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Adventures while visually observing asteroidal occultations


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About this observation
Peter Anderson
Time of observation
20/11/2013 - 14:00
Observatory interior
Observing location
The Gap, Brisbane, Australia
Standard camera
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The image is to support the following account and shows the layout of the twin section sliding roof observatory prior to 2016 when I went totally SCT. 

For well over 40 years I have been visually timing the occultation of stars by the Moon and have to date bagged well over 9,000. In the late 1970’s I also began  observing occultations of stars by asteroids, but the baselines are many times longer and the asteroids are relatively small.  As a result earlier asteroidal occultation predictions were not at precise with less than 5% success rate. For the others the angle, time, and distance of closest approach could be reported but that was all.  

Accuracy has been greatly increased and success rates are now vastly improved.  Many observers now use low light video cameras that provide a permanent and very accurate record and if employing longer integration times, can reach magnitudes well beyond the visual observer.

Nevertheless there is an excitement in the ‘hands on’ experience of visual observing, actually seeing a star ‘wink out’ and ‘wink back on’ some seconds later. At asteroidal distances the apparent stellar disc is no longer tiny and most asteroids are also irregularly shaped so even if an event is ‘square on’ it may not be quite instantaneous but take a small but noticeable fraction of a second, and occasionally much longer with slow fades and reappearances. This adds to the interest of these observations.

Some years ago a friend of mine recorded two distinct sets of events for one asteroid.  Perhaps it was highly irregular or concave in shape or even a close or contact binary – like the recent images of ‘Ultima Thule’. We postulated its possible shape, a figure of 8, or even as a joke, the famous ‘V’ for victory hand gesture of Winston Churchill.

On another occasion I observed a short but definite occultation a few minutes early. I was instantly suspicious. It did not disappear or re-appear quite the way others had.  Examining the eyepiece I discovered a tiny spider nested inside.  It had found its way into the tube of the Newtonian telescope. I determined that this occultation was real enough, but happened about 15mm away and not around 300 million kilometres. (A factor of 1:20,000 billion.)

But the most memorable event involved the family cat. He was very inquisitive.  I was out one evening in the twin section roll-off roof observatory behind the house with the slot open above the telescope and was monitoring the star that was soon to be occulted. This was before the computer ‘go to’ era, and finding and positively identifying a faint star took a little time. Standing on the ladder at the eyepiece of the 16” F6 Newtonian, stopwatches in hand I was also listening to the radio time signal counting off the seconds when I heard a ‘thump scrabble’. The cat had jumped onto the running rails for the roll off roof. Silence, then ‘thump’, it had jumped from there onto the roof itself.  Seemingly wearing hob nailed boots, it walked across the roof towards the open slot. Out of the corner of my eye a head appeared looking down and I heard a “mrrowww” in greeting.

The telescope tube angled up less than 30 degrees and the cat then daintily stepped down onto the end of it. The telescope had a friction clutch and by applying upwards pressure I was able to support the weight of the cat and hold the star in the field of view, if only the cat would stay still! Absolutely not!  After about 20 seconds, probably feeling bored, he started walking down the tube and with the constant variation in the balance point there was no way that I could maintain the star in the field and no possibility of quickly finding it again.

I searched for an appropriate description in the report of my disappointing partial monitoring of this event. The best I could come up with was ‘cattus interruptus’.

 Postscript: I do not presently have a cat!


Tim Haymes's picture

That is a nice account of your observing activities Peter.  We have a few visual observers in England.  I was visual  up to about 1991 (having seen no asteroid occultations) and then converted to video. The mag limit at 25fps was not much better, but at least it was a permanent record. Then along came the integrating video cameras and I now use a WATEC 910HX.  My first positive was Elektra in 2010 (10.7 sec). The observing team in the Asteroids and Remote Planets Section all use video and this year (so far) we have recorded chords across 16 minor planets.

With improvements in prediction quality the visual observer now has much improved chances, and the observing time interval is really only about 30s.  It was 30 min back in the 70/80s !  I see you visually observed an occultation of a 10th mag star by (433) Eros on April 6th this year - congratulations.

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