BACKSCAL keyword value

From: Dave Smith (dasmith@astro.umd.edu)
Date: Thu Aug 31 2000 - 14:29:00 EDT

Hi,

I have extracted spectra using dmextract and a source region file, and
wish to know how the BACKSCAL parameter is derived in the resulting
spectral file. My understanding is that the BACKSCAL parameter is the
area of the region divided by the total area of the detector. So, for a
circle of radius 8 pixels,

BACKSCAL = 8^2 * pi / ( 6 * 1024^2 ) = 3.19E-5 (ACIS-S configuration).

This is a factor 10.66 larger than the actual BACKSCAL keyword value. Is
there something I have misuderstood?

Dave.

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