[fitsbits] Plate coordinate tolerances

[K. L. Tah]

Hi Mark,
You're right, the keywords CRDERia and CSYERia are not in my fits header.
Is there a way to retrieve this info from dss?  Also, it looks like EPOCH
and DATE-OBS give me pretty much the same numbers
(1992.425 as opposed to 1992.430). I've attached the fits file, which I
hope
is of more help.
Thanks,
KL

On Tue, 10 Aug 2004, Mark Calabretta wrote:

>
> On Mon 2004/08/09 13:14:20 MST, "K. L. Tah" wrote
> in a message to: fitsbits at nrao.edu
>
> >I was wondering if someone could tell me the accuracy with which one
> >can determine positions on the fits images?
>
> This depends on the particular image.  The FITS keywords CRDERia and
> CSYERia may be used to record the random and systematic error in the
> world coordinate for axis-i (though it is unlikely that you will find
> these in your header).
>
> >I'm trying to predict the path of  stars over a course of several years in
> >to the future and have written a program that plots this on the fits
> >image.
> >I had first retrieved the fits images centered on the 2000 coordinates of
> >star, and converted this to pixel coords using sky2xy. As a check, I
> >extrapolated  paths back the epoch (which I'm
> >assuming is when the pictures were taken and is given in keyword EPOCH).
>
> EPOCH, now deprecated in favour of EQUINOX, was used to record the
> equinox of (dynamical) equatorial coordinate systems, e.g.
> Bessel-Newcomb/FK4 or IAU1984/FK5 - look for an RADESYS card to
> determine which.  The date of observation would have been recorded in
> DATE-OBS, or as a modified Julian date in MJD-OBS.  Of course it's
> possible that whoever wrote this FITS file confused these times.
>
> >If there are no errors, I would expect this to
> >lie exactly on the center of the star in the image. However, I have
> >found
> >that these are off by several pixels in most cases (see, eg. GJ729
> >attached). Is this to be expected? If so, is there any info in the fits
> >header that might help me quantify this?
>
> If the header has DATE-OBS or MJD-OBS, try plotting the star's position
> at that time.  You might also try interpreting EPOCH, which I assume from
> your attachment was given as 1992.4235, as the equinox of the equatorial
> coordinate system, presumably IAU1984/FK5.
>
> Without seeing the FITS header itself there's not much more I can say.
>
> Mark Calabretta
> ATNF
>
>
>
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