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Re: [TASS] FITS keywords, time

On         Thu, 2 Dec 1999 08:41:32 -0700, "Arne A. Henden" <aah@NOFS.NAVY.MIL> wrote:
*>Chris wrote:
*>>Arne says "start of exposure" is a convention, so that's what I'll
*>>use in the FITS image files.  I think the database should contain
*>>the heliocentric time of the mid-exposure so a conversion will be
*>>required at some point in the pipeline.
*>  Usually the heliocentric correction occurs after the astrometry
*>step has been performed.  This is fairly early on with Star, but
*>might be a couple of steps later with other reduction packages.
*>Note also that there are several time-based corrections if you
*>want the most accurate timing, along with proper setting of the
*>PC clock as mentioned by Chris.  Since these are all 'corrections',
*>I don't think they should be in the raw fits header, but perhaps
*>should be applied before adding observations to the database.

I reference the previous discussion of this in my TASS TEch note
on "time", Tech NOte #58. I will update that note with Chris's info on how
the Linux Mark III driver sets the time, and I've updated Arne's
previous notes on heliocentric concerns.

My Tech Note simply reviews our prior discussions of these issues, and
offers some summary comments. I suggest we read our history before we
repeat it. My update will be provided to Richmond immdiately.

Herb JOhnson

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