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The GAIA Cookbook

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1. N cS The first step is to retrieve a two dimensional image of the region of sky containing your target region or object Click on the Data Servers menu located towards the right of the menu bar at the top of the main window Choose the Image Servers option and then Digitized Sky at ESO from amongst the list of servers it will probably be the only choice If the Digitized Sky at ESO is not amongst the choices offered then see Section 9 1 below http stdatu stsci edu dss http ledas www star le ac uk DSSimage http www nofs navy mil http www wfau roe ac uk sss SC 17 1 Retrieving Remote Images and Catalogues 16 Digitized Sky at ESO 1 Figure 3 Window to retrieve a remote image of a region or object from the DSS A window similar to Figure 3 and titled Digitized Sky at ESO 1 should appear it allows you to specify the region of sky to be retrieved You can simply enter the name of your target object in the Object Name box if you know it GAIA will attempt to resolve the name of the object and look up its coordinates Here simply enter nge 1275 any embedded spaces are ignored However often you will need to enter the coordinates directly The units and formats required are Right Ascension sexagesimal hours with a colon as a separator Declination sexagesimal degrees with a colon as a separator In the present example the values required are a 3 19 48
2. 19 R F Warren Smith 11 January 2000 SUN 33 7 NDF Routines for Accessing the Extensible N Dimensional Data Format Starlink 20 R F Warren Smith and D S Berry 23 October 2001 SUN 210 11 AST A Library for Handling World Coordinate Systems in Astronomy Fortran Version Starlink 21 R F Warren Smith and D S Berry 23 October 2001 SUN 211 11 AST A Library for Handling World Coordinate Systems in Astronomy C Version Starlink 22 R F Warren Smith and D S Berry 17 July 2000 SSN 20 3 Adding Format Conversion Facilities to the NDF Data Access Library Starlink 23 D C Wells E W Greisen and R H Harten 1981 Astron Astrophys Suppl 44 pp363 370
3. ASCII Header format It is also possible to save a copy in one of the other catalogue formats described in Section 4 2 Click on the File button on the left of the the menu bar along the top of the catalogue window and choose the Save as option A window allowing you to specify the filename required will appear The format in which the catalogue is saved depends on the file type specified at the end of the file name again see Section 4 2 Catalogues saved in the FITS tables TST or STL formats can subsequently be imported into CURSA see SUN 190 5 which provides additional catalogue manipulation facilities 39 SC 17 1 Automatic Object Detection Figure 14 The object detection dialogue box test ASC T Figure 15 Window showing a catalogue of automatically detected objects SC 17 1 Measuring Instrumental Magnitudes 40 14 Measuring Instrumental Magnitudes This recipe shows how to use GAIA to measure instrumental magnitudes for objects in an image The technique used here is aperture photometry which involves positioning a circular cursor over the object to be measured and comparing the total intensity within the aperture with the intensity in a similar area of blank sky There are various techniques for measuring instrumental magnitudes and they are discussed along with related matters in SC 6 The CCD Photometric Calibration Cookbook 16 If you are not familiar with photometric techniques then SC 6 is a
4. NDF slice that is a portion of an image rather than the whole thing this facility can be useful for very large images For example type gaia hdscontainer ndf_1 200 500 100 700 See SUN 33 19 for further details of specifying NDF slices This notation can also be applied to FITS files and other foreign formats gaia file fits 300 700 300 700 Note however that here the FITS file will now be accessed as a foreign format that is it will automatically be converted to an NDF prior to being read 5 3 Demonstration mode GAIA has an automatic demonstration mode which shows many of its facilities To run this demonstration move to an empty directory and start GAIA as described above Then click on the Image Analysis menu in the menu bar at the top of the window and choose the Demonstration mode item A window will appear asking if you wish to copy the files for the demonstration into your current directory Click Yes A further window with preliminary information about the demonstration will appear Click on the Start button The demonstration will now run completely automatically without further intervention Annotation describing the facilities being demonstrated will appear in the information window as the demonstration proceeds Note that the demonstration contains flashing images abort it if these might affect you 9 SC 17 1 Obtaining Assistance If you encounter a problem with GAIA and p
5. from the DSS It will be very similar to the image that you created in the recipe in Section J9 and you could substitute your own image if you prefer Load file ngc1275dss sdf into GAIA and adjust the colour table so that you can see the stars and galaxies The display should look something like Figure 10 1 Reading off the Right Ascension and Declination It is straightforward to read off the approximate Right Ascension and Declination of any object in the image 1 The x y pixel positions and the Right Ascension and Declination of the current cursor position are shown in the boxes labelled X Y a and in the control panel in the middle of the upper portion of the main window To find the approximate coordinates of a star simply position the cursor over it and read them off 2 In a DSS image J2000 equatorial coordinates are shown by default It is possible to configure GAIA to display other celestial coordinates Click on the Image Analysis button in the menu bar along the top of the main window and choose Change coordinates and then Celestial coordinates A window appears which allows you to specify the coordinate system Choose the one required perhaps ecliptic coordinates and then click the Accept button Henceforth coordinates are displayed in the chosen system but note that the coordinate boxes are still labelled w and even if ecliptic or Galactic coordinates have been selected Also the coo
6. 2020 and there are three papers describing the FITS WCS proposals papers i and I You should be aware however that all of these documents contain far more detail than you need to know in order to use GAIA and moreover they are not for the faint hearted 5 ftp ftp cv nrao edu NRAO staff egreisen wcs ps gz 6ftp ftp cv nrao edu NRAO staff egreisen ccs ps gz ftp ftp cv nrao edu NRAO staff egreisen scs ps gz SC 17 1 Starting GAIA 6 4 2 Catalogue formats The native format in which GAIA reads and writes the catalogues and tables which it plots as image overlays is the so called Tab Separated Table TST format which is described in SSN 75 6 GAIA can also read and write catalogues in the FITS tables STL and ASCIL HEAD formats GAIA differentiates between these different formats by using the file type at the end of the file name Brief notes on the individual formats and their required file types follow FITS tables a variant of the FITS format for storing catalogues and tables Accepted file types FIT fit FITS fits GSC gsc STL the Small Text List format used by CURSA see SUN 190 5 Accepted file types TXT txt ASCII_HEAD the ASCII Header format used by the EXTRACTOR package see SUN 226 3 Accepted file types asc ASC lis LIS Mixed capitalisations such as Fits are also recognised GAIA interprets all other file formats as indicating TST format catalogues but note that CURSA requires
7. 6 41 30 42 You also need to specify the height and width of the region to be retrieved These values are specified in minutes of arc In the present example values of 7 should be specified which is slightly larger than the field of view of the V band CCD image of NGC 1275 used in the previous recipe Section 8 When the values are set click on the Get Image button After a few moments the retrieved image should be displayed in the main GAIA window A small window entitled FITS HDUs 1 may also appear Click on Close to close this window Then click on Close to close the Digitized Sky at ESO 1 window 3 You may wish to save the retrieved image as a file for future use Click on the File menu the leftmost item in the menu bar along the top of the main window and choose the Save as item A window allowing you to save the image as a file will appear The file will be written in FITS format It is usual to end the file name with a file type of fits or fit 17 SC 17 1 Retrieving Remote Images and Catalogues 4 You may wish to adjust the appearance of the displayed image The most likely items to change are Colors and Magnification in the View menu second from the left in the menu bar along the top of the window For example click on the View menu and select the Colors option A panel will appear Set e the colour scale algorithm to Linear e the colormap to ramp e the intensity to
8. PHOTOM for photometry see SUN 45 10 behind the scenes However the user just sees seamless interaction through the GAIA GUI Graphical User Interface Also GAIA can access images in a wider range of data formats than SkyCat see Section 4 http archive eso org skycat http www eso org Shttp www eso org vlt SC 17 1 Typographic Conventions 2 The following typographic conventions are used in this cookbook Anything that is to be typed into a computer program via the keyboard or output from one via the screen is indicated by a typewriter font like this Lines that are to be typed into the computer are shown beginning with a sign for example gaia The indicates the Unix shell prompt and should not be typed in However items appearing in GAIA s graphical windows are shown in a sans serif font like this 3 SC 17 1 Facilities Available Part I Background Material 3 Facilities Available This section outlines the facilities available in GAIA The hypertext version of the cookbook contains links to outline recipes which summarise how to use these facilities These outline recipes complement the smaller number of detailed recipes in Part II of the cookbook The facilities provided by GAIA divide naturally into three areas e image display e image analysis e remote access to catalogues and image databases via the Internet The facilities available in each of these t
9. TST format catalogues to have a file type of TAB or tab 5 Starting GAIA If you are working at a Starlink site then GAIA should automatically be available to you provided that your account is set up to access Starlink software which will usually be the case No special quotas or privileges are required to run GAIA However it must be run on a workstation console or X display capable of receiving X output In practice GAIA requires a colour display strictly speaking it will run on a black and white one but any image displayed is not visible GAIA is usually run from the Unix shell Simply type gaia amp The amp merely makes gaia run as a detached process so that you can if you wish continue to issue Unix commands from the command line After a few moments the main GAIA window will appear Now click on the File menu which is the leftmost item in the menu bar along the top of the window Click on the Open item A file picker window appears which allows you to choose the image file to be displayed The final appearance should be similar to Figure I See the recipe in Section B for further details Alternatively the required file can be specified on the command line when GAIA is started gaia file name amp 7 SC 17 1 Starting GAIA for example gaia ngc1275jkt sdf amp gaia ngci275hri fits amp The first example is a Starlink NDF file the second a FITS file see Section 4 for the data format
10. configure the range of formats currently available to CONVERT You are unlikely to need to make such a configuration but if you choose to do so then SSN 20 22 gives the requisite details However you should ensure that FITS continues to be one of the formats available because the basic image display functions read FITS files directly A full description of the NDF and FITS formats is beyond the scope of this cookbook The NDF n dimensional Data Format see SUN 33 19 is the native Starlink format The FITS Flexible Image Transport System format is in widespread use in astronomy There is a brief to the FITS format in SC 5 7 In both the NDF and FITS formats and indeed other common astronomical formats the infor mation stored in the file includes more than just the array of values for the two dimensional The original FITS format was proposed by Wells et al 23 in 1981 However it has been developed and enhanced over the years The FITS standard is now maintained and documented by the FITS Support Office of the Astrophysics Data Facility at the NASA Goddard Space Flight Center see URL http fits gsfc nasa gov fits_home html Though FITS is basically an astronomical format it is sometimes mentioned in books about standard image formats See for example Graphics File Formats by Kay and Levinef12 5 SC 17 1 Data Formats image Various other header or auxiliary information describing and annotating the image is also inc
11. in the Image X and Image Y fields within the object picker dialogue box Figure 12 Raise the object picker dialogue box Figure 12 You should find that the pixel positions of the star have been copied into the X and Y fields The star is now fully specified so press the Enter button After confirmation this will amend the details of the star in the list of reference stars in the Fit astrometry reference positions dialogue box Figure 11 p Repeat the procedure for the remaining reference stars All the dialogue boxes remain open so it is relatively simple to cycle through measuring the positions of the remaining SC 17 1 Astrometric Calibration Figure 13 The GAIA object picker dialogue box 34 35 11 12 13 a SC 17 1 Astrometric Calibration stars Hint double clicking on a star in the reference star list is equivalent to clicking on it and then clicking the Edit button Once you have entered all your reference positions close the dialogue boxes shown in Figures 12 and 13 by pressing the Close button in each one Click on the Marker menu on the menu bar along the top of the Fit astrometry reference positions dialogue box Figure 11 choose the Size option and set it to 21 This option makes the markers a convenient size when plotted on the JKT image You may also need to set the Outline colour option to white depending on which colour table you have set Now click the Fit Test bu
12. neg Then click on the Close button Set the magnification by clicking on the Scale button in the bottom left of the control panel in the centre top of the window and setting it to 2x The display should now look something like Figure 4 which is a a reasonable approxima tion to the appearance of the original photographic plate GAIA Skycat cat304 162 fits 1 Go Image Analysis Data Servers dss22128 filezcat304162 fits 318 5 Y 27 0 Value i Equinox 2969 Max 14585 Auto Cut Low 283 High 14585 Color Map Scale x Z z Sl 2 ta intensity ma E image Q select object scroll image measure WCS Control E select region Figure 4 Retrieved Digitised Sky Survey DSS image centred on NGC 1275 SC 17 1 Retrieving Remote Images and Catalogues 18 5 Various remote catalogues are accessible from GAIA The present recipe extracts objects from the USNO PMM astrometric catalogue 13 and overlays them on the image First click on the Data Servers menu towards the right of the menu bar at the top of the main window Choose the Catalogs option and then USNO at ESO from amongst the list of catalogues it will probably be towards the bottom of the list If USNO at ESO is not amongst the choices offered then see Section 9 1 below A window similar to Figure 5 and titled USNO at ESO 1 should appear it allows you to re
13. polarisation maps can be plotted regions selected from them etc These facilities are intended to be used in conjunction with the POLPACK package for polarimetry and spectropolarimetry see SUN 223 1 Profiles of rectangular regions Interactive averaged profiles along the x and y axes of a rectan gle 3 3 Catalogues and Online Resources Catalogues and image stores Access to remote on line catalogues image stores and archives via the Internet Local catalogues can also be read and written 4 Data Formats GAIA can access images in various different data formats The ones that you are mostly likely to encounter are FITS images and NDF files though there are numerous other possibilities including the IRAF format and old Figaro DST files In GAIA and to anticipate the recipes of Part II you can check which formats are currently available by clicking on the File Type button in the file picker window see the recipe in Section 8 Be careful that you do not inadvertently set the button to specify just one of the formats it is usually best left set to any The basic GAIA image display facilities read FITS files The more astronomical functions usually require files in Starlink s NDF format However any necessary format conversions are performed automatically and invisibly on the fly by applications in the CONVERT package see SUN 55 4 and you will not normally be aware that they are happening It is possible to
14. range of intensities in the image to be displayed click the Auto Cut button in the middle right of the control panel in the centre top of the window and set it to 98 b set the magnification click the Scale button in the bottom left of the control panel in the centre top of the window and set it to 1 2x c set the colour table click the Color Map button in the lower right of the control panel in the centre top of the window and set it to heat SC 17 1 Displaying an Image GAIA Skycat ngct275jkt sdt 1 Image Analysis Data Servers Output from FLATCOR filezngcl275jkt sdf 1339 0 Yi 136 0 Value Equinox 82 1939 Max 66892 1 Auto Cut Low 301 773 High jioo7 o2 Color Map Scae 17x Z z S z 14 Intensity Map 1724 Display Figure 1 A V band CCD image of the galaxy NGC 1275 The image was obtained with the Jacobus Kapteyn Telescope JKT on La Palma SC 17 1 Displaying an Image 14 4 5 6 The display should now look something like Figure 1 You can inspect the header information associated with the image see Section 4 by clicking on the View menu second on the left on the menu bar along the top of the main window and choosing the Fits Header option Note that this option will work with images in the NDF format as well as those in the FITS format because the NDF format can contain FITS like header information which G
15. see if you are using the colour table set up above 11 Click on the Draw Contours button Contours of the X ray image will now be drawn superimposed on the X ray image The appearance of the plot should be similar to Figure GAIA Skycat ngct275dss sdt 1 Graphics Go Image Analysis Data Servers Object dss27172 filetngcl275dss sdf X 308 0 Yi 134 0 Value Equinox 2969 Max 14585 Auto Cut Low 9 High fj Color Map Scale x Z z S 2 tl Intensity Map Contour key E image Q select object Q scroll image B measure WCS Control select region Figure 9 Contours derived from the ROSAT HRI X ray image of NGC 1275 superim posed on a DSS image of the same region 27 SC 17 1 Superimposing and Contouring Images It is immediately obvious that the X ray emission is centred on NGC 1275 and also that it extends beyond the optical limits of the galaxy recall that the faintest contour drawn corresponds to quite a bright level in the X ray image 12 When you have finished inspecting the image click on the Close button and the contours will disappear 11 1 Contouring the displayed image Sometimes you might wish to plot contours generated from the displayed image rather than superimposing those from another image The procedure is very similar 1 Display the required image 2 Click on the Image Analysis button on the menu bar a
16. which can be useful in some circumstances Images extracted from the Digitised Sky Survey DSS have a WCS and this recipe will use the DSS image of NGC 1275 retrieved in Section 9 Contours constructed from an X ray image of the same region of sky will be drawn on top of it The X ray image is contained in file ngc1275hri fits It was obtained with the HRI High Resolution Imager on board the ROSAT X ray astronomy satellite R ntgensatellit The cop used here was retrieved from the LEDAJ data archive service at the Deparment of yi University of Leicester The image is in FITS format see Section 4 for details of the data formats available to GAIA and it too contains a WCS Proceed as follows 1 It is useful to examine the X ray image of the NGC 1275 region before trying to contour it You could skip these steps if you were already familiar with the properties of the image to be contoured Start GAIA and load file ngc1275hri fits 2 Adjust the colour so that the very nucleus of the X ray emission is clearly visible For example set the Auto Cut to 100 Then click on the View menu and select the Colors option A panel will appear Set the colour scale algorithm to Logarithmic the colormap to heat and the intensity to ramp The display should now appear similar to Figure 6 It is immediately obvious that the sky appears very different at X ray and optical wave lengths Also by moving the cursor from one edge of the image t
17. AIA can access GAIA has many other functions and options and you may want to spend a while exploring some of them Similarly you might also like to examine the other images included in the examples On line help is available from the Help menu at the extreme right of the menu bar at the top of the window If you click on this menu and choose the Help topics index item then the introductory page of the GAIA on line help information will appear see Figure 2 From this page you can follow hyper links to further pages giving notes on how to perform numerous common tasks with GAIA You are likely to find this help information very useful Help fnetfreaop0d facdscratch star bin gaa help gaaet index fim Figure 2 The introductory GAIA help page When you have finished close GAIA by clicking on the File menu and choosing the Exit option 15 SC 17 1 Retrieving Remote Images and Catalogues 9 Retrieving Remote Images and Catalogues When using remote facilities such as those described in this recipe you should ensure that you are both familiar with and comply with any obligations which they impose on you such as correct acknowledgement of the use of data or services etc This recipe demonstrates how to retrieve an image of a specified region of sky from a remote archive and to overlay it with objects extracted from a remote catalogue In both cases the data are retrieved via the Internet As an example a region centered on t
18. SC 17 1 Starlink Project Starlink Cookbook 17 1 A C Davenhall amp P W Draper 31st December 2001 Copyright 2001 Council for the Central Laboratory of the Research Councils The GAIA Cookbook SC 17 1 Abstract ii Abstract GAIA Graphical Astronomy and Image Analysis is an interactive astronomical image display and analysis tool It includes a comprehensive suite of facilities for displaying and manipulating images It also has extensive facilities for the astronomical analysis of images including astrometric calibration automatic object detection and aperture optimal and surface photometry GAIA can access images in most of the data formats common in astronomy and can also retrieve copies of remote images and catalogues via the Internet This cookbook is an introduction to GAIA It describes the facilities that GAIA provides and gives some simple examples of their use It refers to GAIA version 2 6 or higher Who Should Read this Cookbook This cookbook is aimed at astronomers who are new to GAIA but who think that it might be useful in their work It is not aimed at experienced users of GAIA for whom the GAIA manual SUN 214 or the on line help within GAIA itself might be more useful Copyright 2001 Council for the Central Laboratory of the Research Councils ii je Contents 1 1 GAIA and SkyCat 6 hd eed be PS aS ES 2 Typographic Conventions I Background Material 4 Data Formats 4 1 World coor
19. an object 5 If you click on the Parameters button in the Aperture photometry magnitudes dialogue ma box the appearance of the box changes to resemble Figure 18 You can now set various parameters such as the Photon data per unit image bias level default sky level etc By default the statistic used to estimate the sky background is the mean Usually it is preferable to use the mode because it is less affected by contamination due to faint stars To select the mode click on the Sky estimator button and select the mode see Figure 18 You should measure all the stars that you are interested in the current frame Click on the Aperture button prior to resuming measuring if necessary When the job is done click on the File menu in the menu bar along the top of the Aperture photometry magnitudes dialogue box not the one in the main GAIA window and select Save measurements to save the results in a file of your choice The instrumental magnitudes are listed in this file GAIA does not include any facilities for calibrating instrumental magnitudes into standard photometric systems CURSA see SUN 190 5 contains some simple functions for this purpose see SC 6 16 for an example of using them and further details 43 SC 17 1 Measuring Instrumental Magnitudes Figure 18 The GAIA Aperture photometry magnitudes dialogue box with the options to set parameters selected SC 17 1 Measuring Instrumental Magnitud
20. are legible typically by inverting the image to make the stars white against a dark background as in Figure 10 rather than vice versa Print out a paper copy of the image by clicking on the File option the rightmost option in the menu bar along the top of the main window and choose the Print option followed by Image Use the ensuing dialogue box to save the image as a postscript file which you can then print SC 17 1 Astrometric Calibration 275 02207958 GAIA Skycat ngc1275dss sdf dss27172 3 19 48 17 41 30 41 2 J2000 acd Jan 03 2002 at 14 12 29 Figure 10 DSS image with reference stars marked 30 31 SC 17 1 Astrometric Calibration 9 Save the catalogue of selected objects Click on the File menu in the USNO at ESO 1 dialogue box the leftmost item in its menu bar and choose the Save as option You should specify a file type of lis so that the catalogue is saved in the ASCII Header format see Section 4 2 You now have a list of suitable reference stars You can quit GAIA at this point but it is better to proceed directly to the next stage of the recipe 12 2 Creating a preliminary transformation This stage of the recipe uses the five reference stars identified in the previous stage as fiducial marks to define a preliminary astrometric calibration However before starting a superficial glance at the DSS image for example in Figure f and the JKT image Figure 1 reveals that they
21. are rotated with respect to each other by 180 Sometimes uncalibrated images show such gross rotations with respect to the standard orientation sometimes they do not the standard orientation has north at the top east to the left and Right Ascension increasing from right to left that is the wrong way round If they do then it is best to rotate them before attempting the astrometric calibration This recipe assumes that GAIA is still running and that the catalogue of reference stars created in the previous stage is still available If not then start GAIA and load either the local catalogue of reference stars that you created in the previous stage or file ngc1275usno tab which is the equivalent example file 1 Load image ngc1275jkt sdf into GAIA 2 Adjust the colour table and magnification a click the Auto Cut button for 98 in the bottom right of the control panel in the centre top of the window b set the magnification click the Scale button in the bottom left of the control panel in the centre top of the window and set it to 1 2x c set the colour table click the Color Map button in the lower right of the control panel in the centre top of the window and set it to heat Also set the Intensity Map button directly below the Color Map button to default 3 Rotate the image to the standard orientation Click on the button marked with two horizontal arrows and then the one marked with two vertical a
22. articularly if it does not appear to be available at your site then in the first instance you should contact your local site manager GAIA has a home page at URL http www starlink ac uk gaia which includes a current list of known bugs and work arounds to circumvent them If your problem is not covered by this list then you can send a bug report or query by electronic mail to gaia star rl ac uk Suggestions for improvements and enhancements to GAIA are also welcome and should be submitted by the same route SC 17 1 Obtaining Assistance 10 11 SC 17 1 Introduction Part II The Recipes 7 Introduction This part of the cookbook provides a set of detailed recipes for performing some simple tasks with GAIA The topics covered are displaying an image Section 8 retrieving remote images and catalogues via the Internet Section Ph using World Coordinates Section 10 superimposing and contouring images Section m astrometric calibration of an image Section 12 automatic object detection Section 13 e measuring instrumental magnitudes Section 14 Strictly speaking these recipes can be read in any order and you can simply turn to the ones that seem most relevant to your work However the first five form a natural sequence Also the first is particularly simple and introductory Example data are provided with the cookbook so that you can work through the recipes yourself On Starlink syste
23. belled as NGC 1275 Before contouring an image it is useful to have some idea of the range of numbers that it contains One possibility is to draw a slice through the image Click on the View button on the menu bar along the top of the main window and choose the Slice item You can interactively define a slice through the image which is then plotted as a graph see Figure 17 in Section 14 below for an example Alternatively click on the View button on the menu bar along the top of the main window and choose the Pixel Table item A table showing the values of a small grid of pixels centred on the current cursor position is displayed You can move the cursor over the image examining the values For the present purposes the largest grid permitted 9x9 is the most appropriate You will probably want to experiment with these options for a while You are now ready to proceed with contouring the X ray image on top of the DSS one Load the DSS image ngc1275dss sdf into GAIA If you prefer you can substitute the DSS image that you retrieved whilst working through the recipe in Section 9 they should be very similar Set the colour table and magnification as in Section 9 click on the View menu and select the Colors option Set the colour scale algorithm to Linear the colormap to ramp and the intensity to neg Then click on the Close button Set the magnification by clicking on the Scale button in the bottom left of the control panel in
24. contain an approximate astrometric calibration and in such cases you can skip the first two stages of the recipe and proceed directly to the third to create an accurate astrometric calibration Comparison images retrieved from the SuperCOSMOS surveys rather than the already have an object catalogue attached making it un necessary to retrieve a separate catalogue from the PMM but remember that the SuperCOSMOS surveys are currently only available south of Declination 3 Alternatively you may not need to retrieve an object catalogue because you already know accurate celestial coordinates for a set of reference stars in your image they might for example be listed in a scientific paper associated with the image If you know an approximate astrometric calibration typically the orientation plate scale and approximate central coordinates for an image then you can simply type in the values use the Astrometry calibration item from the Image Analysis menu and choose the Type in known calibration option If you have a series of similar images all overlapping the same area of sky you could determine an astrometric calibration for the first using the method described For the remaining images 37 SC 17 1 Astrometric Calibration you can copy the WCS for the first image and then tweak it to fit the host image use the Transfer button in the Fit astrometry reference positions dialogue box You can transfer a set of reference stars with meas
25. df a reduced V band CCD image of NGC 1275 obtained with the JKT see the recipe in Section 8 for its provenance The image contains a mixture of stars and galaxies Proceed as follows 1 Start GAIA and load image ngc1275jkt sdf 2 You may wish to adjust the appearance of the displayed image The most likely items to change are the colour table click on the Color Map button in the lower right of the control panel in the centre top of the window and the magnification click on the Scale button in the bottom left of the control panel 3 Click on the Image Analysis button on the menu bar along the top of the main dialogue box and choose the Object detection option A dialogue box similar to Figure 4 should appear 4 To generate a catalogue of the objects in the image simply click on the Detect objects button The object detection dialogue box Figure 14 has numerous options but you do not need to adjust any of them the default settings will generate a reasonable catalogue After a few moments ellipses corresponding to the objects found will be drawn over the image in the main window and a dialogue box similar to Figure 15 will be created showing the catalogue of objects detected 5 The catalogue dialogue box Figure 15 has various options for manipulating the catalogue such as sorting and selecting objects the on line help gives the details The catalogue created is automatically written as a file in the EXTRACTOR
26. dinate systems 4 2 Catalogue formats 5 Starting GAIA 5 1 Using GAIA from IRAF 2 Multiple image files and image sections 5 3 Demonstration mode 6 Obtaining Assistance 69 01 I The Recipes 7 Introduction 8 Displaying an Image 9 Retrieving Remote Images and Catalogues 9 1 Restoring catalogue access 2 Using a proxy server 10 Using World Coordinates 10 2 Showing celestial coordinate grids 10 3 Measuring angular separation 11 Superimposing and Contouring Images 11 1 Contouring the displayed image 12 Astrometric Calibration 12 1 Finding astrometric reference stars 12 2 Creating a preliminary transformation 12 3 Creating an accurate astrometric calibration 12 4 Variations NO SC 17 1 Contents 11 11 12 15 19 19 20 20 21 21 22 27 SC 17 1 Contents 13 Automatic Object Detection 4 Measuring Instrumental Magnitudes Acknowledgements iv 38 40 44 45 v SC 17 1 List of Figures List of Figures 1 AV band CCD image of NGC 1275 coo ei we ee Ea ee ee ee ES 13 Dh yee dee Se ee ie ERE ENE eee ae 2g 14 Perey Ah Gee oe ee 16 4 Retrieved DSS image centred on NGC 1275 2 ee ee 17 oe eee ee ee oaa ee 18 6 An X ray image of NGC 1275 wc ka oe teen Rhee EEE OD EG RHA EES 23 ee ee ee ee ee ee 25 REISE CPST CeCe eT e
27. e displayed into an existing GAIA window or a new window will be created as required Note that because GAIA is not a native IRAF image display task it does not support any of the IRAF cursor commands SC 17 1 Starting GAIA 8 5 2 Multiple image files and image sections Some data formats notably FITS and NDF can include more than one image in a single file If you encounter such a file then you need to specify which image is to be displayed In a FITS file each image occurs in a separate extension which is identified by a sequential integer number To display a FITS extension image either open the disk file and choose the extension from the HDU selector window that appears HDU or Header and Data Unit is FITS jargon for an image and its associated auxiliary information or add the extension number to the disk file name gaia mef_file fits 2 the quotation marks embedded in the file name in this example are to prevent the square brackets from being interpreted by the Unix shell The first image in the FITS file is called the primary array and is numbered 1 A similar mechanism exists for NDFs stored in container files at other than the top level con tainer file is NDF jargon for a file which contains one or more NDFs gaia hdscontainer ndf_1 In this case any other NDFs stored at the same level in the container file will also be shown in a selector window It is also possible to access an
28. e required and then click the Accept button Then repeat step 1 above and a grid will be drawn in the new system 10 3 Measuring angular separation To measure the approximate angular separation between two objects simply position the cursor over the first of them and then click and hold down mouse button three Continuing to hold down this mouse button move the cursor until it is positioned over the second object A rubber band cursor is drawn as the mouse moves It shows the vector between the original and current positions and also the offsets in two orthogonal coordinates The separation and offsets are all labelled with their size in minutes of arc with a sexagesimal subdivision into seconds The cursor disappears when the mouse button is released SC 17 1 Superimposing and Contouring Images 22 11 Superimposing and Contouring Images This recipe is an example of superimposing two images in order to compare them GAIA has two mechanisms for superimposing images displaying two or more images and rapidly blinking between them in a manner analogous to the blink comparator used to compare photographic plates and displaying one image as a set of contours superimposed on a display of the other This recipe demonstrates the latter technique GAIA aligns images by using their World Coordinate Systems WCS see Section 4 1 Images that do not have a WCS are aligned by assuming that their pixel coordinates are coincident
29. eduction package Starlink 10 N Eaton P W Draper and A Allan 30 December 2000 SUN 45 12 PHOTOM A Photome try Package Starlink 11 W Jaffe 1998 Astronomical Images Twin Press Vledder CD ROM To contact Twin Press send an electronic mail message to G Kiers gkiers twinpress n1 See URL twinpress nl index htm 12 D C Kay and J R Levine 1995 Graphics File Formats second edition Windcrest McGraw Hill New York See in particular Chapter 18 pp235 244 13 D Monet A Bird B Canzian H Harris N Reid A Rhodes S Sell H Ables C Dahn H Guetter A Henden S Leggett H Levison C Luginbuhl J Martini A Monet J Pier B Riepe R Stone F Vrba and R Walker 1996 USNO SA1 0 U S Naval Observatory Washington DC See also URL http aww nots navy mil P 14 R Morris and G J Privett 6 December 1996 SUN 166 4 SAOIMAGE Astronomical Image Display Starlink 15 R Morris G J Privett and A C Davenhall 2 December 1999 SG 12 2 An Introduction to IRAF Starlink 16 J Palmer and A C Davenhall 31 August 2001 SC 6 4 The CCD Photometric Calibration Cookbook Starlink SC 17 1 References 46 17 G H de Vaucouleurs A de Vaucouleurs H G Corwin R J Buta G Paturel and P Fouqu Third Reference Catalogue of Bright Galaxies 1991 Springer New York 3 a 18 P T Wallace 21 June 1995 SUN 56 10 COCO Conversion of Celestial Coordinates Starlink p
30. ee 25 9 X ray contours superimposed a DSS image of NGC 1275 26 thie Stone Ae eee ee ON ane oe 30 Aiea 32 a 33 Lae en SNE REE TE REE ERE 34 bie et SEE Eee e ey Beek SEE 39 espa mes 39 e aan al 17 A GAIA Slice display panel oe PRS eee Se ah eee eee 42 18 Setting parameters in the Aperture photometry magnitudes dialogue box 1 SC 17 1 Introduction 1 Introduction While the incidental scraps of theogony in Homer name Okeanos Ocean as the origin of the gods thedn g nesis in Hesiod it is Earth Gaia who gives birth to Heaven Ouranos and then marries him they engender the Titans among them Okeanos and Kronos Saturn in the Roman interpretation Comparative Mythology Jaan Puhvel 1987 GAIA Graphical Astronomy and Image Analysis is an interactive astronomical image display and analysis tool broadly similar to SAOIMAGE for which see SUN 166 14 It includes a comprehensive suite of facilities for displaying and manipulating images panning zooming setting the colour table etc It also has extensive facilities for the astronomical analysis of images including astrometric calibration automatic object detection and aperture optimal and surface photometry see Section 3 GAIA can display two dimensional spectra but has no facilities for specifically spectroscopic analyses GAIA can access images in most of the data formats common in astronomy including Starlink NDF files and FITS image
31. es 44 Thanks to Martin Bly and David Berry for useful discussions and or comments on the draft The recipe in Section 12 is based on one originally written by David Berry and the one in Section 14 on one originally written by John Palmer Karen Moran kindly unearthed the contact details for Twin Press Any mistakes of course are our own 45 SC 17 1 References References 1 D S Berry and T M Gledhill 26 October 2001 SUN 223 6 POLPACK An Imaging Polarimetry Reduction Package Starlink 2 E Bertin and S Arnouts 1996 Astron Astrophys Suppl 117 pp393 404 3 A J Chipperfield and P W Draper 14 September 2001 SUN 226 4 EXTRACTOR An Astronomical Source Detection Program Starlink 4 M J Currie G J Privett A J Chipperfield D S Berry and A C Davenhall 1 November 2001 SUN 55 16 CONVERT A Format conversion Package Starlink 5 A C Davenhall 4 November 2001 SUN 190 10 CURSA Catalogue and Table Manipulation Applications Starlink 6 A C Davenhall 26 July 2000 SSN 75 1 Writing Catalogue and Image Servers for GAIA and CURSA Starlink 7 A C Davenhall G J Privett and M B Taylor 16 August 2001 SC 5 3 The 2 D CCD Data Reduction Cookbook Starlink 8 P W Draper N Gray and D S Berry 13 September 2001 SUN 214 9 GAIA Graphical Astronomy and Image Analysis Tool Starlink 9 P W Draper M B Taylor and A Allan 22 October 2001 SUN 139 15 CCDPACK CCD data r
32. ese options allow you to perform aperture photometry The former displays the results in magnitudes the second in counts An Aperture photometry magnitudes dialogue box see Figure 16 will be displayed You can drag this dialogue box off the display panel if necessary You should set the Frame zero point mags to an improbable value typically 30 so that the instrumental magnitudes are not inadvertently confused with calibrated ones Clicking the Help menu in the bar at the top of the photometry dialogue box followed by On Window will bring up a window with a pretty comprehensive description of how to use the aperture photometry facilities The measurement process is straightforward Following the instructions in the on line help proceed as follows a Click the Define object aperture button b c d Move the mouse sideways until the circle contains all the star b Place your cursor on the image over the star that you wish to measure Press down and hold down mouse button 1 41 SC 17 1 Measuring Instrumental Magnitudes e Release the mouse button f Click the Calculate results button g Inspect the Current object details displayed in the upper middle portion of the dia logue box and view the instrumental magnitude of the star You can change things like the inner and outer radii of the annulus for measuring the sky background by moving the sliders in the Aperture Photometry magnitude
33. h the astrometric calibration in a new file perhaps called ngc1275jktpre sdf This is most simply done by entering the new file name in the Selection box and pressing OK Do not worry if a message is displayed saying that the WCS could only be saved as an AST native representation 12 3 Creating an accurate astrometric calibration In some cases the approximate astrometric calibration derived in the previous stage will be adequate However it is possible to use additional astrometric reference stars to refine it Assuming that you already have an approximate astrometric calibration proceed as follows 1 Query the USNO at ESO catalogue to find the objects that overlay the JKT image following the second part of the recipe in Section 9 Alternatively you can load the example local catalogue ngc1275usno tab 2 Click on the Image Analysis button on the menu bar along the top of the main window Choose the Astrometry calibration item and then Fit to star positions from the next menu The Fit astrometry reference positions dialogue box Figure 11 should appear SC 17 1 Astrometric Calibration 36 3 Import the catalogue of reference stars by clicking on the Grab button and choosing the appropriate catalogue 4 Click on the Clip button to remove reference stars which fall outside the image 5 Delete all the catalogue objects which correspond to galaxies nebulae blended double star images etc none of which make good astrometr
34. he galaxy NGC 1275 will be used Recall that the J2000 coordinates of this galaxy are a 3 19 48516 5 41 30 42 1 A region will be extracted from the on line version of the DSS Digitised Sky Survey at ESO The DSS is a photographic survey which covers the entire sky It was constructed by the Space Telescope Science Institute STScI by digitising and combining surveys conducted with the Palomar and UK Schmidt telescopes a KK nef is available as part of the data archive service at the University of Leicester The DSS image will then be overlaid with objects extracted from the USNG PMM astrometric catalogue 13 The SuperCOSMOS image surveys 7 also available from GAIA return images which have catalogues of objects detected in the images already attached and so combine the two stages into one operation However the SuperCOSMOS surveys currently only cover the sky south of Declination 3 If the coordinates of your target region or object are not for epoch and equinox J2000 then you should convert them to this system COCO see SUN 56 18 is available for this purpose Though the target coordinates can be converted within GAIA it is probably better to convert them prior to starting it 1 Start GAIA Type h gaia amp The ampersand amp is simply to run GAIA as a detached process so that you can continue to issue Unix commands from the command line After a few moments the main GAIA window should appear
35. hree areas are briefly outlined below 3 1 Image Display Displaying images Opening files displaying more than one image maximising the display area Changing the zoom position detail and colours Panel readouts interactive slices pixel table Creating resizing moving deleting saving and restoring Image and overlay graphics colour ramp 3 2 Image Analysis Astrometry applying and displaying celestial coordinates to an image Photometry interactive measurements of selected stars using a circular or elliptical aperture Automatic object detection automatic detection of the objects in an image A set of parameters positions brightness ellipticity etc are computed for all the objects detected Surface photometry Surface photometry of galaxies including determining radial profiles and fitting elliptical isophotes Image patching interactive removal of image blemishes and contaminating objects from an image For example foreground stars might be removed from a galaxy image Properties of image regions statistics extraction and removal of shaped regions of an image SC 17 1 Data Formats 4 two images can be compared by either plotting one as a set of contours su perimposed on the other or by blinking between the two images cf the blink comparator used to compare two photographic plates Object position and statistics two dimensional Gaussian fits to single or multiple objects Polarisation plotting
36. ic reference objects The procedure to delete an object is e click on the symbol for the object in the main window so that the object is highlighted in the catalogue window e click on the Delete button and then confirm that the object is to be deleted 6 When you are happy that you have removed all unsuitable objects clear the catalogue markers click on the Graphics menu in the main window s menu bar and choose Clear 7 Click on the Centroid button in the Fit astrometry reference positions dialogue box to refine the pixel positions for all the reference stars New markers should be drawn over the JKT image showing the revised positions of the reference stars Q0 we Click on the Fit Test button and the revised calibration using all the reference stars will be created If any unsuitable objects remain amongst the reference stars or the fit is in some other way unsatisfactory then repeat the steps above to remove the offending objects and repeat the fit When the fit is acceptable click the Accept button 9 Finally save a version of the image with the revised calibration Click on the File menu button in the main window choose the Save as item and use the resulting dialogue box to save the image as a new file perhaps called ngc1275jktast sdf 12 4 Variations The preceding recipe has described just one of the numerous different ways to apply an as trometric calibration to an image using GAIA Many images already
37. jects will be saved as a file The format in which this catalogue is saved depends on the file type specified at the end of the file name see Section 4 2 Catalogues saved in the FITS tables TST or STL formats can subsequently be imported into CURSA see SUN 190 5 which provides additional catalogue manipulation facilities When you have finished click the Close button to close the USNO at ESO 1 window 6 To close GAIA click on the File menu the leftmost item in the menu bar along the top of the main window and choose the Exit option 9 1 Restoring catalogue access It is possible to configure the set of catalogues and sky surveys which GAIA can access though the details are not germane here However if the Digitized Sky at ESO survey or USNO at ESO catalogue do not appear in the lists of Image Servers and Catalogs respectively then the most likely reason is that the default list of catalogues and surveys has been substituted with one which does not include them The simplest way to restore access is to revert to using the default list GAIA configuration files are kept in subdirectory skycat of your top level directory To restore the default catalogues and surveys you should delete or rename file skycat skycat cfg and then restart GAIA 9 2 Using a proxy server Access to the World Wide Web at your site may be restricted by a firewall or maybe just by policy so that it is only available through a web cache or pr
38. line resources available to GAIA allow the process to be automated and simplified Proceed as follows 1 You need to extract a region of the Digitised Sky Survey DSS roughly corresponding to the region imaged in the CCD frame The first part of the recipe in Section gives exactly the procedure required Either repeat this procedure load the image that you created when previously working through Section 9 into GAIA or load file ngc1275dss sdf into GAIA the last is the required region already extracted from the DSS 29 SC 17 1 Astrometric Calibration 2 Adjust the colour table until the image appears as in Figure f Click on the View menu and select the Colors option A panel will appear Set the colour scale algorithm to Linear the colormap to ramp and the intensity to neg Then click on the Close button Set the magnification by clicking on the Scale button in the bottom left of the control panel in the centre top of the window and setting it to 2x 3 Overlay the DSS image with objects selected from the USNO at ESO catalogue following the second part of the recipe in Section 9 4 The next step is to choose five stars in the image to act as reference stars These stars x S should be e easily identified e reasonably bright e stars rather than galaxies because star images have more precisely defined centres e isolated from other stars and galaxies to avoid images which blend together e
39. logue in the list Once the catalogue has been imported the reference stars should be listed under Reference positions in the Fit astrometry reference positions dialogue box Figure 17 You now need to measure the positions in pixels of these stars in the JKT image Click on the first reference star in the list make a mental note of its star name and click on the Edit button The dialogue box shown in Figure 12 should appear Please edit the data for the object below ia 275021964568 E y y y lE rajozz 33458 dec at 3017 50 x F43 158237350425357 S y 1 o643550268263881 lt __Enter ose Reset Pick object Figure 12 The GAIA dialogue box holding details for a reference star Now click the Pick object button The dialogue box shown in Figure 13 should appear The black box in the upper portion of the window displays a section of the main image centred on the position of the cursor Referring back to your paper copy of the DSS image or Figure 10 position the cursor over the corresponding first reference star the star names should match If there are any other significant features visible in the box you should reduce the size of the box using the zoom buttons Z and z immediately below the image box Once you are happy with the sample size re position the cursor over the star and press the left mouse button The pixel coordinates at the centre of the feature are displayed
40. long the top of the main window and choose the Contouring item as before A window similar to Figure 7 should appear 3 Do not click on the Choose file button to specify the image to be contoured Rather proceed immediately to specifying and generating the contour levels by clicking on the Generate button and then proceed as before SC 17 1 Astrometric Calibration 28 12 Astrometric Calibration This recipe describes how to apply an astrometric calibration to an image In this context an astrometric calibration is a mathematical transformation relating the x y positions of pixels in the image array to celestial coordinates on the sky Applying an astrometric calibration is just another term for creating a World Coordinate System WCS see Section 4 1 for the image with the World Coordinates being celestial coordinates GAIA provides numerous options for astrometric calibration only one of which will be used here though Section 12 4 below gives a few hints about what else is available In most of the techniques stars occurring in the image are used as fiducial marks and a transformation is defined between their x y pixel positions measured in the image and their celestial coordinates obtained from an astrometric reference catalogue The most interactive and problematic part of the procedure is identifying a given object in the image with the corresponding entry in an astrometric reference catalogue The principal info
41. luded Typical auxiliary information might include the instrument and telescope used the date and time of observation details of the instrumental set up etc In the jargon of computer science such header information is often called metadata though this term is rarely used in astronomy Sometimes you may wish to examine this auxiliary information In GAIA and again anticipating Part II you can display the headers of a FITS image by clicking on the View menu on the menu bar along the top of the main window and choosing the Fits Header item see the recipe in Section 8 Some alternative ways of listing the header information for the NDF FITS and a couple of other formats are given in Appendix B 2 of SC 6 16 Data files usually contain just a single image However both the FITS and NDF formats allow files to contain more than one image and occasionally you might encounter such a file Section 5 2 includes some notes on how to access a given image inside a file in this case The format of a data file is often indicated by specifying a file type at the end of the file name NDF files have file type sdf When accessing NDF files with GAIA the file type may optionally be omitted FITS files usually have a file type of fit or fits 4 1 World coordinate systems Astronomical images often contain a so called World Coordinate System WCS The WCS is a prescription for converting pixel position indices that i
42. ms these example data are kept in directory star examples sc17 All the files in this directory should be copied into your current directory before attempting any of the recipes You might wish to create a new empty directory for this purpose You should also ensure that your terminal or workstation is configured to receive X output All the recipes in this part of the cookbook use observations of the type I Seyfert Galaxy NGC 1275 also known as 3C 84 The J2000 coordinates of this galaxy are a 3719 48516 41 30 42 1 Its V magnitude is 12 5 and its morphological classification Ep these details were obtained from SIMBAD Shttp simbad u strasbg fr Simbad SC 17 1 Displaying an Image 12 8 Displaying an Image This introductory recipe demonstrates displaying an image which is the simplest use of GAIA The image used is file ngc1275jkt sdf a reduced V band CCD image of the galaxy NGC 1275 obtained with the Jacobus Kapteyn Telescope JKT on La Palma It was derived from observations included on the CD ROM Astronomical Images by Jaffe 11 However the same data are publicly available from the ING Isaac Newton Group data archive maintained at the Institute of Astronomy University of Cambridge See URL http archive ast cam ac uk For technical reasons there are some differences in the arrangement of the FITS headers in the archive and CD ROM versions of the files but both contain identical astronomical info
43. o the other and noting the change in Right Ascension or Declination it is apparent that the image is rather larger than the 7 field extracted from the DSS 3 Aside you might be interested to check that the X ray image really is centred on NGC 1275 a Click on the Data Servers menu towards the right of the menu bar at the top of the main window and choose the Catalogs option Select the RC3 at CADC catalogue which is a version of the Third Reference Catalogue of Bright Galaxies 17 provided by the CADC Canadian Astronomy Data Center gt A window similar to Figure p should appear Click on the Search button in the bottom left of the window and after a couple of moments the galaxies in the RC3 which overlay the X ray image will be listed in the Search Results box and plotted in the main window http edas www star le ac uk 1Shttp cadcwww dao nrc ca 23 SC 17 1 Superimposing and Contouring Images Figure 6 An X ray image centred on the galaxy NGC 1275 The image was obtained with the HRI instrument on the ROSAT satellite SC 17 1 Superimposing and Contouring Images 24 4 7 8 9 b One of the plotted objects is almost exactly coincident with the centre of the X ray emission If you click on this symbol the line for NGC 1275 is highlighted in the Search Results box c If you double click on the line for NGC 1275 in the Search Results box the correspond ing symbol in the image is la
44. oxy server If you are in this situation you will need to configure GAIA so that its remote catalogue access will work Proceed as follows 1 Start GAIA select the Data Servers menu in the menu bar along the top of the main window and choose any remote catalogue 2 The catalogue window will appear similar to Figure 5 Click on the Options menu in its menu bar and choose the Proxies item 3 a Proxies window will appear You should fill in the various fields consult your system manager if you are unsure what to enter Then click on OK SC 17 1 Using World Coordinates 20 10 Using World Coordinates This recipe demonstrates the simple use of World Coordinates in GAIA Recall that a World Coordinate System WCS see Section 4 1 relates the positions of pixels in an image to celestial coordinate systems on the sky In practice it allows you to display and annotate images in terms of celestial coordinates Some images include a WCS as part of the auxiliary information that they contain others do not For example ngc1275jkt sdf the JKT image used in the recipe in Section does not possess a WCS but images retrieved from the DSS as in the recipe in Section D do It is possible to use GAIA to add a WCS to an image and the recipe in Section 12 is an example doing so The present recipe merely gives some examples of using a WCS The recipe uses file ngc1275dss sdf which is an image centred on the galaxy NGC 1275 extracted
45. rdinates are only updated when the cursor moves Thus if it is already positioned over the star that you are interested in then you need to nudge it off the object and return it to the required position 3 Depressing the Caps Lock key will prevent the x y and w 6 values being updated as the cursor moves This facility is useful for example if you wish to preserve the coordinates whilst moving the cursor to another window 4 A list of positions in an image can be selected and saved as a text file by selecting the Positions item from the Image Analysis menu in the menu bar along the top of the main window 21 SC 17 1 Using World Coordinates 10 2 Showing celestial coordinate grids GAIA can superimpose a celestial coordinate grid over the image 1 Click on the Image Analysis button in the menu bar along the top of the main window and choose Overlay axes grid A window will appear with numerous options You can change some of these if you wish but there is no need to do so Simply click on the Draw button and an axis grid is drawn in the current celestial coordinate system The grid is removed when you click Close to close the window 2 To show a grid in a different celestial coordinate system click on the Image Analysis button in the menu bar along the top of the main window and choose Change coordinates and then Celestial coordinates A window appears which allows you to specify the coordinate system Choose the on
46. rmation ngc1275jkt sdf was created by reducing the raw observations using CCDPACK see SUN 139 9 following the recipe in SC 5 7 The data reduction is not considered further here The image is in the NDF data format see Section 4 for details of the data formats available to GAIA If you prefer you could use an image of your own rather than this one Assuming that you have copied all the example data files for the cookbook into your current directory as described in Section 7 above and configured your terminal or workstation to receive X output then proceed as follows 1 Start GAIA by typing gaia amp The ampersand amp is of course simply to run GAIA as a detached process so that you can continue to issue Unix commands from the command line A window displaying a start up message should be displayed shortly followed by the main GAIA window If these windows do not appear then GAIA is not properly installed at your site in the first instance seek assistance from your site manager 2 Load file ngc1275jkt sdf by clicking the File menu leftmost of the options in the menu bar at the top of the window selecting Open and using the file picker which appears to choose the appropriate file 3 The file should open but the main display window will probably be mostly dark with just a few white dots corresponding to the brightest parts of the image Set some of the display options as follows a select the
47. rmation which you need to know about an image before attempting astrometric calibration is the approximate position on the sky corresponding to the centre of the image and the size of the field of view The former can usually be obtained from examining the auxiliary information included with the image see Section 4 The latter may also be found in the auxiliary information or from the documentation for the instrument or telescope If you have no prior information whatsoever about the region of sky observed then astrometric calibration will usually be impossible In this recipe an astrometric calibration will be created for the V band CCD image of NGC 1275 obtained with the JKT which was used in the recipe in Section 8 The field of view of this image is about six minutes of arc in each axis The process of creating the astrometric calibration divides naturally into three stages 1 finding suitable astrometric reference stars 2 creating a preliminary astrometric calibration 3 creating a final accurate astrometric calibration Each stage is described separately in individual sub recipes below 12 1 Finding astrometric reference stars The purpose of this stage is to find a small number of astrometric reference stars imaged on the CCD frame which can be used to define the preliminary astrometric calibration Traditionally finding reference stars is a long winded task involving consulting printed atlases and catalogues However the on
48. rrows these buttons are located on the bottom row of the control panel towards the top of the main window It should now be straightforward to identify objects in the JKT image with the corre sponding objects in your print out of the DSS image created in the previous stage or in Figure 4 Click on the Image Analysis button on the menu bar along the top of the main window Choose the Astrometry calibration item and then Fit to star positions from the next menu The Fit astrometry reference positions dialogue box shown in Figure HM should appear SC 17 1 Astrometric Calibration GAIA Fit astrometry reference positions 1 Figure 11 The main GAIA dialogue box used to create an astrometric calibration 32 33 SC 17 1 Astrometric Calibration 5 Indicate the celestial coordinate system in which your star coordinates are expressed O ma AJ MT na a You do so by specifying suitable values using the coordinate system Equinox and Epoch buttons The coordinates will usually be FK5 equinox J2000 or for older catalogues FK4 equinox B1950 The epoch defaults to J2000 for FK5 coordinates and B1950 for FK4 coordinates For the present recipe the defaults of FK5 equinox J2000 are correct To import the catalogue of selected reference stars click on the Grab button A selection box allowing you to specify the catalogue required will appear The selected reference stars will usually be the first cata
49. s accessible to GAIA If you have already started GAIA and want to display a different image from the Unix command line then type gaiadisp file name It is also possible to plot an image in a given window by specifying its clone number gaiadisp file name clone number for example gaiadisp ngc1275 sdf 2 The plot will be displayed in a window titled GAIA Skycat file name 2 If this window does not exist then it will be created 5 1 Using GAIA from IRAF GAIA can be run from the IRAF command language c1 see SG 12 15 for an introduction to IRAF on Starlink systems However before this option is available the GAIA package must have been included in the version of IRAF that you are using This customisation will usually already have been performed if you are using IRAF at a Starlink site Assuming that the IRAF GAIA package is available your first step must be to initialise it From the IRAF cl type cl gt gaia Alternatively if you think that you will often want to use GAIA from within IRAF then you could include this step in your IRAF login c1 file To display an image type cl gt gaiadisp image plane number image is the image to be displayed and plane number is the number of the IRAF display plane in which the image is to appear which is identical to the GAIA clone number For example cl gt gaiadisp dev pix imh 1 will display the default IRAF image in display plane 1 The image will either b
50. s indices into the two dimensional array representing the image into physical units In practice for direct images of a region of sky the WCS will usually transform pixel indices into some celestial coordinate system such as the Right Ascension and Declination for some equinox and epoch However the underlying concept of a WCS is much more general For example it is possible to have a WCS which transforms pixel indices into x y positions in micron on the face of the CCD chip which detected the image Alternatively for spectroscopic data one index might be transformed into a wavelength in ngstr m The WCS is stored with the other auxiliary information for the image such as the instrument and telescope used the date and time of observation etc GAIA handles all the details of manipulating the WCS automatically ultimately by using the Starlink AST library All that you really need to know is that an image might or might not contain a WCS and an image with a suitable WCS can be annotated and examined in terms of celestial coordinates rather than pixel indices The actual way in which the WCS details are stored in the auxiliary information for an image is rather arcane There are several conventions in use for FITS files none of which are standard There are proposals for FITS WCS standards but these are still under discussion at the time of writing If you are interested to find out more the AST library is documented in SUN 210 20 and SUN
51. s see Section 4 It can also retrieve copies of remote images and catalogues via the Internet This cookbook is an introduction to GAIA It describes the facilities that GAIA provides and gives some simple examples of their use It refers to GAIA version 2 6 or higher The structure of the cookbook is Part I introduction and getting started with GAIA Part II a set of recipes describing in detail how to perform some useful tasks with GAIA The cookbook is aimed at astronomers who are new to GAIA but who think that it might be useful in their work It is complemented by the GAIA User s Manual SUN 214 GAIA Graphical Astronomy and Image Analysis Tool 8 which gives detailed information about the tool Experienced users of GAIA are more likely to find this latter document useful than the present cookbook Extensive on line help information is also available from within GAIA itself 1 1 GAIA and SkyCat GAIA is a customisation and enhancement of the SkyCaf image display tool developed by Teh Allan Brighton and colleagues as part of the IVLT project GAIA s basic display and remote catalogue and archive access facilities are largely the original SkyCat However the facilities for astronomical analysis such as astrometric calibration object detection and photometry are all enhancements unique to GAIA Many of these enhancements are implemented by causing GAIA to automatically invoke conventional Starlink packages such as
52. s dialogue box Apertures are drawn around stars as they are measured GAIA Aperture photometry magnitudes 1 L a Eccentricity Position angle Figure 16 The GAIA Aperture photometry magnitudes dialogue box 4 Clicking on the Options menu in the bar at the top of the Aperture photometry magnitudes dialogue box will allow you to alter settings by using push buttons that are labelled e Use annular sky regions e Use circular apertures SC 17 1 Measuring Instrumental Magnitudes 42 e Keep apertures same size By de selecting the first button here Use annular sky regions you can use interactive apertures to measure the sky background and by de selecting the second you can use ellipses instead of circles Because GAIA is acting as a front end to PHOTOM most of the parameters which can be set in PHOTOM can also be set in GAIA There is a nice feature in GAIA that is of use when deciding how big to make the aperture radius By Clicking on the View menu in the main window and selecting the Slice option it is possible to obtain a cut or slice across any star image on the fly see Figure 17 This option can usefully be used to estimate how far out from the star useful signal exists Pixel Values 5 10 15 Distance along slice X 72 0 Y 56 0 Value 500 0 Print Save as Qose Figure 17 A GAIA Slice display panel showing a slice through
53. spread reasonably uniformly over the image not very close to the edge of the image later you will need to to identify the cor responding stars in the JKT image and the two areas of sky are not exactly the same Figure 10 shows five suitable stars in the DSS example image To select the stars hold down the shift key and click on each star in turn without releasing the shift key As you do so the selected star is highlighted in both the image and the catalogue windows You can add as many stars as you like but five is adequate Copy the selected stars to a new table dialogue box by clicking on the Options menu in the catalogue dialogue box and choosing the Extract selected item A new catalogue dialogue box listing just the selected objects will appear Henceforth you will work with this catalogue dialogue box To double check which objects you have selected click on the Graphics menu in the main window and choose Clear All the catalogue object markers will disappear Re plot the selected objects by clicking on the Plot button towards the bottom of the new catalogue dialogue box Label the chosen objects by clicking on the Options menu in the catalogue dialogue box menu bar and choosing Label all objects It is useful to print out a copy of the image with the reference stars marked for use in the next stage of the calibration If you are using the colour table described above you will need to adjust it so that the star identifications
54. suitable place to start The aperture photometry facilities in GAIA are provided by invoking the PHOTOM package see SUN 45 10 behind the scenes though you will just see seamless interaction via the GAIA windows Before you attempt to measure instrumental magnitudes from a CCD frame any instrumental effects cosmic ray events and other blemishes should already have been removed This process is described inSC 5 The 2 D CCD Data Reduction Cookbook and in SUN 139 the manual for the CCD data reduction package CCDPACK and is not considered further here SC 5 is a good introduction The image used in this recipe is ngc1275jkt sdf a reduced V band CCD image of NGC 1275 obtained with the JKT see the recipe in Section 8 for its provenance Proceed as follows 1 Start GAIA and load image ngc1275jkt sdf 2 You may wish to adjust the appearance of the displayed image The most likely items to change are the colour table click on the Color Map button in the lower right of the control panel and the magnification click on the Scale button in the bottom left of the control panel in the centre top of the window 3 You can now proceed to measure instrumental magnitudes Click on the Image Analysis menu on the menu bar along the top of the main window Choose the Aperture photometry item Two further items will be presented Results in magnitudes and Results in data counts Choose the former Results in magnitudes Both th
55. the centre top of the window and setting it to 2x The appearance of the display should now be similar to Figure f Click on the Image Analysis button on the menu bar along the top of the main window and choose the Contouring item A window similar to Figure 7 should appear Click on the Choose file button and load file ngc1275hri fits Click on the Generate button on the left hand side of the window immediately below Levels A window similar to Figure 8 should appear Set the following values Number 5 Algorithm linear Start 100 Increment 75 The window should now appear exactly like Figure In this recipe the required contour levels are already prescribed however the first time you contour an image you will probably need to experiment to find suitable levels One way to do so is to set the 25 SC 17 1 Superimposing and Contouring Images GAIA Contouring 1 Figure 7 Window for defining contour details GAIA Contouring 1 Percentiles Figure 8 Window for defining contour levels SC 17 1 Superimposing and Contouring Images 26 Algorithm to automatic and allow GAIA to generate the levels itself If these levels are not satisfactory you can adjust them manually Click on the Generate button and the appearance of the window should revert to that in Figure 10 For all five of the specified contours click on the colour boxes and set them to red which makes the contours easier to
56. trieve objects from the catalogue The central position and minimum and maximum radius should be already filled in the values have been obtained from the two dimensional image The Brightest min and Faintest max boxes allow these quantities to be set if desired However in the present example they can be left blank and all the objects in the PMM which overlay the image will be selected USNO at ESO 1 ae Figure 5 Window to retrieve objects from a remote version of the USNO PMM catalogue Simply click on the Search button in the bottom left of the window After a couple of moments the selected objects are listed in the Search Results box and overlaid on the image in the main window A useful feature for identifying objects in the list with the corresponding plotted symbol is that if you position the cursor over either a plot symbol or a row in the table and click with the left mouse button the corresponding row and plot symbol are highlighted To save the selected objects as a text file click on the File menu in the USNO at ESO 1 1Shttp www nofs navy mil 19 SC 17 1 Retrieving Remote Images and Catalogues window the leftmost item in its menu bar and choose either Save as or Print In both cases a window will appear which allows you to save the list Note that though both options produce text files they are in different formats If you choose the Save as option the catalogue of ob
57. tton in the same dialogue box GAIA uses the known celestial co ordinates and measured positions of the reference stars to define an astrometric calibration It then uses this calibration to work out the pixel positions corresponding to each of your reference coordinates and displays markers in the main image at these pixel positions You should find that a marker is drawn more or less on top of each of your reference stars If the markers are not properly aligned then you have probably measured the wrong star or entered an incorrect Right Ascension or Declination value You should correct the reference positions and then press the Fit Test button again To correct the reference positions Edit the details of the offending star as above then press Fit Test again to re calculate the astrometric calibration Cheat you can load a file containing the celestial coordinates and measured positions for the five stars marked in Figure 10 Click on the File menu in the menu bar along the top of the Fit astrometry reference positions dialogue box Figure 11 and choose the Read positions from a file option A file picker appears Use it to load file refstars prelim Then click the Fit Test button as before Once you are happy with the calibration press the Accept button Finally save the astrometric calibration by clicking the File menu button in the main window and then the Save as menu item Use the resulting dialogue box to save the image wit
58. ured positions from a DSS or SuperCOSMOS calibration image to the target image then move the markers for the stars onto the corresponding objects in the target image move the cursor to the appropriate reference star marker hold down the left mouse button move the cursor to the required position and then release the mouse button and measure the positions in the target image click on the Centroid button A reference star can be deleted by positioning the cursor over the appropriate marker holding down the Control key and clicking on the left mouse button After confirmation the corresponding reference star is deleted SC 17 1 Automatic Object Detection 38 13 Automatic Object Detection This recipe shows how to use GAIA to automatically detect objects in an image A catalogue of the objects detected is assembled tabulating various properties for each object position flux ellipticity etc The catalogue can both be examined within GAIA and saved as a file for further analysis The object detection facilities of GAIA are provided by invoking the EXTRACTOR package see SUN 226 3 behind the scenes though you will just see seamless interaction via the GAIA windows EXTRACTOR is a version of the source extraction program by Bertin and Arnouts 2 It is particularly well suited to the analysis of large extragalactic survey data but also performs well on other sorts of astronomical images The image used in this recipe is ngc1275jkt s

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