Getting Started in Asteroid Occultation Observing

These eight pictures tell a story. Starting with the geometry of an asteroid occultation event, this is followed by a typical prediction which will include links to finder charts (e.g. 80-Sappho). Timing equipment from a simple audio tape-recorder with coninuous clock ticks, is followed by some other clocks, watches and Digital Tape. We then move to a camcorder tape using the internal clock, and finaly a complete video system with GPS time stamps. The analysis of Video results will produce a light curve (with a dip if your lucky) and the results of several observer's can form of an asteroid profile published by EAON. Your observation could be one of them. The bit missing from the story is the telescope. Anything from 4 to 14" will work.

Star / asteroid / observer geometry
Track for 80 Sappho
Tape-recorder and ticking clock
Digital Tape and clocks
Camcorder tape and clock
A complete video set up
Video analysis
Published result for Sappho

Why not Observe?
You may be looking for an astronomical pursuit that gets almost instant results, are time and space predictable and can be selected to fit in with other interests. Preparation takes about 30min from a garden location with the telescope set up ready to go. Its satisfying to see an event (if your fortunate and the shadow passes over) and the timings are highly valued by scientific professionals world wide. The geographical space occupied by amatures with their telescopes cannot be underestimated.

So what's involved:
The aim is to find the star (not necessarily the asteroid, as it it is usually much fainter) and to monitor (watch or record) the star to see if it disappears or not; and with suitable equipment make a timed observation of the start and end of the event with an accuracy of half a second or better relative to UT.

Each observation takes about 5 mins, but is unique to your geographical location. No other observer will detect exactly the same event. This make your observation particularly useful to science, because the combined observations from many observers will produce a outline shape of the asteroid. This is one of the objectives. Previously unknown satellites and double stars are also discovered. We use the application OccultWatcher to help us select and plan observations. Its interesting to know if observer in England, Spain or the US plan to monitor the same event for example.

Where do I start?
1) Visual Observation:
Firstly. Find your telescopic visual limiting magnitude in typical sky conditions and magnification. I would recommend an 6"-8" or larger aperture for visual observation and a field of view of say 0.5 degrees. Lets say its magnitude 12. I suggest an occulted star needs to be 1.5 magnitudes brighter (say v10.5) otherwise you will find visual detection of a disappearance and reappearance more of a challenge, and the result may well be less certain (Did I see it or not?). Uncertain observations are not of great value on their own unless confirmed by other observers. This is because a NEGATIVE result (no occultation) has the same importance as a POSITIVE. No harm in pushing the limits though, as one needs to find what the set-up is capable off in different conditions (seeing, magnification, sky brightness etc).

Second. Use Google Earth and find the longitude and latitude of the observing site ( to 0.1" arc), and get an estimate of the height above sea level. Check this on an OS map. The nearest 5m should be fine. There is more info here

Third. If you don't already have star mapping software on your computer, get one of the free programs such as Cartes du Ciel or C2A. For occultations I prefer C2A because it interfaces with OccultWatcher (generates a finder chat). Carte du Ciel can control telescopes (via ASCOM) so it might be worth while to have both softwares installed.

Four Find a predictions from Global asteroid occultation predictions by Steve Preston, OR look on my page on Future Events, OR best of all download OccultWatcher and configure it to your needs, e.g. to present events brighter than 11, with tracks within 100 Km say, and duration longer than a few seconds. There are several feeds to choose from. The IOTA feed is a good start. There is even a feed for the UK.

Fifth Check the altitude and azimuth of the target is within your horizon range, and print off the finder charts.

Sixth Find the star by star hoping (long time) or GOTO (preferred). Double check the star field. If tracking, come back 5 min before the event time, dark adapted and relaxed - use a seat.

Last. Now we can observe! Use a suitable timing method, one you have tested and tried and are familiar with. Record the observation times as accurately as possible. Report if seen (+ve) or a miss (-ve)

2) Timing methods
In essence the time must be UT using a clock synchronised to a reliable time signal. More on my web page here. The most reliable method is to have a continuous time signal recorded with the observation. One easy option is to use a video camcorder and use the sound track to record verbal observation, while the internal clock is running. The clock can be syched to a phone or radio signal before observing. To obtain times, replay the tape with the date and time displayed and interpolate to a fraction of a second.

Several stop watches can be used (if you are daring) and stopped on a phone or atomic clock pip.

3) Video Observations
Video through the telescope is the preferred method of recording and timing, and requires specific equipment, often in addition to the average observer's facilities. 1) An integrating video camera, e.g. WATEC or MINTRON, a video text time overlay (several available) and a video recorder/digitiser e.g. laptop.

The writer uses a F/4 reflector (8" or 12") a WAT 910HX, GPSPRITE2 GPS time inserter OR alternatively the IOTA VTI , and a Sony TRV22E DV camcorder for recording. Digital tape is copied by fire wire to a the PC as AVI. . This is replayed and analysed at leisure.

4) A useful half-way-house, is to combine video and visual observation. The camera displays the star field in real time and the observer watches the screen, timing the event by what ever means. The visual limiting magnitude is overcome, but reaction time and observer alertness still limits the observation, particularly if a small magnitude drop (less that 1 mag) is expected. The benefit is a much improved display of fainter objects.

5) Drift Scan
Sounds complicated, but is quite a simple in concept. You may have a 4" refractor or 6" reflector, perhaps even a Dobsonian.
Focus the star with a DSLR and open the shutter, alowing the star to drift ( Siderial motor off). An occultation will appear a break in the trailed star image. Knowing the start time of the exposure and the rate of drift ( via the declination) we can get all the data from the image.

6) Reporting.
Event times are reported in a standard format (see observing page ) and sent to a coordinator (BAA Asteroids and Remote Planets section) and to the PLANOCCULT list server for Europe. OccultWatcher has a report preparation tool that prefills certain fixed data which ovecomes human error. The most recent option is to import analysed results as well.

7) In Conclusion.
DSLR, CCD Drift Scan (Great Shefford Observatory) and planetary web cams are also being used more widely now. Potential observers are encouraged to investigate different recording methods provided the timing operation is made good enough. The more asteroid occultation observers, the better. These links contain information useful for the beginner and more experienced observers.

UKoccultations , Asteroids and Remote Planets , EAON European Results , IOTA International Occultation Timing Association

Drift Scan method, RASNZ beginners guide


Tim Haymes,
August 2014