APPHOT Package Update

The apphot tasks daofind, center, fitpsf, fitsky, qphot, polymark, radprof, and phot were modified to support the IMDKERN graphics kernel, used to draw graphics overlays on the image display. These tasks will mark the positions of detected or selected objects and draw the sky, photometry, and fitting apertures as appropriate if the user 1) runs the task in interactive mode, 2) sets the task parameter display to "imd", and 3) sets the appropriate graphics parameters, e.g., ``mkcenter'', ``mksky'', and ``mkapert'' to "yes".

The star finding task daofind and its algorithm parameter sets have undergone major changes.

• The parameter convolution specifying the name of the output density enhancement image has been renamed starmap, and a new parameter specifying the name of the output background density image skymap has been added. The sum of these two images models the input image except at the image edges and in regions containing bad data.
• daofind has been modified to read the datapars parameters ``datamin'' and ``datamax''. These parameters are used to reject bad data from the computed density enhancement and background density images.
• The object detection parameters ``nsigma'', ``ratio'', ``theta'', ``sharplo'', ``sharphi'', ``roundlo'', ``roundhi'', and ``mkdetections'' have been moved to the new findpars parameter set task.
• The datapars parameter ``threshold'' has been moved to the findpars parameter set and defined in units of the datapars parameter ``sigma'' rather than units of counts. Together ``threshold'' and ``sigma'' set the brightness of the faintest star that can be detected by the daofind task.

Two new keystroke commands `a' and `o' were added to the photometry tasks qphot, phot, polyphot, radprof, and wphot. The `a' keystroke command computes the sky value by averaging sky values computed at several cursor positions, and the `o' keystroke command uses the computed value to do the photometry. The new commands add an offset sky capability to the apphot photometry tasks which can be useful for measuring extended asymmetric objects like comets.

The sky fitting algorithms and associated parameter set fitskypars used by the fitsky, qphot, phot, polyphot, radprof, and wphot tasks have undergone major changes.

• Two new parameters ``sloclip'' and ``shiclip'' have been added to the fitskypars parameter set. ``sloclip'' and ``shiclip'' define the percentage of pixels to be clipped from the low and high sides of the sorted sky pixel distribution after bad data is rejected but before the sky value is computed by the photometry tasks. Their default values are 0 and 0, respectively.
• The fitskypars parameter ``skreject'' specifying the k-sigma rejection limit has been replaced with two new parameters, ``sloreject'' and ``shireject'', specifying the low and high side k-sigma rejection criteria, respectively.
• The default value of the fitskypars parameter ``salgorithm'' was changed from "mode" to "centroid" to minimize sky determination problems for data taken in regions with very confused background (e.g., nebulosity) and with data taken where the background noise is poorly sampled.
• The definition of the median of the sky pixel distribution was changed from the average of the central 10 pixels to the average of the central 5% of the pixels. Both definitions were intended to help protect against quantization effects in the data but the newer definition gives a more consistent definition and better results.
• The bad sky pixel rejection code has been modified to provide better protection against deviant pixels in the sky distribution. In the first bad sky pixel rejection cycle, the data is limited to values between [sky-min (sky-mindata, maxdata-sky, sloreject*skysigma)] and [sky + min (sky-mindata, maxdata-sky, shireject*skysigma)] instead of values between [sky-sloreject*skysigma] and [sky + shireject*skysigma] as before. This change provides better protection against deviant pixels in cases where the user forgets to set ``datamin'' or ``datamax'', and against defects like cosmic rays which are inside the good data limits but can sometimes bias the computation of skysigma. The value of sky in the above expressions is the median of the original sky pixel distribution except in the case of the "mean" algorithm where it is the mean or the original distribution.
• The size and resolution of the sky pixel histogram required by the "histplot", "centroid", "gauss", "crosscor", and "ofilt" sky fitting algorithms is no longer determined by the value of the datapars parameter ``sigma'' if it is defined. Instead the computed median and standard deviation of the sky pixels, the values of the ``khist'' and ``binsize'' parameters, and limits defined by [median-min (median-mindata, maxdata-median, khist*sigma)] and [median + max (median-mindata, maxdata-median, khist*sigma)] are always used to determine the total width and binwidth of the sky histogram.

Users who have not yet upgraded to IRAF 2.10.3BETA or who are running IRAF on platforms for which IRAF 2.10.3BETA or a later version is not yet available, can acquire the updated version of apphot by retrieving and installing the external package digiphotx from the IRAF network archive.

Lindsey Davis

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