Introduction to Observing Occultations of stars by the Moon or Asteroids

Last updated, 2015 March

see also my : Quick Start Guide

From time to time during the course of its orbit around the Sun, an asteroid (or Moon) will appear to pass in front of a star or another Solar System body as seen from Earth. Such a passage is referred to as an occultation. As the asteroid passes in front of the star the latter will appear to dim or disappear altogether. The track width of such an occultation is very narrow – approximately the same width as the asteroid i.e. 10 to 100 kilometers (or more for the largest asteroids). When the track intersects the Earth there will be few potential observers situated along it. The situation is improved for Lunar occultations. Except near grazing events, lunar occultatations can be seen from most places.

A star will disappear behind the Moons limb during a Lunar occultation reappearing up to an hour later. But for an asteroid ( a much 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, presents real opportunities for further discovers to be made ( by amateurs) while observing and recording these events.

The author use a 30cm F/4 Newtonian allowing observation of 10-11th magnitude stars with the eye, or 13 -14th magnitude stars by video. Here is the "Once in a life time" occultation of a naked eye star by asteroid Roma recorded by Jan Manek (cz)

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

OccultWatcher is the planning tool ( for Windows) used by occultation enthusiasts, and is recommended to anyone wishing to take asteroid occultation a bit further. Many additional predictions are listed, and what is displayed can be customised to local conditions.

I use a stopwatch and the "Speaking Clock" for timing Lunar occultations, and some observers use network time protocols, which I'm informed are now good for visual and video detection. Dimension4 software is recommended.

A PC application written and maintained by astronomer Hristo Pavlov "Beeper Sync 3.3", is a well characterised method and provides reliable 1 sec ticks.suitable for starting and stopping stopwatches and for sychronising timer equipment.

Radio code analogue clocks with accurate 1 sec ticks are guaranteed to be accurate, and can be used to check other time sources the observer may be using. Some clocks with liquid crystal displays howver have a display time lag of 50 to 100ms, or more if used in cold conditions (eg outside in Winter)

Accuracy and certainty is improved when recording events by video camera with a GPS video time inserter (VTI). The WATEC 120N+ ( and similar MINTRON cameras) can record at 25fps for Lunar events or set to integrate (stack) frames for asteroid events. The VTI overlays the exact UT ( to 1ms ) on each field of the recording which can then be replayed to get the event time and its duration. Here is an example of one frame taken from a video of an asteroid occultation (41-Daphne). The drop brightness between "Occulted and uot 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 GPSBOXSPRITE2 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.
Video (VHS) is one recording method, and the write uses DV tape. Is also possible to record dirrect to a laptop using a frame grabber. Peter Birtwhistle's ( observations use the CCD Driftscan technique.

Drift scan of brighter stars can be obtined with quite modest equipment such as a 4" refractor or 6" reflector, coupled with a DSLR. When the star drifts across the image of a stationary camera (shutter open), it leaves a trail. The occultation will appear as a short break in the trail. A stopwatch would also be needed for timing the start of the exposure relative to a UT signal.

Digital web cams are usable for bright objects but not for occultations generally. BUT this will change as developments ( in timing and analysis hardware+software allow the use of digital web cams (planetary imagers) which operate in 12, or 14 bit.

QHY5L-II has attracted interest from occultation enthusiasts recently (2014) using the EZplanetary software, which has built-in time stamping. Time calibration is based on the Windows clock, but carefull sychronisation with a time server suggest a promissing option worth invesigation if you allready have this device. Its not very expensive ! [ added 2014 ix 15]

A Image Source planet webcam can also be controlerd with FireCapture, and the most recent version has a time stamp facility.

HristoPavlov's web page ( is particularly helpful, because the analysis software is used by me and many others.

Negative and Positive events
Asteroid occultations can be short. 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.

Negative observations are typical for asteroids, because their orbits and dimensions are uncertain (hence a reason for observing them) and the occultation shadow is not where its predicted in most instances. The star positon also has errors. The errors all add up,so its alwat a good idea to observe it.


The main instrument is an Orion Optics (Crew, UK) 30cm F/4 reflector (1/8th PV) on an EQ6-pro controlled by EQMOD. I also have an LXD55 SN-8 as a mobile telescope for occultations and Lunar graze occultations. The video detector is an integrating WATEC 120N+ ( 0.02 to 10s exposure), the recorder a Sony TRV22E DV camcorder (tape), and the time insertion on the video frames (VTI) is provided by a GPS time inserter - the GPSBOXSPRITE2-U by Blackbox camera ( All of this runs from a 12V battery.

Other observers using video prefer a USB video digitiser and recording to a PC directly using Virtual Dub (Open source). Everyone has there own preferences. There is a purpose build VTI designed by IOTA ( and made by Videotimers (

USB video digitisers: (added Dec 2014)
[ converts analogue video (VHS, WAT, MINTRON etc) to AVI]
VCAP 800 (Climax Digital) + Largirth Lossless Video Codec
Dazzle DVD Recorder HD into VirtualDub using Huffyuv lossless codec. Preserves fields.

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 is recorded on a Sony TRV22E DV camcorder (video/in)

The predictions and observing opportunities are obtained from Steve Preston ( or by using OccultWatcher software (
For example:

(56)Melete on Jan 17, 2014:

Occultwatcher detail: The position of this observatory is in the shadow (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, and OccultWatcher is used day-to-day. A new UK-occultation feed UKOCL for OccultWatcher, provides low probability events for the keen observer to monitor.

The BAA web pages are

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
OCCULT4 by D Herald