NAME

warmpix -- Flag and/or correct hot pixels in WFPC2 images

USAGE

warmpix input masks tables rej_thresh fix_thresh var_thresh

DESCRIPTION

This task flags and/or fixes hot pixels in an image. It requires an input table(s) containing the locations and dark counts for hot pixels that existed around the time of the observations. Each table typically spans an interval of time between decontaminations, and contains hot pixel data derived from dark files taken at several epochs within that period.

"warmpix" will do the following:

(1) If a hot pixel has high (or highly variable) dark counts, the pixel value in the science image remains unchanged, unless the parameter "rej_val" is set. The corresponding pixel in the image's data quality file (DQF) will have its 10th least significant bit set to indicate that it is a "bad" pixel (i.e. logically OR'ed with 512).

(2) If the hot pixel has a moderate dark count, its value can be fixed in the image. A modified dark value is calculated by extrapolating the hot pixel's value between two epochs that cover the time of observation for the image. In addition, the corresponding pixel in the image's DQF has its 11th least significant bit set (i.e. logically OR'ed with 1024) to indicate that the pixel value was modified, unless the parameter "fix_dqval" is set to a different value.

(3) If a pixel has a low dark count, its value and its data quality pixel are not modified.

PARAMETERS

input = "" [string]

Input image name(s). These must be WFPC2 images.

masks = "" [string]

Input data quality file name(s) for the WFPC2 images specified above.

tables = "" [string]

Hot pixel table name(s), it/they can be STSDAS table(s), FITS table(s), or text table(s). The table(s) must have the following five columns:

CHIP	detector number, from 1 to 4 (short).
X	X coordinate of the hot pixel (short).
Y	Y coordinate of the hot pixel (short).
EPOCH	time when the dark measurement(s) was taken, in MJD (double).
DARK	dark count measured at EPOCH, in DN/sec (real).  A value 
	of -100 indicates when a decontamination was performed.

The table must also be sorted in the following order: CHIP, Y, X, EPOCH. Tables downloaded from STScI's WFPC2 Documentation web page are sorted properly.

Since sorting large tables are very time consuming, we encourage using only one table to process file(s) covering the epoch range of the table, instead of using multiple tables to process many files covering a wide range of epochs.

rej_thresh = 0.[real, min = 0.]

The rejection threshold (DN/sec) for not fixing a hot pixel value. Pixels with dark counts higher than this parameter value at EPOCH either immediately before or immediately after the date of the observation will not be corrected. The science image will remain unchanged, and a value of 512 will be logically OR'ed to the corresponding pixel in the image's data quality file to indicate that this is a bad pixel.

fix_thresh = 0.[real, min = 0.]

The threshold (DN/sec) for determining if a hot pixel should be fixed. Hot pixels with dark counts lower than this level both before and after the observation are considered to be fine, and will not be changed in the image and its data quality file.

Pixels with dark counts that fall between the "fix_thresh" value and the "rej_thresh" value will be fixed. But the fix will only occur if the difference between the hot pixel values before and after the observation is less (in absolute value) than the "var_thresh" parameter value. A new dark value will be calculated using the hot pixel table, and the science image pixel will be modified accordingly.

If the dark and flat field reference files used to calibrate the science image (as specified in its header) are available, the calibrated pixel value will be recalculated, using the new dark value. If they are not available, the old dark will be assumed to be 0 and the old flat field will be assumed to be 1.

The data quality pixel corresponding to the "fixed" image's hot pixel will be logically OR'ed by the value specified by the parameter FIX_DQVAL (see below).

var_thresh = 0.[real, min = 0.]

The "variation" threshold in DN/sec. See the description for "fix_thresh".

(fix_dqval) = [short, min = 0]

Data quality value for fixed pixels. This option allows users to specify how the data quality file pixel flags are set for a "fixed" hot pixel. Some software packages do not recognize WFPC2-specific data quality flags. Therefore, this parameter allows users to set the flag to some value that is compatible with their software (for example, most good pixel flags are set to 0). Use fix_dqval=0 if no flagging of the fixed pixels is desired.

(rej_val) = INDEF [real]

Pixel value for high dark count hot pixels. If this parameter is set to INDEF, the original pixel values are preserved in the image. This option allows users to assign a particular value to flag uncorrected hot pixels in an image so that these pixels will be recognized as "bad" by software that does not use data quality files.

(runagain) = no [boolean]

If an image was processed by this task before, it will be skipped if this parameter is set to no. The user can force an image to be reprocessed by this task by setting this parameter to yes.

(verbose) = yes [boolean]

Print out verbose messages?

EXAMPLES

1. Fix hot pixels in images a.c0h and b.c0h (that are accompanied by their data quality files a.c1h and b.c1h) using the hot pixel tables hp1.tab and hp2.tab.

wf> warmpix "a.c0h,b.c0h" "a.c1h,b.c1h" "hp1.tab,hp2.tab" 0.1 0.003 0.003

REFERENCE

BUGS

SEE ALSO