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From: "B Monard" <bmonard@mweb.co.za> Date: Sat, 2 Aug 2003 14:36:31 +0200 Subject: [vsnet-chat 6744] Demolishing UCAC1 / a refute... Hi, I read with interest about the tests of J Greaves, his conclusion and personal recommendations, followed by comments by others. I wonder in how far UCAC2 is different from UCAC1...... To derive V magnitudes from UCAC is indeed more tricky since O-V is much larger (about 3 times) than O-R (O magnitudes from UCAC), so you end up with a 3x larger conversion error. The accuracy of the conversions is proportional to the accuracy of the knowledge of the colour term of the stars in question. What do you use for that: USNO-A2.0 or perhaps USO-B1.0, what else? How accurate is that? and how precise between different fields? Despite what is in the readme.txt, UCAC is based on 'filtered' measurements of the real sky, not of archival photographic plates and therefore much more true than the data in USNO-A, B-1,2.. How can someone say or hint that USNO-A, B could be possibly better. Referring to the large numbers of test stars in John's tests, there could by no means be a distinction based on colour terms. It's all in one pot. That's not what you do in a comparison sequence. Here you select stars of the proper colour before you work on them. At least that's how IMO sequences need to be made. I have quite some experience with UCAC1 and made several R comparison sequences which have stood good comparison to later dedicated field photometry. Therefore my Q above since John tested using UCAC2 material. I have no experience with ASAS nor TASS magnitudes but I can hardly see soundness in using the latter above magnitude 12.5. I still believe that Tycho and Hipparchos give good B and V magnitudes for up to 11.5V. Perhaps ASAS can fill a gap up to magnitude 13.. Properly converted UCAC1 magnitudes can IMO be used up to magnitude 15 (from 11 upwards), assumed a proper colour selection is done. The expected uncertainty for 15 magnitude stars, derived that way is better than 0.2 magnitude in R, about 0.5 in V. Have no illusions, the other database will not give you anything better in accuracy, although you might think so. Precision and accuracy are not the same. For deriving R magnitudes above 12R, as for use in unfiltered photometry, UCAC1 is far better a dataset than the others. Back to the test range for John, possibly using UCAC1 values this time and calibrated fields, and red ears for Mike and Sebastian..... ;-)] Berto Monard
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