Written on 2015 March
Updated: 2025 November

In the course of its orbit around the Sun, an asteroid (or The Moon) will appear to pass in front of stars or another Solar System body. Such a passage is called an occultation. The star will appear to dim or disappear altogether. The shadow width of an asteroid occultation is quite narrow – approximately the diameter of the asteroid i.e. 10 to 100 kilometers. When the shadow path intersects the Earth there will be potential observers situated along the path who are observing it. The event is recorded by video with time inserted on the frames.

A star disappearing at the Moons limb during a Lunar occultation will reappear some time later at the oposite limb. But for an asteroid ( a smaller object ) the star will reappear within seconds (1 to 30s) after the asteroid has passed.

The remarkable discovery of rings around asteroid (10199) Chariklo, and minor planets with satellites, present real opportunities for further discoveries to be made by amateurs while observing and recording these small body events occulting stars. This also includes unresolved double stars.

Prediction sources for asteroidal occultations such as these by Steve Preston and Observation Calls by Oliver Klos, allow an observer to select the most interesting events to record. The objective with any occultation is to measure the start and end time as accurately as possible within the limitations of the observer's equipment and sky conditions, and to report the observed times, along with an estimate time accuracy.

OccultWatcher is the planning tool ( for Windows) used by occultation enthusiasts, and is recommended to anyone planning to observe asteroid occultations. Many additional predictions are listed, and these can be customised to local conditions.

Occult Watcher Cloud (OWC)is a new resourse for predictions. The information can be filtered more easily. https://cloud.occultwatcher.net/login

Timing

Accuracy and reliability is improved when recording events by video camera with a GPS video time inserter (VTI). The WATEC 910HX/RC can record at 25fps for Lunar events or set to integrate (stack) frames for asteroid events (0.08 to 5sec) . The VTI overlays the exact UT ( to 1ms ) on each frame of the recording which can then be replayed to get the event time and duration. Here is an example of one frame taken from a video of an asteroid occultation (41-Daphne). The drop brightness between "occulted and not occulted" may look small on the insert, but is very clear when measured in the image on the right.

Light curve (above) obtained from video showing the 13.6 second dip in brightness (blue) caused by the occultation of a star by asteroid Daphne, compared to a comparison star (yellow). [ TANGRA 1.4 ]

The video frame above using a GPSBOXSPRITE time inserter was recorded on: 2012 Feb 23 at 2045hr 22.5799 sec. Times are reported (rounded) to the nearest 10 ms

Recording and analysis.
See this page for recording equipment

HristoPavlov's web page (https://www.hristopavlov.net/Observations/Positives/index.html) is particularly helpful, because the analysis software is used by most observers.

Negative and Positive events
Asteroid occultations can be short, typically a few seconds. The event duration is in the range ZERO to 30 seconds depending on the asteroid size and where the observer is inside the shadow. A ZERO event is really a "miss" or "NEGATIVE" i.e. no occultation was seen or detected. We always report NEGATIVE events. Most observations result in seeing a "Miss", but improvements in predictions gives the observer with mobile equipment, an opportunity to travel toward the predicted track, similar in many respects to observing a lunar graze occultation. However most observers use their telescops at home.

For rare events and those of high sceintific value, observers do travel. This is quite common in Europe, and the US.


Equipment (Video or CMOS)

Observations are encouraged to use the equipment they may already have, however the larger the aperture the better for asteroidal occultations. A 10 - 15cm reflector would be a minimum, better still a 20cm F4. SCTs are quite common instruments to use from C6 to C9.25. The C11 possiblty repesents the best in terms weight/appeture. It can be supported by an EQ6 with little problem. Both the EQ and SCT are transportable.

The detector and timing arrangement typically would be:

a) Analogue 8bit WAT-910/HX with GPSBOXSPRITE3 (Time overlay), video digitiser, and laptop with IOTA Video Capture

b) QHY174 CMOS, or any with IMX174. These are relatively inexpensive (due to being 1st Generation), with global shutter and big pixels.
Any other camera that will give up to 20 fps, with 2x2 binning option. The QHY174mGPS costs about £1000 euros, and is a "Jack-of-all-trades"

c) Timing with NTP controlled with Meinberg software, or better, a laptop with GPS via USB.

d) A suitable GPS/USB timer is the TIMEBOX-II from Shelyak Instruments

USB video digitisers: (added Dec 2014)
[ converts analogue video (VHS, WAT, MINTRON etc) to AVI ]

Dazzle DVD Recorder HD into VirtualDub2 using Largarith Lossless Video Codec

Here is a mobile camera for bright star asteroid occultations down to magnitude 10, consisting of a 200mm F3.5 telephoto lens (Pentax screw thread) adapter (M42 to C-mount) and a Watec 120N+ integrating video camera. Video was recorded on a Sony TRV22E DV camcorder (video/in), or laptop.

(56)Melete on Jan 17, 2014:

Occultwatcher detail: The position of this observatory is in the shadow (shaded blue), and indicated by the black bar. The relative position of other stations are indicated. A wide dispersion of observers is needed for best results. Not all will be successful though.

The British Astronomical Association Handbook contains predictions a year in advance calculated by E.Goffin. These are used for medium term planning of bright events. OccultWatcher should used for day-to-day for updates nearer the predicted time

The BAA web pages are https://britastro.org/asteroids/Occultations.htm

Lunar occultation predictions are available in the Lunar Section Circular downloadable from the BAA web site, or predicted using free software. These programs are the result of continuous development by the author/programmer:

LOW: Lunar Occultation WorkBench http://www.softpedia.com/get/Others/Home-Education/Lunar-Occultation-Workbench.shtml
OCCULT4 by D Herald http://www.lunar-occultations.com/iota/occult4.htm