TIME_REQUIREMENTS · BUGS · SEE_ALSO
allstar -- group and fit psf to multiple stars simultaneously
allstar image photfile psfimage allstarfile rejfile subimage
- The list of images containing the stars to be fit.
- The input photometry files containing the initial estimates of the positions, sky values, and magnitudes of the stars to be fit. There must be one input photometry file for every input image. If photfile is "default", "dir$default", or a directory specification, then ALLSTAR looks for a file with the name image.mag.? where ? is the highest existing version number. Photfile is usually the output of the DAOPHOT PHOT task but may also be the output of the PSF, PEAK and NSTAR tasks, or the ALLSTAR task itself.
- The list of images containing the PSF models computed by the DAOPHOT PSF task. The number of PSF images must be equal to the number of input images. If psfimage is "default", "dir$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 photometry files. There must be one output photometry file for every input image. If allstarfile is "default", "dir$default", or a directory specification, then ALLSTAR will write an output file with the name image.als.? where ? is the next available version number. Allstarfile is a text database if the DAOPHOT package parameter text is "yes", an STSDAS table database if it is "no".
- The list of output rejected photometry files containing the positions and sky values of stars that could not be fit. If rejfile is undefined, results for all the stars in photfile are written to allstarfile , otherwise only the stars which were successfully fit are written to allstarfile and the remainder are written to rejfile. If rejfile is "default", "dir$default", or a directory specification ALLSTAR writes an output file with the name image.als.? where ? is the next available version number. Otherwise rejfile must specify one output photometry file for every input image. Rejfile is a text database if the DAOPHOT package parameter text is "yes", an STSDAS binary table database if it is "no".
- The list of output images with the fitted stars subtracted. There must be one subtracted image for every input image. If subimage is "default", "dir$default", or a directory specification, then ALLSTAR will create an image with the name image.sub.? where ? is the next available version number. Otherwise subimage must specify one output image for every image in image .
- datapars = ""
- The name of the file containing the data dependent parameters. The parameters scale , datamin , and datamax are located here. If datapars is undefined then the default parameter set in uparm directory is used.
- daopars = ""
- The name of the file containing the daophot fitting parameters. The parameters psfrad and fitrad are located here. If daopars is undefined then the default parameter set in uparm directory is used.
- wcsin = ")_.wcsin", wcsout = ")_.wcsout", wcspsf = ")_.wcspsf"
- The coordinate system of the input coordinates read from photfile
, of the
psf model psfimage
, and of the output coordinates written to
respectively. The image header coordinate
system is used to transform from the input coordinate system to the "logical"
pixel coordinate system used internally, from the internal logicial system to
the PSF model system, and from the internal "logical" pixel coordinate system
to the output coordinate system. The input coordinate system options are
"logical", tv", "physical", and "world". The PSF model and output coordinate
system options are "logical", "tv", and "physical". The image cursor coordinate
system is assumed to be the "tv" system.
- Logical coordinates are pixel coordinates relative to the current image. The logical coordinate system is the coordinate system used by the image input/output routines to access the image data on disk. In the logical coordinate system the coordinates of the first pixel of a 2D image, e.g. dev$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 = yes
- Cache all the data in memory ? If cache is "yes", then ALLSTAR attempts to preallocate sufficient space to store the input image plus the two image-sized working arrays it requires, plus space for the starlist, in memory. This can significantly reduce the total execution time. Users should however beware of creating a situation where excessive paging occurs. If cache = "no", ALLSTAR operates on subrasters containing the group currently being reduced, and writes the intermediate results to temporary scratch images. This option will work on any-sized image (unless a single group becomes the size of the entire image!) but can become slow of there are a large number of disk accesses. Users may wish to experiment to see which mode of operation suits their system best.
- 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".
- verify = ")_.verify"
- Verify the critical ALLSTAR task parameters. Verify can be set to the daophot package parameter value (the default), "yes", or "no".
- update = ")_.update"
- Update the critical ALLSTAR task parameters if verify = "yes". Update can be set to the daophot package parameter value (the default), "yes", or "no".
ALLSTAR computes x and y centers, sky values, and magnitudes for the stars in photfile by fitting the PSF psfimage to groups of stars in the IRAF image image . Initial estimates of the centers, sky values, and magnitudes, are read from the photometry list photfile . ALLSTAR groups the stars dynamically, performing a regrouping operation after every iteration. The new computed centers, sky values, and magnitudes are written to allstarfile along with the number of iterations it took to fit the star, the goodness of fit statistic chi, and the image sharpness statistic sharp. If rejfile is not null (""), only stars that are successfully fit are written to allstarfile , and the remainder are written to rejfile . Otherwise all the stars are written to allstarfile . Allstarfile and rejfile are text databases if the DAOPHOT package parameter text is "yes", STSDAS table databases if it is "no". An image with all the fitted stars subtracted out is written to subimage . In effect ALLSTAR performs the combined operations of GROUP, GRPSELECT, NSTAR, and SUBSTAR.
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 allstarfile and rejfile 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.
By default ALLSTAR computes new centers for all the stars in photfile . However if the DAOPARS parameter recenter is "no", ALLSTAR assumes that the x and y centers in photfile are the true centers and does not refit them. This option can be quite useful in cases where accurate center values have been derived from an image that has been through some non-linear image restoration algorithm, but the photometry must be derived from the original unrestored image.
By default (groupsky = "yes") ALLSTAR computes the sky value for each group by averaging the individual sky values in photfile for all the stars in the group. If groupsky = "no", the sky value for each pixel which contributes to the group fit is set equal to the mean of the sky values for those stars for which the pixel falls within one fitting radius. If the DAOPARS parameter fitksy is "yes", then ALLSTAR recomputes the individual sky values before averaging over the group, by, every third iteration, subtracting off the current best fit for the star and using the pixel values in the annulus defined by the DAOPARS parameters sannulus and wsannulus to recompute the sky. The actual sky recomputation is done by averaging forty percent of the sky pixels centered on the median of the distribution. Recomputing the sky can significantly reduce the scatter in the magnitudes in regions where the sky background is varying rapidly.
Only pixels within the good data range defined by the DATAPARS task parameters datamin and datamax are included in the fit. Most users set datamin and datamax so as to exclude pixels outside the linearity regime of the detector. By default all the data is fit. Users are advised to determine accurate values for these parameters for their detector and set the values in DATAPARS before beginning any DAOPHOT reductions.
Only pixels within the fitting radius parameter fitrad / scale are included in the fit for each star. Fitrad is located in the DAOPARS task and scale is located in the DATAPARS task. Since the non-linear least-squares fits normally compute three unknowns, the x and y position of the star's centroid and its brightness, the value of fitrad must be sufficiently large to include at least three pixels in the fit for each star. To accelerate the convergence of the non-linear least-squares fitting algorithm pixels within fitrad are assigned weights which are inversely proportional to the radial distance of the pixel from the x and y centroid of the star, falling from a maximum at the centroid to zero at the fitting radius. Fitrad must be sufficiently large to include at least three pixels with non-zero radial weights in the fit for each star. ALLSTAR arbitrarily imposes a minimum number of good pixels limit of four. Values of fitrad close to the full-width at half-maxima of the PSF are recommended.
ALLSTAR computes a weighted fit to the PSF. The weight of each pixel is computed by combining, the radial weighting function described above, with weights derived from the random errors ALLSTAR predicts based on the detector noise characteristics specified by the DATAPARS parameters readnoise and epadu , and the flat-fielding and profile interpolation errors specified by the DAOPARS task flaterr and proferr parameters. Both to obtain optimal fits, and because ALLSTAR employs a conservative formula for reducing the weights of deviant pixels (parametrized by the clipexp and cliprange parameters in the DAOPARS task) which do not approach the model as the fit proceeds, which depends on readnoise , epadu , flaterr , and proferr , users are strongly advised to determine those parameters accurately and to enter their values in DATAPARS and DAOPARS before beginning any DAOPHOT reductions.
By default for each group of stars to be fit during each iteration, ALLSTAR extracts a subraster from image which extends approximately fitrad / scale + 1 pixels wide past the limiting values of x and y coordinates of the stars in the group. Fitrad is the fitting radius specified in the DAOPARS task. Scale is the image scale specified by the DATAPARS task. Fitrad may be less than or equal to but can never exceed the value of the image header parameter "PSFRAD" in psfimage .
If the cache parameter is set to "yes" then ALLSTAR attempts to store all the vectors and arrays in memory. This can significantly reduce the system overhead but may cause excessive paging on machines with a small amount of memory. For large images it may be necessary to set cache to "no", and use the disk for scratch storage. Users should experiment to see what suits them best.
As well as the computed x and y centers, sky values, and magnitudes, ALLSTAR outputs the number of times the PSF fit had to be iterated before convergence was achieved. The minimum number of iterations is four. The maximum number of iteration permitted is specified by the maxiter parameter in the DAOPARS task. Obviously the results for stars which have reached the maximum iteration count should be viewed with suspicion. However since the convergence criteria are quite strict, (the computed magnitude must change by less than .0005 magnitudes or 0.10 sigma whichever is larger and the x and y centroids must change by less than 0.002 pixels from one iteration to the next), even these stars may be reasonably well measured.
ALLSTAR computes a goodness of fit statistic chi which is essentially the ratio of the observed pixel-to-pixel scatter in the fitting residuals to the expected scatter. Since the expected scatter is dependent on the DATAPARS task parameters readnoise and epadu , and the DAOPARS parameters flaterr and proferr , it is important for these values to be set correctly. A plot of chi versus magnitude should scatter around unity with little or no trend in chi with magnitude, except at the bright end where saturation effects may be present.
Finally ALLSTAR computes the statistic sharp which estimates the intrinsic angular size of the measured object outside the atmosphere. Sharp is roughly defined as the difference between the square of the width of the object and the square of the width of PSF. Sharp has values close to zero for single stars, large positive values for blended doubles and partially resolved galaxies and large negative values for cosmic rays and blemishes.
ALLSTAR implements a sophisticated star rejection algorithm. First of all any group of stars which is more than a certain size is not reduced. This maximum group size is specified by the maxgroup parameter in the DAOPARS task. Large groups may run into numerical precision problems during the fits, so users should increase this parameter with caution. ALLSTAR however, in contrast to NSTAR, attempts to subdivide large groups. If the group is too dense to reduce in size, ALLSTAR throws out the faintest star in the group and tries to rereduce it. If two stars in a group have centroids separated by a critical distance currently set arbitrarily to 0.37 * the FWHM of the stellar core and their photocentric position and combined magnitude is assigned to the brighter of the two and the fainter is eliminated. Any star which converges to magnitude 12.5 magnitudes greater than the magnitude of the PSF is considered to be non-existent and eliminated from the group.
After iteration 5, if the faintest star in the group has a brightness less than one sigma above zero it is eliminated. After iteration 10 if the faintest star in the group has a brightness less than 1.5 sigma above zero it is eliminated. After iteration 15, or whenever the solutions has converged whichever comes first, if the faintest star in the group has a brightness less than 2.0 sigma above zero it is eliminated. After iterations 5, 10 and 15 if two stars are separated by more than 0.37 * FWHM and less than 1.0 * FWHM and if the fainter of the two is more uncertain than 1.0, 1.5 or 2.0 sigma respectively the fainter one is eliminated.
ALLSTAR replaces the functionality of the GROUP, GRPSELECT, NSTAR and SUBSTAR task. However the user has little control over the grouping process and does not know at the end which stars were fit together. The grouping process is dynamic, as the groups are recomputed after each iteration, and stars can be fit and leave the group at any point after the fourth iteration. Therefore the quality of the fits may vary over the image as a function of crowding in an unknown way. However ALLSTAR is in most cases the routine of choice. NSTAR is the task of choice when a user wants to maintain control over the composition of the stellar groups.
If verbose = yes, a single line is output to the terminal for each star fit or rejected. Full output is written to allstarfile and rejfile . At the beginning of these two files a header listing the current values of the parameters is written. For each star fit/rejected the following quantities are written to the output file.
id xcenter ycenter mag merr msky niter sharpness chi pier perr
Id is the id number of the star. Xcenter and ycenter are the fitted coordinates in pixels. Mag and merr are the fitted magnitude and magnitude error respectively. Msky is the individual sky value for the star. Niter is the number of iterations it took to fit the star and sharpness and chi are the sharpness and goodness of fit statistic respectively. Pier and perror are the photometry error code and accompanying error message respectively.
If no errors occur during the fitting process then pier is 0. Non-zero values of pier flag the following error conditions.
0 # No error 1 # The star is in a group too large to fit 2 # The sky is undefined 3 # There are too few good pixels to fit the star 4 # The fit is singular 5 # The star is too faint 6 # The star has merged with a brighter star 7 # The star is off the image
1. Fit the PSF to a list stars in the test image dev$ypix. Good stars for making the PSF model can be found at (442,410), (348,189), and (379,67).
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> allstar dev$ypix default default default default default ... verify the prompts ... the results will appear in ypix.als.1 and ypix.arj.1 da> pdump ypix.als.1 sharpness,chi yes | graph ... plot chi versus sharpness, the stars should cluster around sharpness = 0.0 and chi = 1.0, note that that the frame does not have a lot of stars da> display ypix.sub.1 2 ... note that the psf stars subtract reasonably well but other objects which are not stars don't
2. Repeat example 1 but refit the sky using an annulus with an inner sky radius of 3.0 and an outer radius of 15.0.
da> allstar dev$ypix default default default default default fitsky+ \ sannulus=3.0 wsannulus=12.0 ... verify the prompts ... the results will appear in ypix.als.2 and ypix.arj.2 da> pdump ypix.als.2 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> display ypix.sub.2 2 ... note that the psf stars subtract reasonably well but other objects which are not stars don't
3. Run allstar on a section of the input 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> allstar dev$ypix[150:450,150:450] default default default default \ default wcsin=tv wcspsf=tv wcsout=tv ... answer the verify prompts ... fit the stars ... the results will appear in ypix.als.3 and ypix.arj.3 da> display dev$ypix[150:450,150:450] 1 ... display the image da> pdump ypix.als.3 xc,yc yes | tvmark 1 STDIN col=204 ... mark the stars on the original image da> display ypix.sub.3 2 ... display the subtracted image section
4. Run allstar exactly as in example 1 but submit the task to the background. Turn off verify and verbose.
da> allstar dev$ypix default default default default default verbose- \ verify- & ... the results will appear in ypix.als.4 and ypix.arj.4
4. Run ALLSTAR exactly as in example 3 but turn cacheing off.
da> allstar m92 m92.grp.1 m92.psf.1 default "" default verb+ veri- \ cache- > allstar.out &