[vsnet-chat 3598] re NSV 12041 [UCAC1 data]

[crawl@zoom.co.uk]

_Re the offset between USNO A2.0 and 2MASS positions [again]_

I have belatedly thought to look in UCAC1 for this star.  UCAC1 is complete
to a magnitude of around 16, and is a recent CCD based astrometry survey
conducted by USNO.  The magnitudes are for an unfiltered CCD, and are thus
to a "passband" laying between Johnson V and Cousins R (actually 579 to 642
nm): these are considered to be usually consistent to within 0.1to 0.3
magnitudes for any particular half degree square observed. UCAC1 is
primarily an _astrometric_ catalogue [UCAC URL
http://ad.usno.navy.mil/ad/ucac/ ]

Due to the brighter limiting magnitude of UCAC1 the star we all seem to be
in agreement on as being NSV 12041 [ie the one 0.04 Min West of BD -21
5395] is the _only_ one seen by UCAC1 near this position [note UCAC1 uses a
running number identification]:

1UCAC 24392763  19h 27m 53.57s  -21d 35' 52.6"  (ICRS)  +/- 0.038"
RAcosdec, +/-0.009" dec

This position is within 0.0s of RA and 0.1" of declination of the 2MASS
value, which is an improvement over the USNO A2.0 situation of nearly an
arcsec of declination difference.

[On a more general note, the fact that UCAC1 and 2MASS were generated using
similar sized and pixel densitied CCDs on 1 metre class telescopes, and
both pretty concurrently measured within the past few years, whereas USNO
A2.0 is from digitised Schmidt camera survey plates, may be relevant here.
You'd have to ask someone more knowledgeable in this area!]

NOW: I've just thought to double check the USNO A2.0 data, and found that
the blue plate is dated Sept 1976 whilst the red plate is dated June 1987.
As this object is variable, we now have an explanation for the discrepant
USNO A2.0 colour of this object: non-contemporaneous plates.

But this also enables me to use the UCAC1 proper motions of RAcos(DEC)
+35.1 mas/yr [+/-11.6] and DEC +50.5 mas/yr [+/-11.5].  I'm going to assume
the USNO A2.0 position is for the mean epoch of the two plates, giving
about 1982.1, which is a difference of 17.5 years from the mean UCAC1 epoch
of 1999.6.

This 17.5 years amounts to +0.04s of RA and +0.88" declination of proper
motion between USNO A2.0 and UCAC1 positions.  End figure differences
between the USNO A2.0 and UCAC1/2MASS positions are +0.05s of RA and +0.9"
of declination, which is good enough for me!!!!  [I note Brian Skiff
mentioned some possibility of a proper motion effect re positions].

Finally, I've written this out in long winded completeness so that anybody
can feel free to double check any of it!!!  For instance, if I've got my
sign conventions muddled things actually look worse, not better!

(the ucac_mag for 1UCAC 24392763 is 11.9 [in comparison BD -21 5395 has
ucac_mag = 9.2])

Hope it's of some use

Cheers

John

John Greaves
UK

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