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Poor astrometric solution with UCAC-4
11-18-2016, 11:18
Post: #1
Poor astrometric solution with UCAC-4
Hello all,

on several fields I came across poor astrometric solution when using UCAC-4. There was always much better solution with UCAC-3 on the same images. The most obvious difference was the unexpected low number of reference stars queried from Vizier database.

Now I had a closer look at the internally generated query url and found a likely source of the problem: I often use ASTREFMAG_LIMITS parameter to choose the brighter reference stars only and this is applied to R magnitudes by default. But the catalog contains regions where the majority of stars have no R data in UCAC-4 and are skipped as soon as you choose a magnitude limit (setting this to 20 mag does not help).

I believe that the choice of R band as default for ASTREF_BAND is not a good choice in case of UCAC-4. Better choices woud be V or even better f (UCAC fit model magnitude in 579-642nm). Unfortunately it is not possible to reference either one or the other by a custom ASTREF_BAND setting.

Currently, the only workaround is to not use ASTREFMAG_LIMITS with UCAC-4 which is not always appropriate (e.g. wide fields, crowded regions).

I'd much appreciate if this could be dealt with in a future version of scamp.

Thomas Lehmann

(P.S.: I have seen the same odd behaviour in both scamp versions 2.0.4 and 2.2.6)
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