sampdiff -- compute first differences of samples in a NICMOS MultiAccum image.
sampdiff image1 outimage
Sampdiff is a CL script that takes the difference between consecutive samples in a NICMOS MultiAccum image. Alternatively, the sample differences between two different images may be computed. The STSDAS task "msarith" is used to compute the sample differences. Output file IMSET 1 will contain the difference between input file IMSETs 1 and 2, output IMSET 2 will contain the difference between input IMSETs 2 and 3, etc.
Each IMSET (or "readout", or "sample") of a NICMOS MultiAccum image is normally the cumulative sum of the total exposure time prior to that readout. As such, the SCI image extensions are not independent. When analyzing NICMOS images, it is sometimes helpful to look at the data collected during each readout interval, independent of any previous accumulations. To do this, the SCI image for each readout must have the SCI image of the previous readout subtracted from it. The sampdiff task automates this process, producing a new MultiAccum format file where the SCI images are "first differences".
The corresponding task "sampcum" can be used to invert this process.
- Input image 1. The full file name must be given. Normally this is a NICMOS _ima.fits file.
- outimage [file]
- Output image name. The full file name must be given. This file will have the same structure and number of IMSETs as the input file.
- (image= "") [file]
- Second input image. If left blank, first differences of IMSETs in image1 will be computed. If a second input image is given, first differences between image1 and image2 will be computed.
1. Compute first differences for the MultiAccum file "n4xj13jwq_ima.fits", storing the results in file "n4xj13jqd_fdiff.fits":
cl> sampdiff n4xj13jwq_ima.fits n4xj13jwq_fdiff.fits
Authors: M. Dickinson, H. Bushouse, STScI