NAME

sky -- Compute sky using the crrej task sky measurement algorithm, plus a correction for histogram truncation.

USAGE

sky input

PARAMETERS

input = "" [string]

Input image list/template.

(masks = "") [string]

Input Data Quality File list/template.

(lower = -99.) [real]

The lower limit of the input data to be used. Data lower than this limit will be excluded from subsequent calculations. This number is ALWAYS in DN even if the image is in countrate.

(upper = 4096.) [real]

The upper limit of the input data to be used. Data higher than this limit will be excluded from subsequent calculations. This number is ALWAYS in DN even if the image is in countrate.

(dq = "") [pset]

Data quality filter pset. This parameter set can be used to exclude pixels with certain data quality bit values.

(subsky = no) [boolean]

Subtract sky from input image(s) ?

(width = 8) [real, min=0.]

Interval width for sky statistics. This number should have the same unit as the image, i.e. if the image is in count, width will be in count, if the image is in countrate, width should be in countrate.

(stat = "mode") [string, allowed values mean | mode]

Sky correction statistics.

(bunit = " ") [string, allowed values cps | | counts]

Is the image in counts or countrate? If this value is blank, it will look for the keyword "BUNIT" in the header. If the header keyword BUNIT has the value of counts/s, c/s, cps, or count/s, the image is treated as in countrate, otherwise in counts. If the header keyword BUNIT does not exist, the image is assumed to be in counts unless this parameter bunit specifies otherwise.

(expname = "") [string, exposure time header keyword name]

If the image is in countrate, this task will look for the header keyword specified by this parameter to get the exposure time.

(skyname = "BACKGRND") [string]

If this is set to a group parameter name in the input image, the sky values calculated in this task will be used to update that group parameter. For WFPC2, this is usually set to "BACKGRND".

(skyvalue) [real]

Output sky value. This is the sky value of the last image/group in the input list.

(verbose = no) [boolean]

Print out results at STDOUT ?

(tempdir = "tmp$") [file]

Directory for temporary files.

DESCRIPTION

This task first finds a rough value for the sky using the same algorithm employed by the stsdas.hstcalib.wfpc.crrej task, but then refines this estimate using the gstatistics program. This task can deal with images either in counts or countrate.

Users wishing to learn more about these algrithms should see the appropriate help files.

This combination algorithm was developed because having zero sky can important in drizzling, and because the routine used in crrej, while fine for cosmic ray removal, may not be sufficient in some cases (typically when the sky is low and the WFPC2 gain undersamples the noise). It is, however, generally robust, and provides a good starting point for the gstatistics program. If after previous processing, the skies presently found in your images vary by less than the sky noise then you do not need to use this task before drizzling.

This task will either write the sky found in an image to a user defined header keyword, or will update the header value, if the task determines that the header value has already been updated by the crrej task.

The main parameter that users must set is the width. This determines the region of the pixel histogram that is used to determine the image statistics. It should be set to include most pixels in the sky (so substantially more than the FWHM of the sky distribution) but not so large as to include a substantial amount of power from objects or cosmic rays.

EXAMPLES

REFERENCE

This task was written by I. Busko following a recipe by A. Fruchter.

BUGS

SEE ALSO