Re: My problem or yours?/test

[Chris Albertson <chrisalbertson90278@yahoo.com>]

John McKendry wrote:
> 

>  And one last FITS-header question: what's the right thing to do with the dark-masked
> regions of the CCD? Surely they contain information that could be processed somehow,
> but I don't see any FITS keywords that seem appropriate.
> 
>  I thought I had more questions than that, but no. I guess the thing that has me most
> stuck is the 2037 rows.
> 
> John McKendry

FITS files are supposed to be self descriptive.  But it has to end
someplace.  Yes, we could describe every one of the over scan pixels,
inventing our own conventions if needed but I don't think we need
to do that.  All we really need is to have one keyword that tells
us where the live, uncovered pixels are.  We need this so that
software can compute image statistics without having the result
polluted by edge artifacts.

What happens if you process an un-trimmed image is that a median
or mean pixel value is computed.  If at no other time then at least
an image display program will attempt to compress the dynamic
range of the data to fit a display screen and come up with a
non-optimal result because it is trying to fit in some non-image
pixels.  In fact the effect can be quite bad if the algorithm use
the range and the edge pixels contain the extreme data values.
Some star detection algorithms may look for points above
the median.

We could also simply forget about header keywords and publish
a FAQ where question #1 is "Where are the image pixels?".

--
   Chris Albertson             
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   home:   310-376-1029   chrisalbertson90278@yahoo.com
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