[vsnet-chat 4545] re vsnet-id 459 : positional accuracy

[crawl@zoom.co.uk]

>Re: [vsnet-id 458] Re: MisV1089 ID
>
>> However, I also found that there are many (possible) correct
>> identifications with very faint 2MASS objects. Here is the list.
>> 
>> How do you think of them? The first one, MisV0002, is an ecliptic
>> variable star. So it is natual that the 2MASS object is faint. But how
>> about the rest?
>> 
>> Most of the MisV stars have been regarded as red variable stars. 
>> However, the following stars can be different type, cannot they?
>> Are there some stars worth being motitored?
>
>   I have examined the case of MisV0035.  There is a much brighter 2MASS
>star, but is slightly offset from the original MISAO position.
>
>   The 2MASS source is easily identified with a MSX5C source.
>
>   From the color of the faint candidate listed, J=13.1 and J-K=0.4 look
>like a normal faint star, which would not easily explain the large
>variability of a bright unfiltered CCD source.  There being a good bright
>2MASS/MSX5C candidate, it may have been that the MISAO positions can
>sometimes have larger errors than were supposed.  Crowding may have played
>an important role; the "nearest" 2MASS counterpart can be sometimes wrong,
>and astrometry from optical CCD images can be more contaminated by dense
>field stars than in IR images on which red variable are usually outstanding.
>
>2MASS 175644.322 -304940.54 (2000.0)  9.839  8.720  7.983
>= 175644.1 -304941 (2000.0) MSX5C_G359.6035-03.0375
>
>175644.2 -304946 (2000.0) MisV0035 14.3 17.0C M? - - 
>
The MISAO project uses USNO A2.0 for astrometric work.

Is there anything known about the behaviour of that survey when it comes to
crowded Galactic plane fields?

For instance, I have seen it mentioned several times (mostly on the mpml)
that UCAC1 is better for astrometry in the south than USNO A2.0.  Not just
because the epoch of UCAC1 is bound to be nearer to contemporaneous images
thus reducing problems due to proper motion, but also because of improved
coverage in crowded fields.

UCAC1 certainly displays more stars than GSC 1.x/ACT for a lot of these
fields, but also seems to match or better USNO A2.0 in crowded fields when
comparing at an equivalent magnitude basis [USNO A2.0 wins out in the end
because it goes fainter].

I can't see any reason why Galactic plane stars should be more prone to
proper motion effects than non-plane field stars, but I can imagine it
being possible to reduce better quality positions from modern CCD exposures
of a crowded field when compared to reductions from digitisation of
photographic plates where even the mid-range brightness stars are probably
overexposed.

I've seen people complain to Bill Gray at Project Pluto that "Guide doesn't
do this" or "Guide got that wrong" such that they have to have it explained
to them that it is the best Guide can do with the data available, and there
is nothing wrong with the system.

So, when it comes to some MISAO positions being a bit more off than usual,
especially in crowded fields, how much of this is the methodology and how
much is the reference source the methodology is using?

Cheers

John


PS I've been looking at bits and bats of GSC2.2 data... ...from this I can
predict some up and coming caveats that will be posted to vsnet-chat in
future on a semiregular basis.  Mostly that the Vmags that it provides are
_not_ V and not necessarily accurate just because they are quoted to two
decimals (as best exemplified by the errors on Vmag often being given as
"99.0", which means _unknown_).  Brian Skiff pointed out some problems with
the GSPC 2.1, chief photometric calibrator for GSC 2.2, the other day, for
instance.  Some may feel the frequent misuse of Tycho VJ and/or VT mags and
the like is a bit scary, well, be afraid etc, as the movie trailers say ;)
, coz I can guarantee some people will just quote them as 'V' instead of
calling them GSC 2.2 Vmags, which would give their true provenance.

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