--- In guide-user@yahoogroups.com, Bill J Gray <pluto@...> wrote:

>
> Hi John, Clay, all,
>
> John beat me to this one. I noticed UCAC-3 had been added to
> VizieR late last night, got it "almost" working, but had to catch
> some sleep. I was able to return to the problem this afternoon,
> and found John's files.

Hi again Bill

It's the old time zones that do it! That's what happens.

I was also doing another TDF today, or addending and amending one, for the first time in ages, so I was kind of already remembering how to do them and involved.

And I've just done a test due to that which is kind of interesting.

Basically there's a Parenago star that's got the same number between two stars, which also are logged as V1612 Ori and V1613 Ori coincidentally, but are otherwise indistinguishable coz of their proximity, it seems. It was the twinning of the Parenago number that was making me wonder, re possible problems that'll cause for that situation.

USNO A2.0 has one merged object here for instance. Probably most plate based surveys will.

However, CMC 14 split and showed the two perfectly.

But there was _no_ UCAC2 object, either merged, or separate, for these objects.

2MASS split them well too, now I think to check, giving two objects.

Well, I've just checked with UCAC3 and they are now there in UCAC as well, with very good agreement to 2MASS and CMC14 in terms of position (but about a "magnitude" fainter in "red"). So that's one small indication of the sort of differences that's going to happen with UCAC3, more crowded field coverage than with UCAC2, as stated by the team themselves.

There's no proper motion data listed for these two objects, but that's natural, all old surveys and catalogues will see this pair as one object, even if they do include it in catalogue lists, likely, and the stars likely don't have much proper motion anyway. So nothing in old data to get proper motion from. Meanwhile other modern surveys, like 2MASS, are pretty much same epoch within a few years, so nothing to measure motion wise in that short time except for the most extreme of proper motions.

But what is shown is that UCAC3 now plots pairs close together (about six arcseconds in this instance) as faint as UCAC "red" mag about 15 and a bit each star, where before UCAC2 plotted nothing at all, and this is a field that was included in UCAC2 coverage.

So the reprocessing difference is evident, even for the previously catalogued stuff, even in just a simple example as this one.

Cheers

John