ie ini 5 5 1 6 I interactive shape AA BER TARE RR ds 5 5 1 7 M measure 0 0 0 000000 a 5 5 1 8 N non interactive shape iis 4 ch eee eee eee ee ee ees 6 Ei A eh ae eS ee ER oS 7 5 1 10 E photon sta Dove eae ace A ae rea 7 A PA TE SICA Ae rr re 8 5 1 12 V values LL 9 5 1 13 _X eXtraction lt srek su ke a ha da hea ai n ya a aa ia a 9 5 2 Defaulted parameters opel boo eb a ed n 9 52 resfilel e tao e e o AR ra Oe eo 9
16. TOPHOTOM IN INFILE OUTFILE Parameters BIASLE _REAL Read The level in data units per pixel of any constant offset in the image This should be defined if the errors are to be calculated from photon statistics If the true value is unknown then return 0 CENTRO _LOGICAL Read Centre the object before measurement or accept the given position as the centre of the aperture This parameter is forced to be true for optimal extraction If this is TRUE the aperture is centered around the object of interest before the measurement is taken The position supplied to the program is taken as a starting point and the position of maximum flux is located within a search box of fixed size If this is FALSE the position supplied to the program is used as the centre of the aperture CLIP REAL Read The clipping radius used for the weight mask if optimal photometry is selected Not to be confused with the mask used to exclude regions from the background estimate USEMASK parameter EXSOURCE LITERAL Read The source of the image exposure time supplied via the ETIME parameter This can take one of the values HDS CONSTANT or HEADER with the following meanings HDS indicates that the exposure value is stored in an HDS object somewhere in the image this presumes that the image is an NDF and corresponds to the original behaviour of PHOTOM prior to the introduction of this parameter CONSTANT indicates that a simple floating point value w
17. a measurement POSFILE FILENAME Read Name of the file containing a list of positions for measurement The file should contain a minimum of three columns the first of which contains an integer index number and the next two contain an x and y position POSITIVE _LOGICAL Read Find the object centroid for image features which are positive or negative with respect to the background This should be set to TRUE RESFILE FILENAME Write Name of the file to receive the results of the measurements SATURE _REAL Read The saturation level in data units for the image If any pixels in the object aperture have values greater than this then the measurement is flagged with an S in the output record SEARCH _INTEGER Read The size of the search box in pixels to be used in locating the object centroid SEE _REAL Read Approximate seeing in pixels used to estimate the FWHM of the point spread function PSE by the optimal extraction algorithm SEMIM _REAL Read The semi major axis of the ellipse defining the aperture in pixel units For a circular aperture this corresponds to the radius in pixel units SUN 45 16 Full routine descriptions 26 PHOTOM SKY _REAL Read A constant value to be used as the sky estimate for subsequent measurements This defines the sky level in data units per pixel This value is used until another estimator is chosen SKYEST _INTEGER Read Select the estimator to be used to evaluate the
18. ariance This is an estimate of the standard deviation in the sky level in data units and is used when SKYEST is 4 TOLER _REAL Read The required positional accuracy in pixels to terminate the centroiding iterations USEMASK _LOGICAL Read Define a mask to exclude regions from the background estimate If this is TRUE an ARD description is requested Contaminating objects such as bright stars can thus be removed from the background estimate Notes e Aperture Extraction The input output file must contain one line per object that has the following information INDEX XPOS YPOS MAG MAGERR SKY SIGNAL CODE MAJOR ECCEN ANGLE POSITIONS SHAPE Where the fields have the following meaning INDEX unique integer identifying this object XPOS Xcoordinate of object YPOS Y coordinate of object MAG current magnitude mean count of object MAGERR current error in magnitude mean of object SKY current estimate of sky value for object SIGNAL current estimate of the total count in object CODE current object status MAJOR length of semimajor axis of aperture ECCEN eccentricity of object aperture ANGLE position angle of object aperture POSITIONS how the sky regions are determined SHAPE shape of the aperture Values that are unknown initially MAG MAGERR SKY and SIGNAL should be set to 0 0 the derived values will be used to replace these fields on exit The CODE field should 19 AUTOPHOTOM SUN 45 16 F
19. ata units If there are any pixels in the object aperture with values greater than the saturation level then this is indicated by an error code S in the final column of the output table The object magnitude is calculated with the saturated pixel included in the result So changing the value of this parameter will not change the results but will alter the number of objects flagged in the output file The CLIP parameter is clipping radius of the weight mask used in optimal extraction This is needed if optimal extraction is enabled and defaults to 5 pixels If optimal extraction is not enabled using the X command then the CLIP parameter will not appear in the list when the options command is issued The SEE parameter is a rough estimate of the seeing in the CCD image in pixels This is used by the optimal extraction algorithm for an initial estimate of the FWHM of the point spread function PSF during fitting This parameter defaults to 2 pixels and again if optimal extraction is not enabled then this parameter will not appear in the list when the options command is issued 5 1 10 P photon statistics This is used to choose between the different ways in which the errors are calculated There are three possible choices selected by the integers 1 to 4 which have the following meanings 1 Errors from photon statistics 2 Errors from variations in the sky aperture 3 Errors from data variance 4 o gt wa Gaussian
20. ation about the sky region associated with it to be given The sky region can be defined in a single annulus each aperture can have a different inner and outer scale or can be defined as a series of other apertures The easiest way to create such a description of the apertures on an image is to use the GAIA display tool This description can then be run on other frames non interactively say for different colours or repeat measurements 7 Altering program parameters When a PHOTOM application is started the initial selection of most of the parameters is taken from the previous run of the routine The parameter values are stored in the GLOBAL sdf or application_name sdf files in the adam directory at the end of a run The current values can be examined using the HDSTRACE facility SUN 102 To clear these values and to revert to the start up defaults the GLOBAL sdf and application_name sdf files have to be deleted The starting values of the parameters can also be specified within the ADAM command language The keyword facility allows the parameters to be given on the command line see SG 4 the ADAM User s Manual The keywords have the same name as the parameters for example the search box for the centroiding can be changed using the command ICL gt photom search 5 From the C shell the keywords can be included on the command line photom search 5 8 Using different image formats Little has been said so far about the image data
21. chosen as the default This choice is controlled by the CONCEN parameter If the positions of the objects is entered by a file of positions command F then the background is automatically taken with the concentric annulus whatever the default value of CONCEN SUN 45 16 Performing interactive analyses 4 5 1 2 C centroid This is a toggle switch which alters whether the object is centered in the aperture before doing the measurement Centroiding is controlled by the parameters SEARCH POSITIVE MAXSHIFT MAXITER and TOLER These cannot be changed from within the program and if alternative values are required they should be given on the command line when starting PHOTOM For instance photom positive f would make PHOTOM centroid on objects whose signal was negative not a very likely choice The choice of mode is indicated by the messages Centroiding in stellar aperture or No centroiding When starting PHOTOM one of these modes will be chosen as the default Centroiding cannot be turned off when using optimal extraction Unless the field under investigation is very crowded or there are other special conditions it is probably best to leave the centroiding option on all the time This is more important if the measurements are being made in non interactive mode command F unless you re certain that the positions are accurate for all images 5 1 3 E exit This command exits PHOTOM 5 1 4 F file of positio
22. cting a representative piece of sky the procedure is repeated a number of times to increase the statistics PHOTOPT selects regularly spaced points within an image puts down an aperture at each of the points and calculates the difference between the central aperture and the sky estimate from the surrounding annulus Each of the three sky estimators offered by PHOTOM are tried in turn on the same set of points so that a comparison can be made between the three methods The output is in the form of two graphs for each estimator The first shows the difference between the central aperture and the sky estimator for each of the samples The sample number forms the x axis and the difference object sky given in photons per pixel forms the y axis The second plot shows a histogram of the differences for the samples The differences are binned into suitable intervals and form the x axis of the histogram with the y axis giving the number of samples in each difference bin PHOTOPT has a number of parameters in common with PHOTOM the aperture shape parame ters SEMIM ECCEN and ANGLE the scaling factors PADU and SATURE and the background annulus size INNER and OUTER The two parameters unique to PHOTOPT are NP the number of points to sample up to a maximum of 100 and the RANGE which defines the bounds of the data value used in the plot The number of points sampled may not exactly equal the number requested as the program automatically positions the poi
23. e RESFILE parameter in order that this file can be accessed by database routines The first line of the output file will contain the details of the PSF star there will be seven columns in this line containing the following information Column Name Description 1 INDEX Index number of star 2 XPOS X position of centre of aperture in pixels 3 YPOS Y position of centre of aperture in pixels 4 FWHM1 1st FWHM of the point spread function 5 FWHM2 2nd FWHM of the point spread function 6 ROT Rotation from perpendicular 7 CODE Error code flag The remaining lines contain details of the measurements each of these lines will have eight columns with the following information Column Name Description 1 INDEX Index number of star 2 XPOS X position of centre of aperture in pixels 3 YPOS Y position of centre of aperture in pixels 4 MAG Magnitude or mean photon count of star 5 MAGERR Error in MAG 6 SKY Sky value in photons per pixel 29 PHOTOM SUN 45 16 Full routine descriptions 7 SIGNAL Total number of photons in aperture due to star 8 CODE Error code flag SUN 45 16 Techniques of aperture photometry 30 B Techniques of aperture photometry In principle aperture photometry of digitized data is a straightforward procedure Put down a computer generated aperture over the grid of data and add up the counts within the aperture In astronomical applications the usual purpose of aperture photometry is to measure the brightness
24. e account of a sloping background then a sky aperture can be selected a number of times and the mean of these values is used The calculation of the mean is only cleared when an object is measured so if a mistake has been made in estimating the mean of the skies then an object measurement has to be made and a note made that the measurement was in error before going back to the estimation of the sky Control stays with the interactive menu until the third mouse button key 0 is pressed The results of the measurements are displayed on the terminal and sent to the file accessed by the RESFILE parameter 5 1 8 N non interactive shape The size and shape of the aperture can be specified from the keyboard by entering values for the semi major axis in pixels the eccentricity and the orientation of the ellipse defining the aperture When using optimal extraction this command is disabled the clipping radius can be changed using the V menu option An eccentricity of 0 gives a circular aperture with a radius equal to the semi major axis The orientation of the ellipse is given in degrees and specifies the rotation of the semi major axis of the ellipse anti clockwise with respect to the vertical axis of the pixel array 7 SUN 45 16 Performing interactive analyses 5 1 9 O options This allows changes of the values of some of the non aperture shape related parameters The INNER and OUTER parameters define the size of the annulus to be u
25. e aperture in pixel units For a circular aperture this corresponds to the radius in pixel units
26. e descriptions 28 PHOTOM The error CODE can take on three possible values One or more pixels in the object aperture is bad One or more pixels in the object aperture is above the saturation level The object aperture intersects the edge of the data array There are other problems with the measurement NOW If a bad pixel occurs in the object aperture then the pixel is not included in the calculation of the object signal The bad pixel is not replaced by an estimate If a saturated pixel occurs in the object aperture then it is included in the calculation of the object signal If the aperture intersects the edge of the data array the object signal is calculated for the reduced area of the aperture Only one of these code letters is displayed even if more than one of the conditions has occurred The codes are in a increasing hierarchy B S E such that S overrides B and E overrides S Optimal Extraction Format of Associated Files As for aperture extraction the file containing the position of objects to be measured com mand F is read in free format as above However in the case of optimal extraction the first line of the file must have an index number INTEGER of 0 along with the x and y position of the PSF candidate star The output on the screen contains column headers to indicate the content of each column of the results These column headers do not appear in the output file given by th
27. e sky count for brighter stars though this means that the extraction will be non optimal Hence we have that _ Lig Py Dij Sij Ei PE SUN 45 16 Calculation of the errors 32 From this we note that if the PSF is wrong there will be no systematic bias in the results provided one is interested in the relative brightness of one star with respect to another in the same frame A full treatment of optimal extarction can be found in Tim Naylor s MNRAS paper An optimal extraction algorithm for imaging photometry MNRAS 1998 296 339 to which the reader is directed for further information D Calculation of the errors The errors are calculated in one of four ways as discussed in the section on command P The first method assumes true photon statistics and the error is calculated from the following definitions Number of pixels in object aperture a Number of pixels in sky aperture as Sum of data in object aperture D Sum of data in sky aperture D Offset in one pixel BIASLE Number of photons per data unit PADU The contribution of the sky in the object aperture can now be calculated Number of photons in object aperture P PADU D as BIASLE Number of photons in sky aperture P PADU D as x BIASLE Number of photons in object aperture due to sky Ps PADU D as BIASLE x ao as The signal due to the object is the difference of the total number of photons in the object
28. ected from the displayed frame If using aperture extraction the size and shape of the cursor should be set up in advance using the I or N commands while the clipping radius should be set using the 0 command when using optimal extraction There are two basic methods for measurement of background Either the background is sampled from an annulus around the object aperture or from a separately chosen area of sky see command A The two cases can be distinguished at this stage from the on screen display SUN 45 16 Performing interactive analyses 6 In the case of the manual sky measurement the middle box is labelled with SKY while for automatic sky measurement it will remain empty The left hand box with either be labelled STAR in the case of aperture extraction or PSF when optimal extraction is selected The remaining right hand box will be annotated RETURN TO KEYBOARD To perform the measurements with the automatic sampling of the sky the cursor is positioned over the chosen object and the left hand mouse button or key 1 is pressed If optimal extraction is selected the first measurement will define the point spread function PSF for the technique It is important to pick a bright star which is unsaturated for this task After this measurement has been taken if you are using a terminal capable of erasable line graphics such as an xoverlay then the left hand box label will change to read STAR to denote that further measurements will be
29. edge of the annular sky aperture in units of the object aperture size The actual dimension in pixels is obtained by multiplying this factor by the object aperture semi major axis in pixels PADU _REAL Read The number of photons for each interval of the data If the true value is unknown use a value of 1 in which case the quoted measurement errors will be wrong by the unknown factor SORT PADU PHOTON _INTEGER Read Select the method for calculating the measurement errors There are three possible choices selected by the integers 1 to 4 which have the following bindings 1 The errors are estimated from the photon statistics in the sky and object apertures The parameters PADU and BIASLE should be set to their appropriate values to convert the data units to photon numbers 2 The errors are estimated from the measured variance in the sky aperture This method assumes that the measured variance is due to photon statistics and estimates the error in the object aperture accordingly The PADU parameter should be set to its appropriate value to convert the data units to photon numbers 3 The errors are estimated from the variance component of the data array 4 The errors are estimated from the measured variance in the sky aperture This method assumes that the errors are Gaussian same value per object and sky pixel and thus requires no knowledge of the values of PADU and BIASLE but can only be considered an upper limit on the error in
30. eed this can only be true if you are storing your images in NDFs see section 88 and if this is the case then PHOTOM will issue the warning Data does not have a variance component if this method is selected The fourth method of calculating the errors is like the second and uses the measured variance in the sky aperture This method assumes that the measured variance is due to some gaussian source and doesn t require any knowledge of the PADU and BIASLE values which are unknown when dealing with data that has been combined using a mean say from a CCD Mosiac dithered on the sky but clearly this can only measure an upper limit as the actual noise in the object will be fractionally less than in the sky The best way to avoid such uncertainty is by propagating data variances through all the stages that produced the combined data and using method three Appendix D gives a full discussion of the calculation of the errors assuming photon statistics 5 1 11 S sky This is used to choose between the different methods of estimating the background level in the sky aperture There are four possible choices selected by the integers 1 to 4 which have the following meanings 1 Simple mean 2 Mean with 2 sigma rejection 3 Mode 4 A constant The simple mean uses all the values in the sky aperture The mean with 2 sigma rejection excludes all those points which are more than 2 standard deviations from the mean Because one
31. efaulted parameters A number of parameters can only be defined when the PHOTOM program starts They all have reasonable defaults but if required can be changed before running the program The way to set a new value for one of these is to use the keyword the name by which the parameter is always referred to on the command line as in photom usemags f resfile flux dat This outputs the values in photon counts i e as modified by the BIASLE and PADU parameters and writes the results of the analysis to the file flux dat 5 2 1 resfile This specifies the name of the results file which makes a permanent record of the measurements 5 2 2 maxshift maxiter search and toler There are a number of parameters that control the centroiding algorithm SEARCH defines the size of the search box to be used in locating the centroid in pixels MAXITER defines the maximum number of iteration steps MAXSHIFT gives the maximum allowable shift in pixels between the initial rough position and the calculated centroid TOLER defines the position accuracy in pixels that will terminate the centroiding iterations SUN 45 16 Performing interactive analyses 10 5 2 3 exsource and etime These two parameters control how a value for the image exposure time is determined The exposure time is used to scale the results as in mag SKYMAG 2 5log 9 signal exposure time This affects the output values for the measured signal in the object and the resul
32. eractive measurements on which the data has been displayed If the device has an overlay plane then this should be selected ECCEN _REAL Read The eccentricity of the ellipse defining the aperture For a circular aperture this should be set to 0 0 EXSOURCE LITERAL Read The source of the image exposure time supplied via the ETIME parameter This can take one of the values HDS CONSTANT or HEADER with the following meanings SUN 45 16 Full routine descriptions 24 PHOTOM HDS indicates that the exposure value is stored in an HDS object somewhere in the image this presumes that the image is an NDF and corresponds to the original behaviour of PHOTOM prior to the introduction of this parameter CONSTANT indicates that a simple floating point value will be supplied for the image exposure time HEADER indicates that the value to be used is stored in the image header i e FITS headers HDS ETIME LITERAL Read A string that according to the value returned for parameter EXSOURCE allows the exposure time of the image to be determined If EXSOURCE is defined as HDS then a fully qualified HDS path to the required object within the NDF should be given For instance if the exposure time is stored in the CCDPACK extension of an NDF under the item ETIME then a suitable return would be more ccdpack etime The HDS structure of an NDF can be viewed using the HDSTRACE utility see SUN 102 CONSTANT then a floating poi
33. graphics are controlled by the keyboard A message from PHOTOM shows which of these options applies to you 5 1 The PHOTOM menu options The following section describes each of the menu items that you can use at the COMMAND prompt 5 11 A annulus This is a toggle switch which alters the way in which the background level is measured There are two methods available The first is interactive and uses an aperture identical in size and shape to the object aperture The aperture is positioned manually to select the region of sky to measure The message Interactive aperture in use will signify that this has been chosen When using the interactive aperture several sky areas can be sampled to improve the estimate of the sky The sky estimates from each aperture are simply summed and the mean of these is used when the object is measured The alternative is to use an aperture which is a concentric annulus around the object aperture In this mode the sky is measured automatically every time a measurement is made The message Concentric aperture in use signifies this choice The size of the sky aperture is specified by the INNER and OUTER parameters These are defined as multiplying factors of the object aperture size or in optimal extraction mode of the clipping radius e g if INNER were 2 and OUTER 3 then the sky aperture would start at two radii from the centre and end at three radii When starting PHOTOM one of these modes will be
34. he measurement is taken The position supplied to the program interactively or from a file of positions is taken as a starting point and the position of maximum flux is located within a search box of fixed size If this is FALSE the position supplied to the program is used as the centre of the aperture CLIP _REAL Read The clipping radius used for the weight mask if optimal photometry is selected Not to be confused with the mask used to exclude regions from the background estimate USEMASK parameter COMMAND _CHAR Read The next action The options are defined by single letter entries and should be one of the following 23 PHOTOM SUN 45 16 Full routine descriptions A nnulus This toggles between using an annular background aperture and an interactive aperture C entroid This switches the centroiding of the object in the aperture on and off E xit This command terminates the current PHOTOM session F ile of positions This command takes positions from a file and performs pho tometry with the current aperture parameters H elp This displays a brief line of help for each command I nteractive shape This allows the size and shape of the aperture to be adjusted interactively on the screen M easure This performs interactive measurements of objects individually se lected from the screen N on interactive shape The size and shape of the aperture in pixels is entered from the keyboard O
35. he optimal extraction algorithim used in PHOTOMJoriginated with Tim Naylor and thanks should go to him for his help during its incorporation into the code Please acknowledge the use of this software in any publications arising from research in which it has played a significant role Please also acknowledge the use of any other Starlink resources hardware or software in such publications The following is suggested as a suitable form of words The authors acknowledge the data analysis facilities provided by the Starlink Project which is run by CCLRC on behalf of PPARC In addition the following Starlink packages have been used PHOTOM SUN 45 16 Full routine descriptions 14 15 AUTOPHOTOM SUN 45 16 Full routine descriptions AUTOPHOTOM Do aperture photometry on a list of objects Description This program performs photometry of a list of objects It is designed to be used non interactively i e by the GAIA SUN 214 photometry tool or script It provides more flexibility than the automated mode of the PHOTOM program by allowing the specification of sky regions other than in annular regions The results of the measurements are recorded in another file which has the same format as the input file and can therefore be passed back to this routine and the same measurements can be repeated on a new frame The format of this file is different for aperture and optimal extraction and is described in the relevant sections Usage AU
36. he variance component of an NDF Two additional definitions have to be given Sum of variance in object aperture V Sum of variance in sky aperture V The error is then calculated from e S0 PADU Vo V 40 45 3 The magnitude error is calculated from differentiating the magnitude equation m 2 510g 0 So thus 2 5 e S pe m nio S The fourth method is like method two but the sky variations are interpreted by a gaussian error source so PADU and BIASLE are not required With guassian errors the source signal is effectively zero since it has the same noise per pixel as the sky so So PADU 03 y a0 4 and So PADU x D D ao as 5 so the PADUs cancel out in the dm calculation SUN 45 16 PHOTOPT examining PHOTOM s performance 34 E PHOTOPT examining PHOTOM s performance PHOTOPT is an auxiliary package to examine the performance of the various sky estimators used by PHOTOM As indicated in section B the best choice of estimator depends on the circumstances The idea behind PHOTOPT is to put down an aperture on a random piece of sky estimate the background from a concentric aperture using one of the sky estimators offered by PHOTOM and subtract this estimated sky from the flux in the central aperture If the sky estimator correctly estimates the sky then the difference of the two will be zero Since an automatic procedure cannot be certain of sele
37. ill be supplied for the image exposure time HEADER indicates that the value to be used is stored in the image header i e FITS headers SUN 45 16 Full routine descriptions 16 AUTOPHOTOM HDS ETIME LITERAL Read A string that according to the value returned for parameter EXSOURCE allows the exposure time of the image to be determined If EXSOURCE is defined as HDS then a fully qualified HDS path to the required object within the NDF should be given For instance if the exposure time is stored in the CCDPACK extension of an NDF under the item ETIME then a suitable return would be more ccdpack etime The HDS structure of an NDF can be viewed using the HDSTRACE utility see SUN 102 CONSTANT then a floating point value should be given HEADER then the name of the associated item should be given e g the FITS item EXPOSURE FIXANN _LOGICAL Read If TRUE then any annular regions in the input description file are interpreted as radii in pixels along the aperture major axis otherwise they are interpreted as scale factors of the major axis FALSE INFILE LITERAL Read Name of the file containing the descriptions of the objects to measure and the positions and nature of any sky regions associated with them See the notes section for the format of this file IN IMAGE Read Name of the image on which aperture photometry will be performed USEMAGS _LOGICAL Read If TRUE then the output values are c
38. l aperture method or using optimal extraction During aperture photometry the aperture can either be circular or elliptical and the size and shape can be varied interactively on the display or by entering values from the keyboard or parameter system During optimal extraction the mask clipping radius can also be varied from the keyboard or via the parameter system The background sky level can be sampled interactively by manual positioning of the aperture or automatically from an annulus surrounding the object PHOTOM is a menu driven application The menu has been designed around single character entries which hopefully have easily remembered mnemonics Many of the options have counterparts in the parameter system and so can be controlled outside the task by the environment Parameters ANGLE _REAL Read The orientation of the ellipse defining the aperture This is defined in degrees going anti clockwise from the positive y axis This is equivalent to a position angle BIASLE _REAL Read The level in data units per pixel of any constant offset in the image This should be defined if the errors are to be calculated from photon statistics If the true value is unknown use a value of 0 CENTRO _LOGICAL Read Centre the object before measurement or accept the given position as the centre of the aperture This is forced to be true for optimal extraction If this is TRUE the aperture is centered around the object of interest before t
39. nd the results files are given with the full PHOTOM descriptionjin appendix A 5 SUN 45 16 Performing interactive analyses 5 15 H help This displays a brief line of help for each command For more extensive information refer to this manual or the on line help 5 1 6 I interactive shape This allows the size and shape of the aperture to be adjusted interactively on the screen to best suit the objects When using optimal extraction this command is disabled the clipping radius can be changed using the O menu option The size and shape of the aperture is governed by the three ellipse parameters the semi major axis in pixels eccentricity and position angle The semi major axis and eccentricity are as usually defined for an ellipse An eccentricity of 0 gives a circular aperture with a radius equal to the semi major axis The orientation of the ellipse is given in degrees and specifies the orientation of the semi major axis of the ellipse anti clockwise with respect to the vertical axis of the screen When this option is chosen five boxes appear on the graphics display The upper two show the parameter that will be changed and its current value The lower three boxes show the functions that are performed by the mouse buttons or keys if your device doesn t support a mouse Mouse button 1 or key 1 changes the parameter to one of SEMI MAJOR ECCENTRICITY or ORIENTATION Mouse button 2 or key 2 changes the value of the parameter and
40. ns This command causes the measurements to be done automatically A file containing the positions is requested and the photometry is performed with the current parameters The name of the file containing the positions is requested through the POSFILE parameter If the file cannot be found or is not in a suitable format then no action is taken The file of positions is an ordinary text file and should specify an index number and the x and y positions in pixel coordinates For every x and y pair in the file an measurement is made sampling the sky with the concentric annulus whose size is specified by the current values of the INNER and OUTER parameters Centroiding in the object aperture is or is not done depending on the current value of the CENTRO parameter which is selected with command C When the input file is exhausted PHOTOM returns to the command level If optimal extraction is enabled the first entry in the the POSFILE should have index 0 and x and y co ordinates corresponding to the chosen PSF star Results of the measurements are shown on the terminal as well as output to the file named by the RESFILE parameter Results are identified by the index number associated with the x and y position in the input file A previous results file can be used as the input file of positions but it should not have the same name as the results file otherwise the program will fail when it tries to open a new results file The format of the input a
41. nt value should be given HEADER then the name of the associated item should be given e g the FITS item EXPOSURE IN IMAGE Read Name of the image on which the photometry will be performed INNER _REAL Read The radius of the inner edge of the annular sky aperture in units of the object aperture size The actual dimension in pixels is obtained by multiplying this factor by the object aperture semi major axis in pixels MASKFILE FILENAME Read Name of the file containing the positions to be used as centers for masking objects from the sky aperture The file should contain a minimum of three columns the first of which contains an integer index number and the next two contain an x and y position MASKRAD _REAL Read The radius in pixels of the circles used to mask out objects from the background estimate A pixel which is inside the sky aperture and inside a masked region is not included in the background estimate MAXITER _INTEGER Read The maximum number of iteration steps to be used in locating the object centroid MAXSHIFT _REAL Read The maximum allowable shift in pixels between the initial object position and the calculated centroid OPTIMA _LOGICAL Read If this is TRUE then optimal rather than aperture extraction is used for photometric measurement The default for backward compatibility reasons is FALSE 25 PHOTOM SUN 45 16 Full routine descriptions OUTER _REAL Read The radius of the outer
42. nts to be on a rectangular grid which evenly covers the whole data array The program also ensures that the density of points does not result in the central apertures overlapping PHOTOPT can be run from ICL or the C shell From ICL use ICL gt photopt From the C shell use assuming photomstart has been executed photopt The full description of PHOTOPT follows 35 PHOTOPT SUN 45 16 PHOTOPT examining PHOTOM s performance PHOTOPT Perform sampling experiments with different sky estimators Description PHOTOPT examines the performance of the three different sky estimators used by PHO TOM on a particular frame It does this by performing aperture photometry on random parts of the frame subtracting the estimated sky level from a concentric aperture from the level in the central aperture If the estimator is good then the expected result is zero as long as there are no objects in the central aperture This is repeated a number of times over the frame to ensure that a fair representation of the frames characteristics is obtained The results are shown in graphical form as a set of difference graphs The histogram of differences will indicate which is the best suited estimator for the frame Parameters ANGLE _REAL Read The orientation of the ellipse defining the aperture This is defined in degrees going anti clockwise from the positive y axis This is equivalent to a position angle DEVICE DEVICE Read The name of
43. of an object without including possible contributions from contaminating sources such as bias levels sky defects or other stars and galaxies Some if not all of these contaminants will always be present in a finite sized aperture and so this background has to be accounted for If it were possible the best place to estimate this background would be behind the object i e in the object aperture with the object not there As this is usually not possible to achieve except for the case of moving objects or supernovae the usual method is to estimate the background from other regions close to the object Estimating the contribution in this background is not always straightforward In the simplest case the histogram of pixel values in the background will have an approximately Gaussian distribution due to random fluctuations and the best estimator is a simple mean It is however common for real astronomical situations to be less straightforward than this Other contributors are likely to be present such as non random noise bad pixels cosmic rays and the presence of other objects in the background Even when the possible contaminating objects are very faint compared to the object to be measured the histogram of pixel values can be sufficiently skewed to result in the mean giving an estimate of the sky too poor for high precision photometry In this case it is usual to use some sort of clipping or filtering to remove the effects of such contamination
44. onverted into magnitudes If FALSE the output values MAG and MAGERR are modified to be a mean photon count and the error in this count the other values remain the same i e the sum of sky corrected photons and the mean sky value Note the SKYMAG value is not used when this is FALSE TRUE MASK LITERAL Read An ARD description of any regions to be excluded from the image before any calcula tions of sky and object are performed The ARD language is described in SUN 183 A filename can be given using the indirection character MAXITER _INTEGER Read The maximum number of iteration steps to be used in locating the object centroid MAXSHIFT _REAL Read The maximum allowable shift in pixels between the initial object position and the calculated centroid OPTIMA _LOGICAL Read If this is TRUE then optimal rather than aperture extraction is used for photometric measurement The default for backward compatibility reasons is FALSE OUTFILE FILENAME Read Name of the file to contain the updated descriptions of the measured objects See the notes section for the format of this file 17 AUTOPHOTOM SUN 45 16 Full routine descriptions PADU _REAL Read The number of photons for each interval of the data If the true value is unknown use a value of 1 in which case the quoted measurement errors will be wrong by the unknown factor SORT PADU PHOTON _INTEGER Read Select the method for calculating the measurement error
45. ou can now start up the main PHOTOM program by typing the command photom 1At one time this program was the only application in PHOTOM hence the now slightly confusing terminology 3 SUN 45 16 Performing interactive analyses this could be from the C shell or ICL The first response from this program is a request for the name of the image you have just displayed This image remains open until you exit from PHOTOM If you need to measure objects in another image then you should exit PHOTOM display the new image and then restart PHOTOM The next request should be for a parameter COMMAND which indicates that you are now in the main command loop of PHOTOM Your response to this prompt should be a single character To see a menu of the possible commands return a h or H The first time one of the interactive graphical options I interactive shape aperture photometry only or M interactive measurement is selected the name of the display device is requested If you are using an image display with overlay capabilities then remember that the overlay device is not the same as the device you displayed your image in e g if you displayed on an X windows device xw then the overlay device is called xov Typically the cursor position is controlled by the mouse and the mouse button meanings are indicated by three menu boxes drawn on the screen If you use an unusual display device without a mouse then the
46. photometric For further measurements using optimal extraction or all measurements using the older aperture method an aperture is displayed where the measurement was made For optimal extraction this aperture will be the size of the clipping radius the CLIP parameter If centroiding is being done command C automatic enabled for optimal extraction then the displayed aperture may not be centered on the cursor position The results of the measurement are printed on the terminal and recorded in the results file Measurements can be continued until the third mouse button key 0 is pressed When using manual selection of the background the middle mouse button key 2 is also used Selecting this button records the sky estimate in an aperture identical in size and shape to the object aperture at the position specified by the cursor On the screen an aperture is displayed at that position No centroiding is done in this aperture even if the centroiding option is on When the measurement of the object is made the most recent value of the sky is used This means that the sky has to be sampled BEFORE the measurement of the object Having a correct background estimate is crucial for optimal extraction to such an extent that PHOTOM will not allow you to make a star or PSF measurement using this method until a background measurement has been provided If the background needs to be sampled in several places around an object to minimise the noise or tak
47. s There are three possible choices selected by the integers 1 to 4 which have the following bindings 1 The errors are estimated from the photon statistics in the sky and object apertures The parameters PADU and BIASLE should be set to their appropriate values to convert the data units to photon numbers 2 The errors are estimated from the measured variance in the sky aperture This method assumes that the measured variance is due to photon statistics and estimates the error in the object aperture accordingly The PADU parameter should be set to its appropriate value to convert the data units to photon numbers 3 The errors are estimated from the variance component of the data array 4 The errors are estimated from the measured variance in the sky aperture This method assumes that the errors are Gaussian same value per object and sky pixel and thus requires no knowledge of the values of PADU and BIASLE but can only be considered an upper limit on the error in a measurement POSITIVE _LOGICAL Read Find the object centroid for image features which are positive or negative with respect to the background This should be set to TRUE SATURE _REAL Read The saturation level in data units for the image If any pixels in the object aperture have values greater than this then the measurement is flagged with an S in the output record SEARCH _INTEGER Read The size of the search box in pixels to be used in locating the objec
48. sed in the automatic sampling of the sky The annulus has the same elliptical shape as the object aperture but is larger by the factors given by INNER and OUTER These two parameters are given in terms of multiplicative factors of the semi major axis of the object aperture Thus an INNER radius of 1 means that the sky annulus starts where the object aperture ends The annulus thus grows and shrinks with changes to the object aperture The PADU parameter defines the number of photons for each interval of the data Multiplying the data value in each pixel by PADU gives the number of photons recorded after correcting for BIASLE If this parameter is unknown then leave it at 1 It is necessary to provide an estimate of this number if optimal extraction is to be carried out correctly The SKYMAG parameter specifies the magnitude to be given to the sky when calculating the magnitude of the object The magnitude of the object is calculated from mag SKYMAG 2 5 log signal where signal is the brightness of the object minus sky in photons This parame ter is not used if USEMAGS is set to FALSE in this case the output is not in magnitudes The BIASLE parameter gives the level in data units of any offset in the bias level per pixel This is needed if there is any non photon source of background and proper photon statistics are required If this parameter is unknown then leave it at 0 The SATURE parameter is the saturation level for the image in d
49. spectra of densities and brightnesses are continuous and a clean rejection scheme is difficult to construct The best scheme would seem to be to take many independent samples using the same sized aperture as for the object A new population is made from the mean sky value in each aperture and the peak of the histogram of values here called the mode is used as the sky value The mode is a maximum likelihood estimator and this scheme ensures that the most likely sky value averaged over the aperture size is used The problem in this case is to get sufficient independent 31 SUN 45 16 Explaining optimal photometry samples close to the object aperture and therefore individual pixel values are usually used to construct the sample population What these arguments are leading to is that there is no unique answer to the question which is the best estimate for the sky it depends on the circumstances PHOTOM offers a choice of three estimators a simple mean a mean with 20 clipping and the mode When running jobs non interactively it is best to use one of the estimators that performs clipping either the mode or the mean with 20 rejection In the presence of positive contamination the mode will in general provide the most rejection the 2 sigma clipping the next and the mean will provide no rejection If the you suspect that one of the estimators may be giving wrong results then try one of the others C Explaining optimal photometry Optimal extrac
50. t centroid SEE _REAL Read Approximate seeing in pixels used to estimate the FWHM of the point spread function PSF by the optimal extraction algorithm SKY _REAL Read A constant value to be used as the sky estimate for subsequent measurements This defines the sky level in data units per pixel This value is used until another estimator is chosen SKYEST _INTEGER Read Select the estimator to be used to evaluate the background level in the sky aperture There are four possible choices selected by the integers 1 to 4 which have the following bindings 1 Mean All pixels in the sky aperture are averaged 2 Mean with 2 sigma rejection All pixels with data values within 2 standard deviations of the mean are averaged 3 Mode The peak of the histogram of pixel values the most likely value in the sky aperture is estimated SUN 45 16 Full routine descriptions 18 AUTOPHOTOM 4 A constant A single value to be used for all measurements A sky variance standard deviation is also requested so that a realistic error can be assigned to the measurements if the error is calculated from the variance in the sky aperture SKYMAG _REAL Read The magnitude assigned to the sky level when calculating the magnitude of the object using the relation OBJMAG SKYMAG 2 5 x LOG10 SIGNAL where SIGNAL is the brightness of the object minus sky in photons Not used if USEMAGS is FALSE SKYSIG _REAL Read A constant value for the sky v
51. t of Associated Files At present the file containing the positions of objects to be measured given by the POSFILE parameter command F is read in in free format The first three columns of the input file have to contain an index number INTEGER and the x and y positions REAL in that order The index number is passed to the output to assist in identification of the objects The output on the screen contains column headers to indicate the content of each column of the results These column headers do not appear in the output file given by the RESFILE parameter in order that this file can be accessed by database routines There are eleven columns in the output file containing the following information Column Name Description 1 INDEX Index number of star 2 XPOS X position of centre of aperture in pixels 3 YPOS Y position of centre of aperture in pixels 4 MAG Magnitude or mean photon count of star 5 MAGERR Error in MAG 6 SKY Sky value in photons per pixel 7 SIGNAL Total number of photons in aperture due to star 8 CODE Error code flag 9 SEMIM Semi major axis of aperture 10 ECCEN Eccentricity of aperture 11 ANGLE Orientation of aperture in degrees The magnitude MAG is calculated from mag SKYMAG 2 5 log signal The error in the magnitude MAGERR is estimated using one of the methods expounded in appendix D note this error is not transformed into magnitudes when USEMAGS is FALSE SUN 45 16 Full routin
52. tant magnitude or flux but it does not change the reported value for the sky in each pixel or the error in the measurement There are three methods for getting an exposure time 1 supply a floating point value 2 supply the name of a FITS keyword which must decode into a floating point value 3 supply the name of an HDS object that exists somewhere in your data file that can be decoded as a floating point value this presumes that you re storing your images in NDFs So for instance if you ve got an image with a FITS type header and the exposure time of the image is recorded in the record with name EXPOSURE then you d use a command like photom exsource header etime exposure A simple floating point value 600 is indicated by photom exsource constant etime 600 An HDS object ext_time in the CCDPACK extension of an NDF is indicated by photom exsource hds etime more ccdpack ext_time The structure of an NDF can be viewed using the HDSTRACE SUN 102 utility The default exposure time is 1 0 5 2 4 usemask This parameter is a logical flag which indicates whether a mask is to be used when estimating the background The purpose of the mask is to block out contaminating objects from the background aperture In this way bright stars can be excluded from the estimation of the sky which would otherwise introduce contamination Note that the sky estimators that perform clipping of the pixel histogram the mode and the mean with
53. tatus POSITIONS how the sky regions are determined Values that are unknown initially MAG MAGERR SKY and SIGNAL should be set to 0 0 the derived values will be used to replace these fields on exit The CODE field should be set to OK initially The POSITIONS field should have one the values annulus or regions to indicate how the sky regions are determined this is ignored if SKYEST is 4 Note that the same clipping radius will be used for all stars this is an entirely proper and necessary restriction under the algorithm Other lines in the file may be comments or definitions of the sky regions Comment lines start with the character sky regions either with ANN or SKY the is used so that other programs can skip over this information If the POSITIONS field of an object is set to annulus then at least one ANN line must be present for this object this defines the scales or sizes for the inner and outer loci of the sky region ANN INDEX INNER_SCALE SEMI_MAJOR OUTER_SCALE SEMI_MAJOR The INDEX value is the identifier of the related object A good estimate of the inner radius of the sky box is about twice the FWHM of the PSF star If POSITIONS is set to regions then as many lines starting with SKY should be present as there are regions circular or elliptical apertures in which to estimate the sky value for this object SKYn INDEX XPOS YPOS SHAPE MAJOR ECC
54. the device to receive the graphical output ECCEN _REAL Read The eccentricity of the ellipse defining the aperture For a circular aperture this should be set to 0 0 IN IMAGE Read Name of the image on which the sampling test will be performed INNER _REAL Read The radius of the inner edge of the annular sky aperture in units of the object aperture size The actual dimension in pixels is obtained by multiplying this factor by the object aperture semi major axis in pixels NP _INTEGER Read The number of points to sample in the image up to a maximum of 100 This number is factorised so that a regular grid of samples is taken The actual number of samples is restricted to ensure that the central apertures do not overlap OUTER _REAL Read The radius of the outer edge of the annular sky aperture in units of the object aperture size The actual dimension in pixels is obtained by multiplying this factor by the object aperture semi major axis in pixels PADU _REAL Read The number of photons for each interval of the data If the true value is unknown use a value of 1 RANGE _REAL Read The limit of the plot in photon units data units x PADU This can be used to limit the range of the plot to the interesting region SUN 45 16 PHOTOPT examining PHOTOM s performance 36 PHOTOPT SATURE _REAL Read The saturation level in data units for the image SEMIM _REAL Read The semi major axis of the ellipse defining th
55. the sum of sky corrected photons and the mean sky value Note the SKYMAG value is not used when this is FALSE Note also that this value may only be set once when PHOTOM is started and must be set either on the command line USEMAGS TRUE or USEMAGS FALSE or in response to a forced prompt command line argument PROMPT TRUE USEMASK _LOGICAL Read Define a mask to exclude regions from the background estimate If this is TRUE a file of positions is requested which define the centres of circles used to block regions from the sky aperture Contaminating objects such as bright stars can thus be removed from the background estimate Examples PHOTOM ARP199 Performs aperture photometry on image ARP199 PHOTOM SEARCH 5 MAXSHIFT 2 0 27 PHOTOM SUN 45 16 Full routine descriptions Defines the centroiding search box to be 5 pixels wide and the maximum shift of the centroid from its initial rough position to be 2 pixels PHOTOM EXSOURCE HDS ETIME MORE EXP_TIME An exposure time for the frame will be found in the primitive component EXP_TIME which is a component of the structure MORE in the data file PHOTOM USEMASK T A mask file will be used to define regions to be excluded from the sky aperture PHOTOM USEMAGS FALSE This will output the photometry results in photon counts so the MAG field will now have a mean photon count and MAGERR the error in this count assuming poissonian statistics are valid Aperture Extraction Forma
56. tion offers serveral advantages over the normal aperture method of photometry Formally optimal extraction is equivalent to profile fitting however it offers more robust error estimation and a freedom of the bias introduced by mis estimating the point spread function PSF It has been found to offer a gain of around 10 per cent in signal to noise over normal aperture photometry A general formula for summing flux F within an aperture is P WD 50 ij where sum is over all the pixels i j within the aperture where the total count in a pixel is Dj the estimated sky level is S j and Wz is the weight given to each pixel For normal aperture photometry this is one within the aperture and zero outside it Finding the optimal value for Wz j for each i j within the aperture is a two step process Firstly a model profile is fitted to a nearby star a 2 D Gaussian has proved adequate for this purpose the resulting estimated stellar proifle PE is normalised to one As shown by Horne PASP 1986 98 609 once the estimated profile is known the best signal to noise is obtained for P EL Vij Wi 455 a LP Vij where Vj is the variance for each pixel Substituting this into our first equation we obtain the basic formula governing optimal extraction However at this stage we make a further assumption that the variance for each pixel is the same For very faint stars this is clearly the case since the counts in each pixel is dominated by th
57. ull routine descriptions be set to OK initially The POSITIONS field should have one the values annulus or regions to indicate how the sky regions are determined this is ignored if SKYEST is 4 The SHAPE field should be set to circle or ellipse to indicate the aperture shape Other lines in the file may be comments or definitions of the sky regions Comment lines start with the character sky regions either with ANN or SKY the is used so that other programs can skip over this information If the POSITIONS field of an object is set to annulus then at least one ANN line must be present for this object this defines the scales or sizes for the inner and outer loci of the sky region ANN INDEX INNER_SCALE SEMI_MAJOR OUTER_SCALE SEMI_MAJOR The INDEX value is the identifier of the related object If POSITIONS is set to regions then as many lines starting with SKY should be present as there are regions circular or elliptical apertures in which to estimate the sky value for this object SKYn INDEX XPOS YPOS SHAPE MAJOR ECCEN ANGLE The n added to the SKY identifier indicates the number of the sky region being defined and is optional The other fields are the same as for an object aperture Optimal Extraction Then the first star in the file must be the PSF In this case the following information must be provided INDEX XPOS YPOS FWHM1 FWHM2 ROT C
58. y the Graphical Astronomy and Image Analysis tool GAIA SUN 214 which integrates the tasks of photometry with an image display tool This allows the detailed inspection of objects and their environments and provides a highly interactive environment for placing rotating and resizing apertures 2 New to aperture photometry If you are new to aperture photometry then you should read the section on techniques of aperture photometry in appendix B You should also read the Starlink Cookbook CCD Photometric Calibration SC 6 If you have any questions beyond what these offer which are elementary texts then you should get friendly with a local expert or be prepared to delve into the literature 3 New to optimal photometry If you are going to be using the optimal extraction features of PHOTOM then it is recommended that you should read the MNRAS paper An optimal extraction algorithm for imaging pho tometry MNRAS 1998 296 339 which discusses this algorithm in depth However a brief of the algorithim is given later in appendix C 4 Running the photom software To initialize the PHOTOM package so that you can run its programs from the C shell you use the command photomstart after setting yourself up to run Starlink software see Setting up your environment in SUN 145 Similarly to set up PHOTOM to run from within use SUN 45 16 Performing interactive analyses 2 ICL gt photomstart 4 1 Getting help

PHOTOM — A Photometry Package

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PHOTOM &mdash; A Photometry Package

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PHOTOM — A Photometry Package