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
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)
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
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.
is a new addition to video time insertion via GPS, and is designed to
give best results for occultations.
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 +/-
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
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,
Assistant Director (Occultations)
Asteroids and Remote Planets Section
- and Lunar Occultation Coordinator
British Astronomical Association.