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Re: Tass reference catalog
I see Tom's point. That a star near the field center effects the
zero point used on a star near the edge. This would be a
problem if _either_ Tyco or TASS images had a gradient. I'd
guess that Tom is not dis-trusting the tycho catalog.
The problem is the software assigns one zero point to the
entire frame. Not only is the flat fielding imperfect but the
PSF is not constant over the field. There have been suggestions to
divide the field into subfields. I don't like the idea of border
effects or that with smaller subfields less Tycho stars would be
in each field
Maybe the way out is to in effect divide the TASS frame into
an infinite number of subframes with each subframe's size
equal to the full frame size. Yeh, I know, geometrically
impossible. What you'd really do is compute a zero point
as a continuous function of (x,y) by using a weighted average
of the tycho stars. I'd weigh them by one over the square
of the distance from (x,y) This would take a fair amount
of computation but time on PCs is cheap.
--- aah@nofs.navy.mil wrote:
> So, Tom, you think that TOM1 does a better job of photometry
> across the CCD than does Tycho2? I think you will find it
> hard to convince me of this. Flatfielding is the biggest
> problem for wide-field systems, so you will almost always
> find systematic errors from center to edge. Trust Tycho2
> first. I'll talk about your other suggestions in more
> detail when I get back from the current trip, though Michael R.
> has already done a good job of description.
> Arne
>
=====
Chris Albertson
Home: 310-376-1029 chrisalbertson90278@yahoo.com
Cell: 310-990-7550
Office: 310-336-5189 Christopher.J.Albertson@aero.org
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