J. Brit. Astron. Assoc., 107, 1, 1997, p. 49

Observing a stellar occultation by an asteroid

After 38 nights of being more or less clouded out, last night proved mainly clear. I included in my schedule the next stellar occultation predicted by the Asteroid Pro software and published in the latest BAA Circular, namely 892 Seeligeria, which was forecast for November 9, 01:46-02:06 UT. The star involved was GSC 4695 543, which was quoted in Asteroid Pro to have a V magnitude of 10.9, i.e. relatively faint.

I used my Meade 20cm Schmidt-Cass telescope, which is mounted as a guidescope on my main instrument. I used a flip-mirror viewer to monitor the star, which was conveniently located, in that it has two other stars fairly closely spaced either side of it. The eyepiece was of 13mm focal length of the illuminated reticle type. I find the weakly-illuminated cross-hairs to be of help in monitoring the fainter stars.

Observation began about 01:42 UT, when a few thin clouds appeared. These soon cleared (the Anti-Spode Law operating on this occasion) and intensive monitoring began around 01:46 UT. The mid-occultation prediction was 01:56 (verified later using Asteroid Pro as 01:55:40). Watching intently and comparing the target star with its two companions, I suddenly hit the button on my stopwatch (Casio Telememo 50 wristwatch) as I reacted to the object seeming to disappear. I immediately looked to the other two stars to check they were visible, which was the case, then switched to averted vision – nothing there! By then my finger had transferred to the 'lap-time' button on my watch. A few more seconds passed and whilst my eye was still, the star suddenly popped back and instinctively I hit the watch a second time. I continued to monitor the region for about 1–2 min., when all 3 stars remained visible. Thinking it was nearly 02:00, I went to the radio intending to press the next 'lap-time' reading on the pips. After 5 min. no pips appeared, so I decided to check the time by stopping the watch and immediately switching to the indicated time – this way I measured the UT of mid-occultation to be 01:50:55 ±2 sec. The duration of the occultation according to the reading on the watch was 4.97 sec. Allowing for the initial shock factor, I estimate that the delay in starting the stopwatch was about 0.2 sec. longer than my reaction in stopping the watch when the reappearance happened, making the estimated duration to be 5.2 ±0.2 sec. The reappearance seemed much more obvious an event than the disappearance – presumably because I was primed ready for it.

(N.B. Disappearances of monitored stars can frequently occur if you catch the star in that part of your vision occupied by your 'blind spot' as you move your eye around the field. With experience you can avoid this or at least learn to recognise it when it happens. When you catch the star in your blind spot you must avert your vision fairly quickly to check that its still there. You must of course avoid starting the stopwatch when this happens. Gazing directly at the star can also seem to cause it to dim considerably. Generally however you should be aware of the rough location of your blind spot and also avoid direct vision).

I was surprised to find that my observation took place almost 5 minutes ahead of the prediction, and knowing the asteroid was moving at a speed of about 12.0 km/sec on the sky this corresponded to a difference of well over 3000 km. I turned to my reliable Guide Version 5.0 software, selected my local latitude and longitude and lo and behold when I selected 01:51 UT, 892 Seeligeria was pinned firmly on the star. The closest approach (about 0.1 arcsec) was predicted at 01:51:35, i.e. only 40 seconds from my actual observation and not the 5 minutes or so given by Asteroid Pro. Indeed, Guide indicated that the asteroid should have been more than 2 arcsec from the star at the time given by Asteroid Pro. The 'observed' minus 'Guide 5.0 predicted' positions amounted to only about 0.3 arcsec.

The observed duration of the occultation translates to a chord of 62.0±2.4 km in length, neatly fitting within the catalogued diameter for 892 Seeligeria of 78.5±1.7 km. The difference indicates that my location in mid-Cheshire was about 20–25km from mid-track. Since Seeligeria was at a relatively low altitude in the sky (<20°), this difference projected onto the Earth's surface actually amounts to about 70–90km from mid-track. It should be noted that this event was relatively favourable from the point of view of the width of the shadow track on the Earth, which was given in the Circular as 260km. Given this situation it is quite possible that a second UK observer also recorded a positive event.

This must be my 4th or 5th serious attempt at observing a stellar occultation by an asteroid this year. The event was not predicted by EAON, the European Asteroid Occultation Network: EAON tend not to include many of the GSC stars, which appear on Asteroid Pro. I have no record of a previous stellar occultation by an asteroid reported by a UK observer.

Richard Miles
Northwich, Cheshire; 1996 November 9