Re: Astrometry errors

[Tom Droege <tdroege@veriomail.com>]

Andrew comments on position errors, fits and the like,

On 001018 MICHAEL was modified for center push focusing.  This reduces the 
tilt in the plane of the CCD as it is moved in focus.  This has improved 
the focus over the field.  Disk 16 is from 000928 and disk 15 is still 
earlier.  Sorry, I know that it is a pain that I keep improving things, but 
that is the problem with experiments.  You have to wait until all the 
technicians go home over the Christmas holliday to get good, consistent 
data.  They are otherwise mucking about and changing everything.

Hopefully, those higher order coefficients will keep decreasing.

Tom Droege

At 08:16 PM 11/5/00 +0000, you wrote:
>On Sat, 04 Nov 2000 15:38:24 +0100, Jure Skvarc
><jure.skvarc@ijs.si> wrote:
>
> >Andrew reported that he gets 0.1" probable error (? is this average error or
> >median?) in RA and DEC with TYCHO 2 catalog and recent Mark IV 
> images.  This is
> >almost too good.
>Surprised me too, after the struggle to get down to 1 arcsec!
>
> >  It would be nice to know more details, such as the maximum
> >magnitude of the stars taken in the comparison and what is their signal, 
> number
> >of stars used for fit etc...
>
>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
>30 brightest (but clearly not saturated) matched Tycho2 sources.
>All stars are refitted using the final PSF; this is an optimum
>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.
>
>For the longer exposures, almost all the Tycho2 sources are matched;
>300 or so per image going to beyond M=12. My analysis of the shorter
>exposures does not go as deep so there are fewer matches.
>
>The figure I quoted is the probable error or semi-interquartile range.
>There are lots of outliers (mostly the result of other close sources,
>I think) so the standard deviations are much worse. The worst
>outliers are pruned as part of the astrometric fit: since this fit
>is least-squares, this pruning is essential. The results are thus
>a little bit optimistic.
>
>Some typical results
>H3R1659.830 (short V-band exposure) 246 matched sources
>p.e. RA 0.204 Dec 0.205 arcsec; Mag 0.0688
>H3R1659.832 (long V-band exposure) 344 matched sources
>p.e. RA 0.138 Dec 0.169 arcsec; Mag 0.1126
>H4R1659.830 (short I-band exposure) 185 matched sources
>p.e. RA 0.283 Dec 0.225 arcsec; Mag 0.1453
>
>And the last two I measured before my previous post:
>H4R1659.832 (long I-band exposure) 272 matched sources
>p.e. RA 0.098 Dec 0.098 arcsec; Mag 0.2377
>H4R1659.836 (long I-band exposure) 367 matched sources
>p.e. RA 0.104 Dec 0.106 arcsec; Mag 0.2009
>
>Why the long exposure I-band images are consistently
>better than the V-band when the PSF is clearly fuzzier
>for the I-band, I do not know
>The larger magnitude errors for the long exposures is
>simply the result of going deeper in magnitude.
>The larger magnitude errors for the I-band compared
>to V-band arises mostly from converting the B and V
>catalog magnitudes.
> >
> >                                       Jure Skvarc
>
> >
> >Stupendous Man wrote:
> >>
> >>   Now, on the other hand, if you are talking about the Mark IV images --
> >> which are stare-mode frames, not scans -- then the main source of
> >> astrometry errors is distortion in the PSF and in plate scale
> >> near the corners of the chip.  I'm not sure which is more important;
> >> perhaps Andrew Bennett or Jure Skvarc could give us an informed opinion.
> >>
> >>                                               Michael Richmond
>I, at least, have no idea! All the errors disappear into
>the cubic fit ... the last time I tried higher order, there
>was a hint of 5th order in there too. The PSF variation is
>certainly highly significant. If forced to pronounce on it, I
>would say it was the dominant error. With brute force fitting
>to good catalog positions, the distinction doesn't matter.
>
>Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.