NOTE!!! PICCRS can be a complicated program. Make sure to understand and
check your results; we offer no guarantees. If your images are not
well-registered, PICCRS won't work!
PICCRS is a program originally written by Ed Groth at Princeton for the
statistical removal of cosmic rays from HST WFPC data. It is useful,
however, as an optimal combiner of frames with rejection of outlying
pixels. PICCRS works by going through each pixel in a stack of images.
For each pixel, the value in each image is compared with the mean
determined from all of the other images in the stack. Using the known noise
properties of the detector, the pixel in question will be rejected if it
falls outside some specified criterion. This continues through the entire
stack. Finally, the weighted mean of all pixels not flagged is taken and
placed in the output image.
The input images are specified in a file given with the LIST= file, with
format exactly like that of BIGMEDIAN. A mean and bias value can be
specified in this input file. There is an optional fourth parameter which
can be specified, unlike BIGMEDIAN, which is an output file name for each
input file; if this name is given, the output file will be identical to the
input file but all of the rejected pixels will be set to BLANK. The output
file will be in WFPC format with a separate header and image file.
By default the input files should be in FITS format, but WFPC input files
can be used if the WFPC keyword is specified.
IMPORTANT: To insure proper operation, the gain and readout noise must be
correctly specified with the GAIN= and RN= keywords. The default values of
GAIN=7.5 and RN=13 apply for WF/PC-1 data (NOT WFPC2). If desired,
saturated pixels can be rejected using the BSAT=n keyword which specifies
that all pixels larger than n will be considered bad on input. IMPORTANT:
by default, all negative pixels are considered bad on input; you can change
the lower boundary for bad pixels with the MIN=x keyword.
Since the original idea of PICCRS was to reject cosmic rays, the program is
designed to allow a less stringent rejection criterion for neighbors of
pixels which deviate by a lot, to allow for the fact that cosmic rays often
take up several pixels, and that pixels next to the brightest cosmic ray
pixels are more likely to be part of the cosmic ray hit than any random
pixel in the image. Consequently, the algorithm goes through the image and
rejects as ;SPMquot;primary;SPMquot; cosmic rays those pixels above the threshold specified
with the TP= keyword (default is 5 (sigma)), then neighbors of these pixels
are tested with a lower threshold specified with the TN= (for adjacent
neighbors) and TD= (for diagonal neighbors) keywords (defaults are 3
(sigma) for both of these).
The whole idea of PICCRS depends on the fact that the input images are all
well registered; if this is not the case, then most of the objects,
e.g. stars, will be rejected as deviant from the mean and your output image
will be horribly incorrect. To account for very small errors in
registration, the computed errors, against which the pixel deviations are
tested for rejection, includes a ;SPMquot;fudge;SPMquot; factor to increase the expected
variance for an increased signal level; this helps prevent bright pixels in
stars with slight misregistration from being rejected as cosmic rays. Note
that if such pixels are rejected, the output image will not be
photometrically correct. The variance computed by PICCRS is given by:
MEDIAN keyword.
BLANK= keyword
NEG keyword.
where the first two terms are strictly correct and the last term is the
fudge factor with variable parameter F. The default for F is 0.05, but this
can be modified with the F= keyword; to turn off the fudge factor, set F to
0, but make sure your (real) bright objects aren't getting clipped.