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UCAC4 and APASS

NMR
Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 08/20/2012 - 17:48

The newly released UCAC4 quotes APASS magnitudes (B V g r and i) for large numbers of stars. But lots of the quoted magnitudes are brighter than magnitude 10 and I thought from the APASS documentation that this means they would be saturated. Can they still be used with confidence?

Martin Nicholson - Shropshire, UK

Sebastian Otero
Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 08/20/2012 - 18:26
V mags in UCAC4

Two things about this.

First, if the stars are in Tycho-2, bright stars in UCAC4 will have Bt and Vt magnitudes instead of B and V. That's not correct since different passbands are being mixed.

And second, yes, some bright stars still show APASS data when they are saturated and they shouldn't be used.

These two points together mean that the V mags in UCAC4 shouldn't be used for bright stars. Use GCPD, Hipparcos, ASAS-3 and Tycho-2 for stars brighter than 10 (in order of preference).

Also if you get a V-mag. by transforming UCAC f and a mags. using 2MASS colors, it seems that UCAC3 is far better than UCAC4. The zero point has changed for the worse. But a deeper analysis on this should be made. I think that the use of APASS for the calibration didn't take into account saturation problems and blending issues.

Cheers,
Sebastian

HQA
Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 08/25/2012 - 12:56
APASS in UCAC4

Hi Martin,

APASS does saturate around 10th magnitude, perhaps a bit brighter depending on the field.  However, the main catalog does include stars that might saturate in some passbands but not others.  These interim data releases will include outliers that will be either improved or rejected in the final APASS catalog.  Use Sebastian's guidelines for stars brighter than 10th.

As for the transformation of UCAC4 magnitudes into V magnitudes: my guess is that new transformation coefficients need to be determined since the field zeropoints are now different from the earlier UCAC3 release.  I'd reserve comment on whether UCAC3 or UCAC4 gives better results until this step has been performed.

Arne