CCD observations to update asteroidal occultation predictions are sought; Priority events for some of 1996 are given in the next section. Measurements of six to eight exposures are useful for averaging to obtain a more accurate final result, especially if your astrometric software only gives positions to 0.01s of R.A. and 0.1" of arc, which is more accurate than can be obtained in absolute coordinates with the GSC. Relative positions, most important for our work, with CCD's can be a few hundredths of an arc second, as described in the article below. So positions to 0.001s of R.A. and 0.01" of Dec. are useful for our work, considering that most of the asteroids have angular diameters around 0.10".
Astrometric updates are requested for the asteroidal occultations listed below. In most cases, these will be "last-second" CCD observations when the two objects are close enough together to image in the same CCD field using the same GSC reference stars usually only a day or even several hours before the occultation. For events with southern declinations, observations from latitudes +40's or better, +30's are sought. Although we have only one active CCD astrometrist for occultations in the southern hemisphere (Gordon Garradd in Australia), fortunately he has been very productive, and has provided most of the astrometric updates so far this year. But it is not always clear at his location in New South Wales, so observations by others are needed, especially with the relatively fast-motion events we have coming up, where it will be rare that the asteroid and target star can be imaged on the same frame more than 2 days before an event.
For many of the events, one or two days before, the objects might be imaged in separate fields but with some reference stars in common, allowing some link between the two and giving better accuracy than completely separate fields. Also possible is the use of wider-field CCD's to obtain secondary (non-GSC) reference star positions, and smoothed positions for the GSC stars, that can be used with narrow CCD fields to obtain better positions for the target star and asteroid that would be better than using separate GSC data alone. Valuable are longer-range meridian circle observations (events suitable for these are indicated with an "x" under the "M" column) and sidereal-rate strip CCD observations (with an "x" under the "S" column, for events with small motion in declination). "V" after "asteroid" is the predicted V-mag. of the asteroid at the time of the occultation. The second table gives additional information, including the angular motion ("mot.") in arc minutes per day. More details about these can be obtained from Edwin Goffin's charts distributed by IOTA and EAON.
Good luck with your observations.
Only North American events during the next few months are listed below. For North American events of interest during the rest of 1996, see the table on p. 69 of the February issue of Sky and Telescope.
1996 Event
Date U.T. Asteroid V Star mag. R.A. (J2000) Dec. M S h m s o ' "
Apr18 9:49 39 Laetitia 11.5 SAO 145292 9.0 211909.048 -091003.94
May19 4:53 372 Palma 12.6 SAO 117650 8.7 92344.212 +083759.13
May20 2:42 14 Irene 11.0 SAO 080587 8.8 90200.623 +252642.07
Jun08 4:44 498 Tokio 12.2 SAO 159939 8.9 162906.310 -144057.87 x
Jun16 5:34 532 Herculina 10.5 PPM 100949 11.3 111032.630 +222336.20
Jun27 5:43 466 Tisiphone 13.5 CMC 413773 11.8 200705.305 -192520.59 x
Jun30 6:17 419 Aurelia 10.4 LN 1637 11.4 195341.334 -135603.93 x
Jun30 7:10 704 Interamnia 11.6 PPM 091751 10.0 23119.004 +295514.52
Jul09 3:39 253 Mathilde 14.8 GSC 49960276 11.6 143119.770 -070456.20 x
Jul15 9:05 511 Davida 11.8 GSC 00590303 11.2 31640.290 +031314.20
Jul17 11:10 12 Victoria 8.7 PPM 203269 9.4 194551.624 -044019.42 x
Jul18 4:48 341 California 13.2 SAO 207552 8.8 161911.983 -303731.72 x
Jul28 9:58 203 Pompeja 13.0 SAO 188231 7.5 193116.584 -254413.16 x
Date UT mot. Notes, nominal region of visibility
Apr18 9 20.42 Central and eastern U.S.A.
May19 4 15.43 Western North America, Mexico
May20 2 24.23 Greenland, Labrador (s.e. Canada, n.e. USA if s. shift)
Jun08 4 14.60 east & south U.S.A., n.w. Mexico
Jun16 5 23.04 west Canada, west U.S.A.; satellite? Not visual event, dm=0.6
Jun27 5 10.46 Canary Is., southern U.S.A.
Jun30 6 9.54 eastern and northwestern U.S.A. Not visual event, dm=0.4
Jun30 7 23.08 Bermuda; eastern U.S.A & Canada if n. shift
Jul09 3 5.10 w.Mexico (s.w. USA if n.shift), Colombia, Amazon; NEAR target
Jul15 9 19.86 Great Lakes region
Jul17 11 12.94 Alaska; w. USA & Canada if s.shift. Not visual event, dm=0.5
Jul18 4 1.49 Mexico, western Plains; very slow motion
Jul28 9 11.96 n.w. Mexico, Hawaii
David Dunham, IOTA, 1996 April 13
The article below is from pages 157 to 158 of the International Occultation Timing Association's (IOTA's) Occultation Newsletter, Vol. 6, No. 7 (June 1995).
David W. Dunham
The growing number of amateur and professional observatories with CCD systems that are connected to PC's are encouraged to help update asteroidal occultation predictions during the night preceding the event when both the star and asteroid can be imaged in the same CCD field. Those who are now involved in this IOTA-organized program need to be augmented with more observers with a greater geographical distribution; for example, weather (mainly) and other problems prevented any updates from being obtained for the May 12th occultation by 74 Galatea whose nominal path crossed the USA centrally from Washington state to Florida. Those with such systems who do not already have astrometric software packages that use the GSC to quickly determine the J2000 R.A. and Dec. of the objects can get information about these from the individuals below who have already contributed observations for astrometric updates to this IOTA-organized program; the Internet address is given following the location:
Dennis DiCicco, Sudbury, Mass.;
dicicco@cfa.harvard.edu
Petr Pravec, Ondrejov, Czech Rep.;
ppravec@asu.cas.cz
John Rogers, Camarillo, Calif.;
72401.3174@compuserve.com
Fiona Vincent, St. Andrews, Scotland;
fv@st-andrews.ac.uk
George Viscome, Lake Placid, New York;
73023.561@compuserve.com
The idea of last-second astrometry using narrow-field CCD's (rather than the more warning one gets from "last-minute" astrometry that can sometimes be made using wider-field photographic plates) was first described in ON 6 (5), pp. 112-114. The title for that article should have been "Last Second Astrometry . .", not "Last Minute Astrometry . .". Although the Eugenia occultation described below was not observed, it was the first time good last-second astrometry was obtained from two observatories. A poor distribution of GSC reference stars resulted in discordant predictions when two last-second astrometric observations were obtained to update the April 22nd occultation by 106 Dione.
George Viscome at Rand Observatory, Lake Placid, NY, obtained 5 CCD images of GSC 1385 00541 and 45 Eugenia 0.6 hour after the May 4th occultation, and my reduction of them shows that the path was only 004 north, or 3/8th path width north, of Edwin Goffin's nominal prediction shown on my map in the February issue of Sky and Telescope, with the time correction only +0.1 minute. This path crossed over northern Quebec, southwestern Labrador, and over western and southern Newfoundland, including Cape Race just south of St. Johns', which was probably near the northern limit. This path was only 0.4 path width, or about 110 km, south of the prediction from Petr Pravec's "last-second" astrometry obtained 6 hours before the event, and I think vindicates the general concept of "last-second" astrometry (although some actual occultation observations would do that even better). The different predictions relative to the nominal prediction, and relative to Viscome's prediction (actually, "postdiction"), are below:
Obs. Rel.to Dist. Relative to Viscome
Date occ'n, from Separation Time,
May,UT days star Observer Widths min.
' "
2.83 -1.26 20.4 Pravec +0.15 +1.4 -0.1
3.20 -0.89 14.4 Rogers +0.28 +2.6 0.0
3.82 -0.27 4.4 Pravec +0.04 +0.35 +0.1
4.12 +0.02 0.4 Viscome 0.00 0.0 0.0
There seems to be a southward shift in the path with time, perhaps due to the GSC-based reference frame being skewed. So in the future when observations with a similar distribution of distances from the star are available, it might be possible to extrapolate to the event time to get a more accurate prediction. But we shouldn't depend on that too strongly, since the mean error of Viscome's path is +/-0".03 from the statistics (some of this might be due to his positions being reported only to a precision of 0.01s in R.A. and 0".1 in Dec.). The predictions might be even better for some of the future events where the motion is slower.
Pravec's observations on May 2.8 consisted of separate observations of the star and Eugenia; the two objects were not yet close enough together to appear on the same CCD frame, but many of the same reference stars could be used.
Rogers' observations were made with a smaller telescope that had a wide CCD field, allowing both objects to be imaged on the same frame. Although the results are less accurate, the increased warning time provided can be valuable, and the wide field of such systems can be used in the future to obtain additional non-GSC field star positions for reductions of narrower CCD fields, and link separate narrow CCD fields of the target star and asteroid.
Pravec's message giving his May 3.8 observations, 6 hours before the occultation, was sent about an hour after the observations were made. Unfortunately, it arrived at our fileserver at 5:17 pm local time, and the PC that handles my e-mail had been shut off for scheduled power maintenance at 5:00 pm. The power was restored shortly after 9 pm, about an hour and a half before the event, but by then I was 50 miles away setting up to video record a grazing occultation that occurred about 40 minutes before the Eugenia appulse (the graze was quite successful). So I received those data the morning after the event.
Meridian circle observations also give promise for updating asteroidal occultations, and can give much more warning, even weeks, than CCD observations. For the 654 Zelinda occultation in the USA in January, meridian circle observations made at USNO-Flagstaff were used by Larry Wasserman to predict the path of the occultation, which was observed from two stations, to an accuracy of 0".02 or so. Unfortunately, three photographic exposures made at Lowell Obs. the night before the event indicated a path about two path-widths (almost 0".3) farther south, and most mobile observers set up according to this erroneous information. Meridian circle updates require that the target star and asteroid be visible on the meridian in a dark sky during the weeks before the event (so the solar elongation of the event should be greater than 90 deg.), and the asteroid must also be bright enough to observe with the meridian circle telescope. After Zelinda, we are becoming more distrustful of photographic astrometry, which has been used to improve the predictions for many other past events (some quite successfully). But photographic observations are more difficult to make than CCD observations, although they have the advantage of being able to use accurate PPM reference stars rather than the much less accurate GSC reference stars, if the field size is large enough.
Another approach to astrometry is described in an article, "Minor Planet Astrometry with a Flat Bed Scanner and a Quadratic Reduction Procedure", by Paul Comba; 1411 Galaxy Lane; Prescott, AZ 86303; published in The Minor Planet Bulletin, 22 (2), pp. 13-15 (1995 April). He describes using a 35mm camera with a 2800mm focal length telescope. A high-precision flat bed scanner, with 1200 dots per inch now available for about $600, is used to scan an enlarged print of the photo. Finally, he describes quadratic reduction software that, with his system, gives smaller mean errors than standard linear reduction algorithms.