Re: [guide-user] Searching URAT1 Catalogue (and Gaia-DR1)

Bill Gray Oct 13 8:22 AM

Hi Dave, all,

On 2016-10-13 04:50, 'DaveGee' dave4gee@... [guide-user] wrote:
>> It would take me an unreasonably long time to download 200 GBytes.
> I don’t know what your goals are but perhaps uses don’t need the
> whole 200Gb shebang. e.g. …

You're right. The occultation folks, for example, are going to be
hard-pressed to get good occultation data for fainter stars ("did it
blink out or didn't it?") For them, Dave Herald's subset may do the

I've swapped a few e-mails with Dave Herald concerning Gaia14 and
Gaia in general. He has mentioned that Gaia14 is on a personal server,
and that it wouldn't be particularly fair to the owner to have lots of
downloads from it. (Which is why Dave hasn't been making the download
address public. I _think_ access to Gaia14 is through the Occult
software only.)

> As I understand it, the Gaia1 lacks proper motion data, so to get
> around this Dave Herald (the author of Occult) has produced a hybrid
> catalogue he calls Gaia14 (i.e. limited to mag.14) in which he uses
> the high quality position data from Gaia and uses pm data from UCAC4.
> We are currently using this with good results for asteroid
> occultation searches.

I think about the only reason you'd want access to "full" Gaia-DR1
would be if you were doing astrometry. For an occultation, you'll have
a specific star or stars in mind; you can get the data from VizieR,
and probably ignore proper motion (the mean epoch for Gaia is 2015.0,
so there hasn't been much motion since then; if you do think motion
may be important, you'd be best off taking the mean positions from
URAT1, UCAC4, and deriving the proper motion from those... I'd bet
that the occultation community will be doing some of that.)

If you're doing a search for occultations, i.e., "what stars
brighter than mag 14 will be occulted by asteroids this month, as
seen from where I am", then Gaia14 will be of interest to you. But
you'll probably be doing that from within Dave Herald's Occult software
anyway. (Which is quite impressive, by the way. It's for Windows,
but can be run on Linux, and presumably OS/X, using Wine, though
some of the dialogs look a little weird; I think there's a Unicode
issue somewhere in there.)

Anyway. If you're doing astrometry, you'll want all of Gaia-DR1,
because going to mag 14 won't suffice; you actually have a need for
the entire billion-star catalog. Those folks would probably have the
most interest in this USB-drive exchange scheme.

Photometrists sometimes work with bright enough objects that a mag
14 cutoff might not bother them; depending on what data are in Gaia14,
they might be quite happy with it.

-- Bill