Monitoring / recording asteroid occultations of stars is highly valued among planetary observers / scientists. It provides precise position and size measurements of small objects with two time measurements, beginning and duration.

Being in the right place at the right time is an advantage that many amateurs have in their back gardens.

Stars of magnitude 10.5 and brighter can be monitored visually for occultation events (6-8"). Fainter targets can be followed with increasing apertures (say 10-12"). The star/asteroid should be sufficiently bright to be seen clearly in the eyepiece so that a disappearance and reappearance are detectable. One should test the visibility of the target with the telescope and eyepiece combination to find a usable threshold. In the event that the asteroid and star are of similar brightness (which happens), its the light drop from the combined magnitude of star and asteroid, down to the asteroid alone that has to be detected. If the magnitude drop is 0.7 or less, then the occultation is unlikely to be seen visually (0.7 is regarded as a cutoff point). Small magnitude drops of 0.2 can be recorded by video, and under favourable conditions ( good signal to noise) and suitable digital processing 0.1 magnitude.

The occultation of a bright star by a faint asteroid will casue the star to vanish for a short while.

Some observers use a video camera and TV monitor as an electronic eyepiece with very good results, since the star and asteroid are "amplified" making visual timing with a stopwatch easier. There is still the observer reaction time, and a video delay to take into account when reporting (video delay = half the integration time)

Audio Recording:
The simplest method is to use stopwatches, or an audio tape recorder and ticking clock. (i.e. radio time signal, analogue quartz clock, electronic metronome etc). The aim should be to produce continuous 1 second ticks overlayed with your verbal observation ("In" "Out" etc.). The clock should be synchronised to a primary time source such as radio ( MSF, DCF, WWV) or BT land-line in the UK (dial 123). Remember to announce a minute marker at the beginning and end of the observation so that the seconds can be identified later. Alternatively use a camcorder as an audio tape and use the internal clock which has been set to UT as accurately as possible.

Video and Video Time Stamps
Observers who have sensitive video cameras (WATEC, MINTRON or SUPERCUITS) will obtain the most reliable results when recorded on tape or computer disk. Video also need a time stamp. A GPS time inserter made by Blackbox is available. The CT230 Time and Date Generator supplied by Voltek is another option (not GPS) and is a useful backup. GPS does not work well in a building, best outside with the ariel placed on a metal plate.

The IOTA-VTI is a new addition to video time insertion via GPS, and is designed to give best results for occultations.

Drift Scan
For exclusively asteroid occultations, the CCD drift scan is a good method, but may be specific to the telescope drive software such as WinScan. An 80mm finder scope and DSLR on a good mounting can be used as drift-scan recorder. A stop-watch with a lap mode timer, together with BT telephone time can time the start and end of the drift (instrument not driving) and event time can be deduced to better than 0.5 sec accuracy. Experimentation will be needed to get reliable results. The drift scan magnitude limit for an 80mm refractor is about 9 using a Canon 20D. Newer cameras have less noisy CCDs and should perform better and a 500 or 600 mm focal length should be about optimum.

A recent positive observation of (261) Primno and a 9.5mag star (2014/09/06) was made with a 6" F/5 and DSLR [ Sergey Tarantsov RU ]. A radio time signal was used, and duration was 5.8 +/- 0.5sec

Event UT:
The predicted time of an asteroid occultation is reasonably well known. (to within 20 seconds in many cases). It's advisable to begin observing about five minutes before the event. (Having already found the star). Asteroids with well defined orbits can be predicted accurately to within a few seconds. Smaller or more distant objects (e.g. TNOs) with less well defined orbits will have UT errors of up to 1 minute or more. Check the prediction for the event UT. To detect unknown satellites of asteroids, observation will need to commence 10 min earlier than the time for the parent asteroid.

Reporting Negative and Positive Asteroid Occultations
All observations are gratefully received. Results will be forwarded to the central body for reducing and archiving. Please contact me for a report form and any questions.

Clear Skies and good observing,

Tim

Author: Tim Haymes
Assistant Director (Occultations)
Asteroids and Remote Planets Section
- and Lunar Occultation Coordinator
British Astronomical Association.

Maidenhead,
UK
December 2010