Re: Astrometry errors

[Jure Skvarc <jure.skvarc@ijs.si>]

I did not study influence of the PSF distortion, but the field distortion (or
maybe a combination of both) is definitely measureable and goes up to 3.5
arcseconds in the image corners. It is drawn in the addendum to TN67
(http://www.tass-survey.org/tass/technotes/tn0067.files/addendum/addendum.html). 
Of course, this can be corrected by using higher order plate constants (or is it
chip constants?).

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.  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...

 
					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