3.   Looking at the Data

IRTOOL generates FITS images, even though there is no FITS extension. The images have dimensions of 256x512 pixels. The bottom 256x256 pixels are from the first read (taken just after the IDLE mode stops) and the top 256x256 pixels are from the second read (taken after the exposure time has elapsed).  You will be able to see the quadrants very clearly because of the differences in DC levels for each quadrant in the controller electronics.

As noted in the Exposure window section, we recommend that you have first created a “raw data” directory where you are storing all the raw data.  You can then create a “working” directory where you can process the data.  Then if there is a problem while you are processing the data, the raw data will remain untouched.  Let’s say you have created a directory called “working” below the raw directory.  Then if you wish to see the full 256x512 image, do the following in the IRAF reduction window (assuming you are in the “raw” directory):

1. cl> copy imagename working/imagename
2. cl> cd working
3. cl> copy imagename imagename.fits
4. cl> display imagename.fits 1

Generally, there is little reason to display the pre-read portion, and so an IRAF script called nproc was written to do the following tasks that will make life easier.  Note:  if you have already created “imagename.fits” in the working directory using the steps above, nproc will abort when you try to run it because the fits image already exists.  So you should delete “imagename.fits”.  (See why you want a working directory?)  The procedure nproc will:

1. copy the image from the raw data directory into the current working directory and add the .fits extension.
2. subtract the bottom half of the image from the top half.
3. generate a new FITS image of that difference and optionally subtract a sky image (if you have accumulated one) – the new image has a "p" appended (for processed)
4. display the differenced (sky subtracted) image into image plane 1
5. optionally invoke the IRAF examination tool, IMEXAMINE after the display

At this point, you should see much less distinction between the quadrants. Any remaining distinction is due to gain differences in the controller electronics.