Testing the Catalogues
We have an active programme testing the veracity of the catalogues we
produce.
Several tests were presented in
Naylor et al (2002), and we are always on the lookout for more.
Internal astrometric consistency
The RMS between the fitted positions of our stars, and those given for
the same stars in whatever astrometric cataologue we use, are dominated
by the error in that catalogue.
It would be nice to know what the internal consistency is, since this
gives a good idea of the mis-centering error likely in fibre
follow-up.
We are in the middle of creating a new RIZ catalogue for NGC2547,
which allows us to compare the positions in that catalogue, with those
in the tables we presented in
Naylor et al (2002).
The answer is encouraging, with an RMS difference of 0.057 arcsecs in
RA and 0.056 arcsecs in declination.
This corresponds to about a tenth of a pixel, which is what one might
expect from the effects of intra-pixel sensitivity variations.
However, these differences are arrived at after removing a global
shift of 0.023 and 0.005 arcsecs in RA and declination respectively.
This could be the effect of 3 years of proper motion, but that's not clear.
There is also a discussion of the
key points of the optimal photometry
software we use for colour magnitude diagrams.
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