Re: Tass reference catalog

[Tom Droege <tdroege2@earthlink.net>]

My point is that the errors on the bigref_cat that we are now using are 
large.  Up to 0.2 mag.  There still may not be enough of them so that the 
zero point error is not affected by the particular set used.

Since we are measuring stars in different positions in the field on 
different days, then the reference stars used can make a difference in the 
reported magnitude.  All OK within the error of the Tycho reference stars, 
but large for day to day comparison purposes.

Now Arne is interested in what Arne is interested in.  Absolute photometry, 
I think.  Possibly I am interested in something else.  If it is just 
searching for new variables, then absolute photometry be dammed.  One wants 
to do relative photometry.  So I think I will make my catalog and try it.

Note that I am not looking at variations across the field.  I am just 
looking at the large errors in our reference catalog and wondering if they 
are causing the day to day jumps mostly in the I data.  I wonder how the 
errors in the Ic data compare to the errors in the V data?  I seem to 
recall that Michael had to compute the Ic mags from the color terms?  So 
possibly they are even larger than we think??

OK, I will try it.  You can't stop me.  ;^)  It will take a few days as I 
have to write code to sort things out and that is slow going for me.  Then 
Rob is going to show up to take ROB away on Sunday.  Further the computer I 
am using to test ROB just died, so it will have to be fixed to show Rob how 
things work.

Tom Droege

At 09:47 AM 8/9/02 -0700, you wrote:
>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
>
>__________________________________________________
>Do You Yahoo!?
>HotJobs - Search Thousands of New Jobs
>http://www.hotjobs.com