Solar System - Occultations

I will not longer provide own occultation predictions of ordinary asteroids (MBA etc). At least two programs (Occult and LinOccult) are free available to do such predictions by everybody as well as there are the nominal predictions made by Edwin Goffin and distributed by EAON annually. From my point of view there is no need for additional resources (which would essentially give the same informations). You find here a list of sites providing occultation predictions.

Therefore I will concentrate my efforts on special targets like Planets, Moons, TNOs etc. and on the archiving / reduction of asteroidal occultation observations.

Prediction uncertainties

The most reliable (and straight forward) way computing the ephemeris uncertainty would be to compute a new/improved orbit for the asteroid. From this least-squares fit the formal error of the solution (covariance matrix) can be propagated to the time of occultation. And out of that we got the uncertainty ellipse (RA/DE) in the sky plane. Other methods to estimate the ephemeris uncertainty are nicely summarized in a mail list message by Steve Chesley (JPL).

While this could be done (and in case of selected updates it is) for single asteroids or special events this approach is not suitable (or at least not easy) in an automatic way necessary to feed the occultation search engine with a large bunch of asteroids. Beside of performance reasons these orbit improvements would have to be done automatically, including weighting and rejecting bad observations. Especially for low-numbered objects (and these are the main candidates for occultations because they are the larger ones), over decades (and more) observed, an automated observation selection procedure could be worse then a comprehenshive, manually examination of the orbit solution and the observational data.

One practical way is to use the daily updated astorb.dat database of orbital elements. The database gives also a current ephemeris uncertainty and the daily change for it. These values can be used to get an ephemeris uncertainty for the time of occultation (plus a formal error of the star position).

NB: in my point of view the automated process of computing the orbits (i.e. rejecting, weighting of the observations) can lead to lower quality solutions compared to the orbits given in mpcorb.dat, especially for low-numbered objects etc.. Thus I would basically prefer mpcorb.dat for prediction purposes, but the individual computed (low-numbered) orbits here might not consider the latest observations (or even latest oppositions). In conclusion it is sometimes recommendable to take both data sets into account (or to compute an individual/updated orbit).

The following table gives you an idea how the prediction uncertainty in arcs (ephemeris + star position) together with the geocentrical distance yields to the path uncertainty on Earth (better: on the Bessel plane). The left column gives the assumed uncertainty in arcs (0.05, 0.1, 0.5,...), the top row the geocentric distance of the object in AU (1.0, 1.5,...,30.0). Then you can pick up the uncertainty on Earth in km.

--	1.0	1.5	2.0	2.5	3.0	3.5	4.0	5.0	10.0	20.0	30.0
0.05	36	54	72	90	108	126	145	181	362	725	1087
0.1	72	108	145	181	217	253	290	362	725	1450	2175
0.5	362	543	725	906	1087	1269	1450	1813	3626	7252	10878
1.0	725	1087	1450	1813	2175	2538	2901	3626	7252	14505	21757
1.5	1087	1631	2175	2719	3263	3807	4351	5439	10878	21757	32636
2.0	1450	2175	2901	3626	4351	5076	5802	7252	14505	29010	43515
5.0	3626	5439	7252	9065	10878	12691	14505	18131	36262	72525	108788

(Last minute) astrometry

In general you have to be familiar with CCD astrometry of Minor Planets. Otherwise you should read the introduction by the Minor Planet Center (MPC) and start with that gathering experience. For updates of occultations very accurate astrometric measurements are needed (better than say 0.2-0.3"). It is difficult to achive that accuracy with small amateur telescopes (short focal length).

I recommend to use the UCAC2 star catalog (mean pos. accuracy about 20mas for 10 < R < 14 and about 70mas for R_lim ~16) for the reduction rather than the USNO-B1.0 (mean accuracy about 0.2") or USNO-A2.0 (mean accuracy about 0.3") if your FOV does allow this. Another pitfall within the context of last minute predictions might be that USNO-B1.0's proper motions mean epoch is about 1975 and that the proper motions are relative, not absolute. The present offset wrt the ICRS is about 0.3" (Schmadel/Stoss 2007) while the occultation target stars are usually HIP,TYC or UCAC2 stars (i.e. very close to the ICRS).

The formal positional uncertainty of the UCAC2 catalog depends on the magnitude as shown in the figure (Zacharias et al. AJ 2004).

Reference Frames / Star Catalogs

A good introduction/overview to the ICRS including further reading (links/papers) is given by George Kaplan (USNO).
Comprehenshive informations and recommendations on star catalogs are given by Norbert Zacharias et al. (USNO). You might also take a look here on some personal comments concerning star catalogs for occultation work.

Credits

In general all individuals / institutes are acknowledged for providing the community with data and/or services for free !

The U.S. Naval Observatory (George H. Kaplan) is acknowledged for the superb NOVAS library which I am using in my programs since many years.

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