Re: [guide-user] Updating mpcorb.dat

Bill Gray Feb 24, 2017

Hi Ted,

You're right; that option to 'download updated comet data' does
that, and only that. It fetches the 'Soft02Cmt.txt' file from the
MPC site, and a similar file from the IMCCE (the former Bureau des
Longitudes, in Paris) to cover less recent comets. It doesn't do
anything at all about MPCORB.DAT.

At present, you have to do what you did, which is to download
MPCORB.DAT separately. However, it occurs to me that this isn't
absolutely necessary... I should be able to add a suitable link,
close to the one you used to get new comet data, that says "Click
here to get an updated MPCORB.DAT file." Except that it should
warn you that MPCORB.DAT is a considerably larger file (about
44 MBytes, decompressing to about 150 MBytes), and the download
may take a little while.

To answer questions you didn't ask : as you probably found,
using a current MPCORB.DAT puts (834) Burnhamia very close to where
it ought to be. If you use Guide's features for displaying an
occultation path, though, it will be only roughly in the right
part of the world, for a couple of reasons.

Asteroid occultations are just about the most demanding events
in terms of accuracy for Guide to do. The orbital elements from
MPC are not entirely up to the job; the folks at IOTA compute
their own elements, being very selective about what observations
they use (only those of high accuracy, dropping most of the
survey observations... unless they're from long ago, extending
the observed arc and giving you a better handle on the mean motion
of the object). Then, for the star being occulted, they'll hunt
around for the best possible position, using UCAC4, URAT1, and/or
the new Gaia-DR1 catalogues. They may find that the star's
position in one or another catalog has discrepancies. Or they might
use (say) the Gaia-DR1 position, which is _very_ accurate but only
tells you where the object was almost two years ago, and combine it
with data from older catalogs to get a really accurate position for
the time of the occultation.

Anyway. You can see that it gets to be quite "fiddly", and not
particularly amenable to being automated. People _have_ generated
highly accurate elements, using Find_Orb or other software, and
combined them with a highly accurate star position, cobbled together
as I've just described, and used that combination in Guide to make
occultation charts that are about as good as one can get. (Even then,
actually seeing the occultation ranges from "not guaranteed" to "hope
you're lucky".)

And a final comment on this ramble: if you look in Tables...
Miscellaneous Tables, and don't see occultations for 2017 listed,
download this file and extract the contents to the Guide folder :

-- Bill

On 2017-02-24 09:43, Ted Blank tedblank@... [guide-user] wrote:
> I noticed that in my Guide 9, asteroid (834) Burnhamia was way out of the position it was predicted to be in for an upcoming occultation.
> I tried going to "Extras -> Asteroid Comet options -> Add MPC comets / asteroids -> Click to download updated comet data and add it to Guide" several times, but things did not get any better.
> I finally realized that my copy of mpcorb.dat was still dated 2013, so I manually downloaded the latest mpcorb.dat file from MPC, placed it into the asteroid folder, and all was well.
> So I'm wondering why that option in Guide did not successfully download the file for me (didn't get any error msgs).
> I also noticed that in Guide Help, clicking on the link at the top for Minor Planet Center (MPC) seems to be broken.
> What is the link to "click to download updated comet data and add it to Guide" actually supposed to do? If the answer is "download the latest mpcorb.dat" I guess that didn't work for me.
> Thanks,
> Ted