[fitsbits] Plate coordinate tolerances

[K. L. Tah]

Hi,
I was wondering if someone could tell me the accuracy with which one
can
determine positions on the fits images?
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).
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?
Thanks,
KL
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