saltcombine

Name

saltcombine – Combine SALT images

Usage

saltcombine images outimage (combine) (reject) (mask) (weight) (blank) (scale) (statsec) (lthresh) (hthresh) (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‘. The input files will be overwritten with the gain-corrected data.

outimage

String. This is the name of the output image to be created. This image is the combined image of all of the input images.

combine

String. Method for combining the data. The different options are average and median.

reject

String. Method for rejecting the data. The different options are None, ccdclip, and sigclip. ccdclip uses the variance in the data to reject pixels based on the expected noise. sigclip rejects pixels that are outside of the thresholds set by hthresh and lthresh.

mask

Bool. If set to ‘yes’, the BPM frames in the data will be used to reject pixels

weight

Bool. If set to ‘yes’, the inverse variances frames in the data will be used to produce a weighted average of the data.

blank

Real. Value to use if there are no useful pixels at a position

scale

String. Type of multiplicative scaling. If it is set to ‘average’, it will use the ‘average’ of the section specified in statsec to scale the images. Likewise for the ‘median’.

statsec

String. Section to use for calculating the value to scale by. Leave blank to use the whole image.

lthresh

Real. Lower threshold to reject pixels for ‘ccdclip’ or ‘sigclip’

hthresh

Real. Upper threshold to reject pixels for ‘ccdclip’ or ‘sigclip’

(verbose)

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

Description

SALTCOMBINE is a tool for combining images into a master image. The program has a number of options for how the images are combine including the combination and rejection methods.

The task assume that all input images are of the same size and type. Options for combining the sources are average or median. For average combination, a weighted average can be applied if inverse variance frames are included with the data. To include inverse variance frames, the extension of the frames should be included using the IVAREXT keyword in the corresponding data extension. Also, if the BPMEXT keyword is set, it will also use the bad pixel maps for rejecting pixels.

Options for rejection only include ccdclip and sigclip. If ‘ccdclip’ is selected, the gain and readnoise will be extracted from the image header and used to calculate the expected variance in the frames. The task will check to see if the data have already been gain corrected and use the appropriate value if they have been. If ‘sigclip’ is selected, the standard deviation of the pixels being averaged will be calculated for use in the rejection. Any source outside of the thresholds set by lthresh and hthresh will be rejected.

Prior to combining the images, the images can be multiplicatively scaled. The user can set the scale statistic to either calculate the mean or the median of a region statsec. The images will then be scaled by that value. If statsec is left blank, the scaling will be based on the statistics of the entire image.

Examples

1. To combine a set of SALT images into a master flat:

   --> saltcombine images='@images.lis' outimage='VFLAT.fits'
   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 can be processed in 0.31 sec.

Bugs and limitations

Not all features available in iraf.ccdred.combine are available for SALTCOMBINE.

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

See also

saltclean saltmosaic