[vsnet-chat 4233] A star near NSV 6160

["Bruce Sumner" <b.sumner@bom.gov.au>]

In reply to Sebastian Otero's query (vsnet-chat-4199).

The question was 'is the Tycho-2 magnitude of GSC 7275 1964 correct?
Here is a table:
 
                    Tycho-2           UCAC1   USNO-A2.0
                 V           B-V        V       V   B-V
GSC 7275 1593  10.34+/-0.04  1.11     10.2    10.5  0.6
GSC 7275 1781  10.61+/-0.06  0.38     10.5    10.9  0.7
GSC 7275 1145  10.83+/-0.06  1.05     10.8    11.2  0.6
GSC 7275 1964  11.07+/-0.09  0.77     10.4    11.1  0.6

In all instances the V and B-V values are derived from measures in 
other passbands and are not actual V or B-V measures. The UCAC1 
magnitudes are calculated from my own conversion coefficients. Noting 
that these four stars have roughly similar colour, the relative UCAC1 
magnitudes should be pretty reliable, even if there is an unknown 
zero offset. The UCAC1 magnitudes imply GSC 7275 1964 is the second 
brightest of these four stars, the same as found by Sebastian, and 
not the faintest as implied by Tycho-2.

It again needs to be pointed out that Tycho-2 magnitudes tend to run 
off the rails when fainter than 10.5, and any Tycho-2 magnitudes 
fainter than 10.5 should be treated with suspicion and used for 
guidance only. 

USNO-A2.0 magnitudes at this brightness are inherently almost useless 
as they are generally derived from saturated images on long exposure 
Schmidt plates. They are only included in the above table for the 
sake of completeness

Looking at the evidence, the UCAC1 magnitudes appear to confirm 
Sebastian's suspicion that GSC 7275 1964 is really about V=10.5, 
rather than V=11.07 as given in Tycho-2.

Another case of 'beware of faint Tycho-2 magnitudes'.

Bruce Sumner

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