Subject: taking out bias pattern of old DSC images

From: Xiaohui Fan

Submitted: Thu, 2 May 96 12:02:11 EDT

Message number: 37 (previous: 36, next: 38 up: Index)

 It's known that DSC has some vertical bias patterns. Right now,
some overscan rows are added so that by taking the median of those
overscan rows these patterns may be taken out. 

 I have some older DSC images which suffered from these patterns, and
there was no overscan row like now. I (suggested by Robert Lupton) tried
to take out the median of every column of a flattened image. After taking out the
medians, the bias patterns are basically gone. And since that bias pattern
affected my sky determination of photometry quite much, esp. when
doing aperture photometry for faint stars, after taking them out, the photometry is
improved significantly. On the web site below, I put two figures, 
both the r' magnitude difference between same stars taken in different
nights, one before substraction of bias pattern, the other after. The
scatter of magnitude differences represents the photometric precision
in both cases. You can see the improvement of the aperture photometry here.

 http://astro.princeton.edu/~fan/paper/qso96.html

-- 
Xiaohui Fan
Princeton University Observatory
Peyton Hall
Princeton, NJ 08544

email : fan@astro.princeton.edu
tel   : (609) 258-5153 (O)
        (609) 683-1315 (H)
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