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Re: change of airmass across can be important

Stupendous Man wrote:
> 
>   Jure brought up the issue of airmass:
> 
> > One thing that might be useful is the airmass.  This would normally
> > be in the header but the field of view is so big that it maybe
> > makes a difference.


I think all you really need is the means to compute airmass in the
star list headers.  I think it is best to just copy all the image
header information to the star list header plus add some HISTORY lines
that explain what was done during processing.  

The raw star lists are not an end product.  The will most likely
not be read by a human but by some program that calibrates and/or
combines them.  They will also likely be shipped around the Internet
so compactness matters too.

The image header will have the ra, dec of the image center point and
the lat, lon of the observer as well as the time/date of observation.
>From this one could computer the airmass of each star as well as refractive
effect Arne mentions.  IMO we should not toss out any header information
at any step.  So later when we have our database and light curves we can
answer questions like "How far above the horizon was this point on the curve?
When was the CCD temp. at this point? and so on."  These questions could
be important to someone who wants to query the data for say, only points
with airmass < 1.4  On the other hand computing the airmass for each star
is redundant with the header information and only makes the file bigger.
It is simple to compute if you have a library like starlib or something.
Just supplying the means to compute it later is IMO good enough.

My choice for star list files is FITS tables.  One reason is that once
we decide on a table format later if we find we need to include one or
two more columns we can do this ++without++ breaking older reader programs
that don't know about the added columns.


   Chris Albertson             home: chris@albertson-home.net
   Redondo Beach, California   work: calbertson@primeadvantage.com