Hi Alan,
(Looks as if this reply perhaps should be cross-posted to the
STEREO comet hunter group.)
OK, I copied/pasted the astrometry, ran it through Find_Orb,
and got a nominal orbit roughly similar to Rob Matson's. It
puts the object on the far side of the sun, at an elongation of
about 110 degrees from the sun as seen from earth. I find it
hard to believe we wouldn't have seen it. However, if I set
a constraint so it has a semimajor axis of 2.5 AU, I get a
much more believable main-belt orbit :
https://projectpluto.com/temp/mpec.htm
(The uncertainties would only be accurate if we had a solid
reason for the semimajor axis being exactly 2.5 AU. So they're
definitely underestimates, though we've no idea by how much.)
I have 'astcheck' software for figuring out which asteroids
might match a set of observations, but it doesn't work for
satellite-based observations. I'm reasonably sure this is a
known main-belt object, but can't tell you which one.
Some other notes/responses to comments on the STEREO comet
hunters group :
The "second lines" giving positions for satellite observations
have long been a pain in the anatomy. I've thought about trying
to write up something that could load up all the "first lines",
send off a query to JPL's Horizons system (which can accept a
list of discrete times), and parse the returned data to create
all the "second lines". But thought is about as far as I've gotten.
As mentioned in my previous message, we don't do nearly as much
with STEREO and SOHO astrometry as we really ought to, and there's
a lot of interesting work to be done in this area. MPC hasn't
done much with it in years; I don't even know if anyone currently
at MPC is familiar with sungrazer astrometry. I computed a lot
of orbits years back, but haven't had time to do much since then.
So I'll warn you that the following is sort of a "best case" scenario,
the way things might go if I (or somebody) had plenty of time :
What I'd like to do is to start by computing orbits for
objects for which we have astrometry from any two of SOHO,
STEREO-A, and STEREO-B: that is, sungrazers for which we
have parallax. For these, we can determine "real" orbits with
the eccentricity (usually) well-determined from observations,
instead of having to assume it's a parabolic orbit. This might
also require going back into the archives, noting that a bright
comet was found in (say) SOHO, and looking for STEREO observations.
A sungrazer seen from two places is a heck of a lot more useful
than two sungrazers, each seen from only one place.
Given these, we could get a better handle on the characteristics
of the various comet groups (Kreutz -- OK, that's well-determined,
but there are sub-groups -- Marsden, Kracht, Meyer). We've a
vague idea as to the overall trends in the orbit for each group.
But having a few objects in each with really solid orbital elements,
with uncertainties, generated without a priori restrictions
(except perhaps the e=1 one), would tell us what the populations
of each group are like.
And _then_ we could use that information to set suitable
constraints for the swarms of objects currently backlogged for
SOHO and STEREO, most of which have orbits that are far too
short to just get a "standard" six-parameter fit. For these,
we'd need to set constraints that match the characteristics of
their host groups.
Before doing all this, I'd ideally want astrometry that included
magnitudes and positional errors. The orbits currently computed
all ignored photometry, and it's a shame; you've no idea even
which objects were brighter than others, much less the data
you'd need to say in what way the comets brighten as they
approach the sun. The lack of astrometric uncertainty means you
can't properly give more weight to more precise observations and
less weight to the "iffier" data. (Sungrazer astrometry has the
added issue that you sometimes have your target in one corner,
and you're basing its position on a few stars in the other corner,
leading to a highly elliptical astrometric uncertainty. That's
not something you encounter in "usual" CCD astrometry, where
there are plenty of reference stars all over the image.)
-- Bill.
On 2018-03-01 20:27, Alan Watson c2009a1@... [find_orb] wrote:
>
>
> Here is the links with background info
>
>
> Other Movers in the fov
> https://drive.google.com/file/d/1AZ80z3rz2q_fTQf9XHXAXAblxZROzExX/view?usp=drivesdk
>
> Track across one month
>
> https://drive.google.com/file/d/1IyUYhOKZiKQcZC6CBhSfodAFohHiY7nB/view?usp=drivesdk
>
> Animated Gif across 4 days, look carefully.
> https://drive.google.com/file/d/1IoQ2N5-s_zgTz7MAkHqbrl6c-tSDlx5s/view?usp=drivesdk
>
> Pdf with stereo hunter background, has position data and some orbit solutions.
>
> https://drive.google.com/file/d/12rO89SRwh323MTYN7pJmrUU-vx0QTIIl/view?usp=drivesdk
>
>
> More questions to follow....
> Regards, Al
>
> On Thu, 1 Mar. 2018, 12:03 pm Alan Watson, <c2009a1@... <mailto:c2009a1@...>> wrote:
>
> Thanks Bill,
> Collecting my thoughts and links to share here.
>
> More info soon, regards Al
>
> On Wed, 28 Feb. 2018, 1:12 am Bill Gray pluto@... <mailto:pluto@...> [find_orb], <find_orb@yahoogroups.com <mailto:find_orb@yahoogroups.com>> wrote:
>
> __
>
> Hi Alan,
>
> Sure, go ahead and post positions. I'm glad to hear you're
> measuring STEREO positions; it seems to me to be an under-used
> resource.
>
> Getting orbits from STEREO and SOHO data tends to be a bit
> painful. You don't get parallax and the positions aren't nearly
> as exact as those from ground-based astrometry. It used to be
> that MPC would produce orbits from SOHO and STEREO data, but
> I think a lot of the knowledge of how to do that was in Brian
> Marsden's head. I computed orbits for a few hundred comets
> some years back, and we now have a tremendous backlog of comets
> for which orbits need to be computed... it's been on my 'to do'
> list for a few years now.
>
> However, if you just have one or a few objects, things ought
> to be somewhat more straightforward.
>
> Also be warned: if you can get the object from _both_ STEREOs,
> or from a STEREO and SOHO, you'll get an _immensely_ better orbit.
> (Which is why I'm glad to hear of STEREO positions being measured.
> Comets measured just using SOHO -- the usual case -- tend to have
> extremely approximate orbits, unless they were bright enough to
> be measured over a couple of days.)
>
> -- Bill
>
> On 2018-02-27 07:35, c2009a1@... <mailto:c2009a1@...> [find_orb] wrote:
> >
> >
> > Gidday All,
> >
> > I joined this forum, as I now seem to be able to measure data from STEREO HI1a images that can be used with Find_Orb. So, sorry if you have answered these questions all before.....
> >
> >
> > So, just to introduce the problem, I can see many known asteroids in STEREO HI1a images. Most I can name from general alignments in Starry Night software with mpc data files. I want to rework the orbit calculation for some of these from measures from HI1A positions. To prove I have a good processing model to get real orbit elements.
> >
> >
> > Well, I have an unknown object ( unknown to me) moving across the fov for a month, have tried with help from Stereohunter forum members to work an orbit. Karl B ( mr sungrazer) suggested contacting Bill for help.
> >
> > I have the input data files available that have GEI position info for the Stereo satellite. Not that I know what a good solution looks like other than low residual numbers. Need to lock a good solution so that the unknown can be named.
> >
> >
> > Can I share the positions file here??
> >
> >
> > Kind regards, Alan Watson
> >
> >
> >
> >
>
>
>
>