[fitsbits] Plate coordinate tolerances

On Mon 2004/08/09 13:14:20 MST, "K. L. Tah" wrote
in a message to:

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

On Mon 2004/08/09 13:14:20 MST, "K. L. Tah" wrote
in a message to:

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

On Tue 2004/08/10 09:27:35 MST, "K. L. Tah" wrote
in a message to: Mark Calabretta
and copied to:

Is there a way to retrieve this info from dss? Also, it looks like EPOCH

Dear K.L.

You did not say that it was a DSS image! The presence of EQUINOX
indicates IAU1985/FK5 equatorial coordinates at equinox J2000.0. EPOCH
here means the epoch of observation, i.e. DATE-OBS, as stated in the
comment. I suggest you consult the literature or ask the DSS authors
concerning the accuracy of the DSS plate solutions.

Mark Calabretta
ATNF

On Tue 2004/08/10 09:27:35 MST, "K. L. Tah" wrote
in a message to: Mark Calabretta
and copied to:

Is there a way to retrieve this info from dss? Also, it looks like EPOCH

Dear K.L.

You did not say that it was a DSS image! The presence of EQUINOX
indicates IAU1985/FK5 equatorial coordinates at equinox J2000.0. EPOCH
here means the epoch of observation, i.e. DATE-OBS, as stated in the
comment. I suggest you consult the literature or ask the DSS authors
concerning the accuracy of the DSS plate solutions.

Mark Calabretta
ATNF