Testing the CataloguesWe 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 consistencyThe 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.