[vsnet-unknown 17] Re: re unknown 13 : AC Sco; AF Sco

[no name <crawl@zoom.co.uk>]

I'd checked the ESO R MAMA/CAI reduced image of the AF Sco field
earlier...

...AF Sco is probably not in USNO A2.0 because it looks like a rather
bad drawing of an out of focus Saturn!

It is difficult to tell whether this is due to superposed stars or
nebulosity or plate fault, but the "density" of the image suggests
superposed stars.

AC Sco's image looks okay though, so it should be in USNO A2.0...

[...wait a sec, just thought of summat...]

AC Sco appears to be in the GSC [at the GCVS & IRAS positions], but not
UCAC1

AF Sco _is_ in UCAC1 [at the IRAS positions], but apparently not in GSC!

So, they "could" be in USNO A2.0 but aren't!


Taichi Kato wrote:

> [Since most of highly overexposed stars are recorded in 2MASS PSC,
> there may be a special reason why these stars or fields were avoided
> simply because of saturation].

Although I seem to have forgotten more than I have read on 2MASS
documentation, I get a feeling that it is likely that exclusion of such
stars from 2MASS may not be a photometric problem after all, but
possibly an astrometric one, for decent astrometry must be a bit tricky
on these objects' images... ...however, as you say, this doesn't always
seem to be the case on other saturated images.

I suppose it may be to do with 2MASS not being a final release as yet
[????????]


John

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