Re: Astrometry errors

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Sun, 05 Nov 2000 20:16:16 +0000, Andrew Bennett <andrew.bennett@ns.sympatico.ca> wrote:
*>
*>The data source is CD15 (short and long exposures).
*>
*>I match Tycho2 stars not flagged as having other stars close
*>by or other pathology.
*>
*>The PSF is estimated by fitting 30 coefficients to the=20
*>30 brightest (but clearly not saturated) matched Tycho2 sources.
*>All stars are refitted using the final PSF; this is an optimum=20
*>least squares fit to X1, X2 and A. As previously reported ad
*>nauseam, the optimum least squares fit is better than aperture
*>methods, especially for weak sources against a high sky
*>background. People with nice dark skies may disregard this comment.
*>

I presume your complete report will have information about the size
of the PSF vs. magnitude or some other metric. For those who still
insist on aperture methods, such data would be a cross-check for the
fixed aperture size they select and the potential penalties for such
a decision. I recall from previous discussions with you, that a
fixed aperture "loses" over your fitted PSF for brighter objects
and for dimmer objects: data from you would give numbers to those
considerations. It's my belief that different methods serve as
cross-checks and diagnostics, so I myself appreciate the use
of different approaches to TASS photometry.

Herb Johnson

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