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Dear IRAF expert,I would like to process CCD images taken with a plate scanning
machine. These are negatives (i.e. black stars on white background)
with a limited dynamic range. I do have FITS files and they display
fine with IRAF. With imexam I can display contour maps but the
"r" radial profile fit crashes.1) Is there a way to "invert" the image (have bright stars on
dark background)?2) How can I change the pre-set "z-scale" to "linear"?Norbert
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Norbert Zacharias, Ph.D.
Chief of Cataloging Division
Astrometry Department
US Naval Observatory voice: 202 762 1423
3450 Mass. Ave. NW fax: 202 762 1516
Washington, DC 20392 e-mail: nz@usno.navy.mil
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The IMEXPR task will allow you to write an expression than can effectively invert the values as you describe. Note you'll need to figure out what are reasonable values for the high/low pixels. Note there is also the DTOI package under IMRED which was (long, long ago) used for dealing with plate-scanned images.The z-scale display algorithm is just a way of picking optimal values to scale the display from 16/32-bit range to the 8-bit display range. Likewise, IMEXPR could also be used to linearly scale your pixels values to the range
0-200 and you could display then with the params "zscale- zrange- ztrans=none" to display raw pixels.-Mike