BUGS · SEE_ALSO
substar -- subtract photometry results from an image
substar image photfile exfile psfimage subimage
- The list of images from which to subtract the scaled and shifted PSF.
- The list of PSF fitted photometry files. There must be one photometry file for every input image. If photfile is "default", "dir$default", or a directory specification, SUBSTAR will look for a file called image.nst.? where the question mark stands for the highest existing version number. Photfile is usually the output of the NSTAR task but may also be the output of the PEAK and ALLSTAR tasks or even the PHOT task. Photfile may be an APPHOT/DAOPHOT text database or an STSDAS table.
- The list of photometry files containing the ids of stars to be excluded from the subtraction. Exfile must be undefined or contain one exclude file for every input image. If exfile is "default", "dir$default", or a directory specification, SUBSTAR will look for a file called image.pst.? where the ? mark stands for the highest existing version number. Exfile is usually the output of the PSTSELECT task but may also be the output of the PEAK, NSTAR and ALLSTAR tasks or even the PHOT task. Exfile may be an APPHOT/DAOPHOT text database or an STSDAS table.
- The list of images containing the PSF models computed by the DAOPHOT PSF task. The number of PSF images must be equal to the number of input images. If psfimage is "default", "dir$default", or a directory specification, then PEAK will look for an image with the name image.psf.?, where ? is the highest existing version number.
- The list of output subtracted images. There must be one output subtracted image for every input image. If subimage is "default", "dir$default", or a directory specification, then SUBSTAR will write an image called image.sub.? where question mark stands for the next available version number.
- datapars = ""
- The name of the file containing the data dependent parameters. The parameters scale , datamin , and datamax are located here. If datapars is undefined then the default parameter set in uparm directory
- daopars = ""
- The name of the file containing the daophot fitting parameters. The parameters psfrad and fitrad are located here. If daopars is undefined then the default parameter set in uparm directory is used.
- wcsin = ")_.wcsin", wcsout = ")_.wcsout", wcspsf = ")_.wcspsf"
- The coordinate system of the input coordinates read from photfile
, of the
psf model psfimage
, and of the output coordinates written to
the standard output if verbose
= "yes". The image header coordinate
system is used to transform from the input coordinate system to the "logical"
pixel coordinate system used internally, from the internal logicial system to
the PSF model system, and from the internal "logical" pixel coordinate system
to the output coordinate system. The input coordinate system options are
"logical", tv", "physical", and "world". The PSF model and output coordinate
system options are "logical", "tv", and "physical". The image cursor coordinate
system is assumed to be the "tv" system.
- Logical coordinates are pixel coordinates relative to the current image. The logical coordinate system is the coordinate system used by the image input/output routines to access the image data on disk. In the logical coordinate system the coordinates of the first pixel of a 2D image, e.g. dev$ypix and a 2D image section, e.g. dev$ypix[200:300,200:300] are always (1,1).
- Tv coordinates are the pixel coordinates used by the display servers. Tv coordinates include the effects of any input image section, but do not include the effects of previous linear transformations. If the input image name does not include an image section, then tv coordinates are identical to logical coordinates. If the input image name does include a section, and the input image has not been linearly transformed or copied from a parent image, tv coordinates are identical to physical coordinates. In the tv coordinate system the coordinates of the first pixel of a 2D image, e.g. dev$ypix and a 2D image section, e.g. dev$ypix[200:300,200:300] are (1,1) and (200,200) respectively.
- Physical coordinates are pixel coordinates invariant with respect to linear transformations of the physical image data. For example, if the current image was created by extracting a section of another image, the physical coordinates of an object in the current image will be equal to the physical coordinates of the same object in the parent image, although the logical coordinates will be different. In the physical coordinate system the coordinates of the first pixel of a 2D image, e.g. dev$ypix and a 2D image section, e.g. dev$ypix[200:300,200:300] are (1,1) and (200,200) respectively.
- World coordinates are image coordinates in any units which are invariant with respect to linear transformations of the physical image data. For example, the ra and dec of an object will always be the same no matter how the image is linearly transformed. The units of input world coordinates must be the same as those expected by the image header wcs, e. g. degrees and degrees for celestial coordinate systems.
- cache = ")_.cache"
- Cache the image pixels in memory. Cache may be set to the value of the apphot package parameter (the default), "yes", or "no". By default cacheing is disabled.
- verify = ")_.verify"
- Verify the critical SUBSTAR task parameters? Verify can be set to the DAOPHOT package parameter value (the default), "yes", or "no".
- update = ")_update"
- Update the SUBSTAR task parameters if verify is "yes"? Update can be set to the default daophot package parameter value, "yes", or "no".
- verbose = ")_.verbose"
- Print messages about the progress of the task ? Verbose can be set to the DAOPHOT package parameter value (the default), "yes", or "no".
SUBSTAR task takes an input photometry list photfile containing the fitted coordinates and magnitudes, and an input PSF psfimage , and for each star in the photometry list scales and shifts the PSF and subtracts it from the input image image . The final subtracted image is saved in the output image subimage .
The input photometry list can be the output from of the PEAK, NSTAR or ALLSTAR tasks or even the PHOT task although most people would not want to use the PHOT output for this purpose.
Selected stars may be ommitted from the subtraction by supplying their ids in the file exfile . Exfile is normally the output the PSTSELECT task and is used to tell SUBSTAR to subtract the PSF star neighbours, but not the PSF stars themselves.
The coordinates read from photfile are assumed to be in coordinate system defined by wcsin . The options are "logical", "tv", "physical", and "world" and the transformation from the input coordinate system to the internal "logical" system is defined by the image coordinate system. The simplest default is the "logical" pixel system. Users working on with image sections but importing pixel coordinate lists generated from the parent image must use use the "tv" or "physical" input coordinate systems.
The coordinate system of the PSF model is the coordinate system defined by the wcspsf parameter. Normally the PSF model was derived from the input image and this parameter default to "logical". However if the PSF model was derived from a larger image which is a "parent" of the input image, then wcspsf should be set to "tv" or "physical" depending on the circumstances.
The coordinates written to the standard output if verbose = yes are in the coordinate system defined by wcsout . The options are "logical", "tv", and "physical". The simplest default is the "logical" system. Users wishing to correlate the output coordinates of objects measured in image sections or mosaic pieces with coordinates in the parent image must use the "tv" or "physical" coordinate systems.
If cache is yes and the host machine physical memory and working set size are large enough the input and output image pixels are cached in memory. If cacheing is enabled and SUBSTAR is run interactively the first subtraction will appear to take a long time as the entire image must be read in before the measurement is actually made. All subsequent measurements will be very fast because SUBSTAR is accessing memory not disk. The point of cacheing is to speed up random image access by making the internal image i/o buffers the same size as the image itself. However if the input object lists are sorted in row order which SUBSTAR does internally and are sparse cacheing may actually worsen not improve the execution time. Also at present there is no point in enabling cacheing for images that are less than or equal to 524288 bytes, i.e. the size of the test image dev$ypix, as the default image i/o buffer is exactly that size. However if the size of dev$ypix is doubled by converting it to a real image with the chpixtype task then the effect of cacheing in interactive is can be quite noticeable if measurements of objects in the top and bottom halfs of the image are alternated.
The SUBSTAR task is most commonly used to check on the quality of the PSF fitting produced by PEAK and NSTAR, to search for non-stellar objects and close binary stars, to generate an improved PSF in crowded fields, and to remove neighbours from bright stars which are to be used to determine aperture corrections.
1. Subtract the NSTAR photometry results for the test image dev$ypix from the image dev$ypix.
da> datapars.epadu = 14.0 da> datapars.readnoise = 75.0 ... set the gain and readout noise for the detector da> daofind dev$ypix default fwhmpsf=2.5 sigma=5.0 threshold=20.0 ... answer verify prompts ... find stars in the image ... answer will appear in ypix.coo.1 da> phot dev$ypix default default annulus=10. dannulus=5. \ apertures = 3.0 ... answer verify prompts ... do aperture photometry on the detected stars ... answer will appear in ypix.mag.1 da> display dev$ypix 1 da> psf dev$ypix default "" default default default psfrad=11.0 \ fitrad=3.0 mkstars=yes display=imdr ... verify the critical parameters ... move the image cursor to a candidate star and hit the a key, a plot of the stellar data appears ... type ? for a listing of the graphics cursor menu ... type a to accept the star, d to reject it ... move to the next candidate stars and repeat the previous steps ... type l to list all the psf stars ... type f to fit the psf ... move cursor to first psf star and type s to see residuals, repeat for all the psf stars ... type w to save the PSF model ... type q to quit, and q again to confirm ... the output will appear in ypix.psf.1.imh, ypix.pst.1 and ypix.psg.1 da> group dev$ypix default default default ... verify the prompts ... the output will appear in ypix.grp.1 da> nstar dev$ypix default default default default ... verify the prompts ... the results will appear in ypix.nst.1 and ypix.nrj.1 da> pdump ypix.nst.1 sharpness,chi yes | graph ... plot chi versus sharpness, the stars should cluster around sharpness = 0.0 and chi = 1.0, note that that the frame does not have a lot of stars da> substar dev$ypix default "" default default ... subtract the fitted stars da> display ypix.sub.1 2 ... note that the psf stars subtract reasonably well but other objects which are not stars don't
2. Rerun the previous example on a section of the test image using the group file and PSF model derived in example 1 for the parent image and writing the results in the coordinate system of the parent image.
da> nstar dev$ypix[150:450,150:450] default default default default \ wcsin=tv wcspsf=tv wcsout=tv ... answer the verify prompts ... fit the stars ... the results will appear in ypix.nst.2 and ypix.nst.2 da> display dev$ypix[150:450,150:450] 1 ... display the image da> pdump ypix.nst.2 xc,yc yes | tvmark 1 STDIN col=204 ... mark the stars da> substar dev$ypix ypix.nst.2 "" default default ... subtract stars from parent image ... the output images is ypix.sub.2 da> substar dev$ypix[150:450,150:450] ypix.nst.2 "" default default \ wcsin=tv wcspsf=tv wcsout=tv ... subtract stars from the nstarinput image ... the output images is ypix.sub.3