saltxtalk

Name

saltxtalk – Correct images for amplifier crosstalk

Usage

saltxtalk images outimages outpref (xtalkfile) (usedb) (clobber) (logfile) (verbose)

Parameters

images

String. List of input images including, if necessary, absolute or relative paths to the data. Data can be provided as a comma-separated list, or a string with a wildcard (e.g. ‘images=S20061210*.fits’), or a foreign file containing an ascii list of image filenames. For ascii list option, the filename containing the list must be provided preceded by a ‘@’ character, e.g. 'images=@listoffiles.lis‘. Input files must have been pre-processed by the task saltprepare.

outimages

String. A list of images. Data can be provided as a comma-separated list, or a string with a wildcard (e.g. ‘outimages=rS20061210*.fits’), or a foreign file containing an ascii list of image filenames. For ascii list option, the filename containing the list must be provided preceded by a ‘@’ character, e.g. 'outimages=@listoffiles.lis‘. This list must be of the same size as the images argument list. If the output is intended for a different directory the absolute or relative path must be supplied with the file name.

outpref

String. If the outpref string is non-zero in length and contains characters other than a blank space, it will override any value of the outimages argument. Output file names will use the name list provided in the images argument, but adding a prefix to each output file defined by outpref. An absolute or relative directory path can be included in the prefix, e.g. ‘outpref=/Volumes/data/x’.

xtalkfile

String. This is the name of an ascii table that contains the CCD amplifier crosstalk coeffcients. The table is used to subtract cross-talk contamination from the CCD amplifier images. Crosstalk is assumed to occur at a constant level across an individual amplifier, but the coefficients are assumed to vary across amplifer pairs. An example of the table format follows:

# PFIS CCD amplifier crosstalk data
# from 20041201 gain = bright distortion image, outer amps duplicated
# Date    VCTM     2       1       4       3        6        5
#         SRC      1       2       3       4        5        6
2004-01-01     .001474 .001474 .001166  .001111  .001377  .001377

The crosstalk-corrected amplifier image is given by SRC - VCTM * coeff.

(usedb)

Boolean. If yes, then the task will use the coefficient values in the xtalkfile. If no, then it will use the values in the headers.

(clobber)

Hidden boolean. If set to ‘yes’ files contained within the outpath directory will be overwritten by newly created files of the same name.

(logfile)

String. Name of an ascii file for storing log and error messages written by the task. The file may be new, or messages can also be appended to a pre-existing file.

(verbose)

Boolean. If verbose=n, log messages will be suppressed.

Description

saltxtalk corrects data listed by the images argument for amplifier crosstalk. Each SALTICAM and RSS CCD has two readout ammplifiers. There is crosstalk between them at the level of ~ 0.1% which results in faint ghost sources across the image. Ghosts appear as faint mirror images across amplifier boundaries of bright sources. Provided images are not saturated or non-linear, crosstalk can be mostly removed by simple subtraction of a scaled image of one amplifier from it’s neighbour. The scaling factors are supplied as an ascii table through the xtalkfile argument or in the header keywords.

The task creates new files with names specified by either the outimages or outpref argument. Existing files of the same name can overwritten if clobber=’yes’.

Coefficients stored in the xtalkfile must be recalibrated on a TBD schedule and made available to SALT users.

Examples

1. To correct for crosstalk contamination in image files:

   --> saltxtalk images='@images.lis' outimages='' outpref='x'
   xtalkfile='/iraf/extern/salt/pfis/data/PFISxtalk.dat'
   clobber='no' logfile='salt.log' verbose='yes'

Time and disk requirements

Individual unbinned full frame RSS image files can be 112MB in size. It is recommended to use workstations with a minimum of 512MB RAM. On a linux machine with 2.8 Ghz processor and 2 Gb of RAM, one 2051x2051 image in 0.85 sec.

Bugs and limitations

Currently no error propagation is performed through the calculations. This can occur once the saltprepare tool writes bad pixel and variance maps to raw data.

Send feedback and bug reports to salthelp@saao.ac.za

See also

saltclean saltslot