Using SExtractor With the Drizzlepac Tweakreg Software for
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1. default wcs for alignment during preliminary cosmic ray rejection and pfoduction of In PyRAF if using line commands rather than the Teal GUkinterface this is the syntax 7 L 1 z D th o i This produces cr cleaned versions of each input image called _crclean fits files on which you run SExtractor The of the initial cr rejection depends on the quality of the original wes alignment and this is for images taken within the 1 and To as needed i at the unix prompt in this case we used for ACS WEC Note the different group numbers and the offset to 0 amp 3 in the example below th re were a number of spurious sources cosmic rays which were not removed in the earlier step since that afea of the CCD chip a strip along the bottom did not overlap any other images to which comparison could be made for automatic identification and removal of the cosmic rays so these were then edited from the catalog by hand before they were fed to the tweakreg task for alignment of the images might have been to use a larger detect_minar a in SExtractor however this too can have its drawbacks since that will result in fewer objects and may also yield a poorer splution in some cases as it did in this one It can be very data dependent so experimentation is key to do this is to mask out the affected area before running SExtractor but you may want to use replacement values which are nef to some mean global sky value if the sky back2. Using SExtractor With the Drizzlepac Tweakreg Software marung for Aligning and Combining Sources in HST ACS Images Hes Ray A LUCAS Bryan Hilbert and Warren Hack 1 Space Telescope Science Institute Baltimore MD ABSTRACT The Tweakreg software which is part of the new STScl Drizzlepac software for aligning and combining images has initially relied upon a daofind like algorithm which is tuned to finding stellar sources to use for image alignment This works well when there are enough stellar sources present in the images However a significant number of images are of higher latitude extragalactic fields with few if any notable stellar images useful for alignment In such cases the use of software such as SExtractor can be useful for the generation of object catalogs which can be used for image alignment In this poster we explore the use of SExtractor with Tweakreg for such cases This poster is the Sadon o awa da a aa e y e aeae Tntoce www stsci edu hst riSiroverview drizzlepac resources ral_aas anchorage 2012 tomplate yv7 _Lucaspc was presented at the 220 AAS Meeting in Anchorage Alaska in June 2012 It is part of a group of posters on Astr of the Astrodrizzle documentation for users This poster will be on line in the same location Image Name POSTARG POSTARG PA_V3 RA of Dec of Ex x in arcsec n arcsec Orientation Aperture Aperture ime degrees sec a Data parameters same visit amp filter 3 using
3. d geometric distortion corrections etc However if that is the case you may also be using older uncorrected _flt fits images instead of either newer _flc fits both CTE amp geometric corrections or _flt fits only geometrically corrected images which have the latest OTFR pipelined corrections We strongly suggest fresh recently retrieved flc fits files as the input for each rumof astrodrizzle The updatenpol task is now applied in OTFR and the data which has been thus corrected for both CTE and geometric distortion updates would be the _flc fits files which are now also composed of 2 groups a 0 header plus groups 1 and 4 for science data groups 2 and 5 for error arrays groups 3 and 6 for DQ arrays and representing the new geometric corrections from updatenpol group 7 for the D2IMARR array and groups 8 11 for the WCSDVARR arrays An STSDAS catfits command on the _flc fits file will show you these details on the individual groups of the newly created _crclean fits images We used the same basic SExtractor default sex parameter values for minimum area and threshold for this test as for the other example and This worked well whereas a much larger DETECT_MINAREA again yielded poorer results However this can still be very data dependent Also an earlier run shown in Poster accidentally used blotted images and yielded a falsely better solution than this one Rather than full line commands as in th
4. e other example using v2 8 we set our SExtractor parameters in the default param amp default sex files and ran commands as sex jabu0 fvq_crclean fits etc This was because SExtractor v2 5 gave an error on EXT_NUMBER when using 0 or 3 in the command line So it ran on all groups We edited down to the 2 separate sci cats from groups amp 4 scil sci2 prec we got results like those in Figure 4 below SExtractor catalog overplotted we used a varis of parameters We first used the with and This resulted in arms of 0 16 slightly higher than we wanted but still good Trying to see if we could improve upon this we tried nclip 5 and nsigma 2 5 and then nclip 8 and nsigma 2 0 and got slightly better rms values each time but we were overclipping such that the trend along 0 in the rms plots was not really evident Since 0 16 is still a very good solution for an image with no real useable stars but only galaxies we went with that one Histograms amp vector plots OK for all so rms plots determined it The x y rms 0 149 0 166 on the left amp good scatter around using default nclip 3 and sigma 3 0 RMS values smaller on right but plots not as good Anderson J amp Bedin L R 2010 PASP 122 1035 1064 Chiaberge et al ACS ISR 2009 01 Gonzaga et al zzlePac Handbook Available on line http document gt i edu hst HST ov documents DrizzlePac Dri ac cover html
5. ground isnot strongly variable since this can otherwise also affect the globali sky level which SExtractor uses in object detection and measurement SExtractor can also be set up to use local sky values around each object but other factors come into play then See notes in purple to right to view the maps that Extractor created as checkimages you may need to insert the SIMPLE T header keyword in the checkimages before you can display and view them use a file relating the images to be updated to the catalog file for each chip of E each image to be ed by tweakreg The name doesn t matter as long as it matches the file name for it in the parameter in tw fain this case we have called it astdriz_catfile list In the automated version of tweakreg it is called coords _catfile lis Similarly the SExtractor catalog names should just be ones that make the most sense to you E THRESH Original and cr cleaned images telling tweakreg which columns in the SExtractor catalogs contained the x and y positions Those columns were determined earlier by the order in which you list them when editing your SExtractor default param file In this case we used columns 5 and 6 Run it first with updatehdr no until you are satisfied rms 0 15 rms plots centered around 0 then run it once more with updatehdr yes In the run below we used default values of minobj 15 searchrad 1 0 arcsec nclip 3 sigma 3 0 fit try rscale and x y positions i
6. n units of pixels Again using line commands in PyRAF DX pixels 200 S23 i 2 3000 HEr BED 2000 2000 1000 O 1000 2000 2000 1000 O 1000 2000 X pixels Y pixels Histogram vector plot and rms residuals plots RMS values of 0 14 0 16 are good plots look centered around 0 SExtractor http www astromatic net software sextractor Bertin E amp Arnouts S 1996 SExtractor Software for Source Extraction Astronomy amp Astrophysics Supplement 317 393 Bertin E SExtractor v2 13 User s Manual at https www astromatic net pubsvn software sextractor trunk doc sextractor pdf Akhlaghi M 2012 Notes on SExtractor 2 8 6 at http www astr tohoku ac jp akhlaghi sextractor_notes html Holwerda B SExtractor for Dummies at http astroa physics metu edu tr MANUALS sextractor Guide2source_extractor pdf erde o a E e MEEA StS Chec ns t HST overvicw clrizzlepac reSOURESS a oi was all taken in the same visit It is best to work with data from the same visit filter first We used the B band images of Abell 370 with rootname jabu01 from program 11507 the SM4 ACS ERO imaging it appears when you type epar astrodrizzle after importing drizzlepac into PYRAF we using the default WCS to create the _crclean fits images to be fed to SExtractor NOTE If you have older versions of the data retrieved from the Archive some time ago you may need to run the drizzlepac task updatenpol first to account for update
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