Timing Strategies for Asteroid Occultations

This page lists some strategies for timing asteroid occultations. Choose the most accurate one you can manage and PRACTICE it well before the event. When possible, incorporate a backup plan.

Video/CCD Observing

Video Recording: The basic idea is to record the event by attaching a video camera to your telescope (shorter f-ratios are best). Record WWV on one of the audio channels to provide a timebase. If you haven't done this before, try to practice beforehand. In the last few years many observers have started using GPS based time-inserters that label each video field with the UT time (very nice).  For more information on the equip and setup for videotaping see the main IOTA page at IOTA Observing Resources. Or post a question to the IOTA Yahoo group.

CCD Recording: Most Astronomy oriented CCD system can be used for timing asteroid occultations via a "drift-scan" technique.  John Broughton has made many observations using this technique and provides and excellent explanation on this webpage:  http://www.users.bigpond.com/reedycrk/driftscantiming.htm

Visual Observing

Best method: Use an audio tape recorder to record your voice and WWV while you observe. Call out a short phrase when the star dims or brightens (e.g. "gone" and "back", or "out" and "in", or "wow" and "wow", ...). Try to keep the tape recorder warm so it doesn't record at an abnormal speed. If your tape recorder is battery powered, put NEW batteries in it. Get a NEW blank audio tape. Practice a few times before the event. If you are not familiar with WWV see the info below.  If you don't have an audio tape recorder, you can use a camcorder to record your voice.

Alternative method 1: If you don't have an audio tape recorder. Use two stopwatches. Start the first when the star dims. Star the second when the star brightens. Listen to WWV (by radio or phone - see below) and stop both watches on the tone for 10 seconds after the start of a UT minute. Note down the UT time at which you stopped both watches and the time recorded on each watch. You can now determine the time of the start and end of the event by subtracting the appropriate stopwatch time from the UT time on which you stopped the watches.

If you don't have two stopwatches you can use a stopwatch with a split timer instead. Start the stopwatch at the beginning of the event, hit the split button at the end of the event. Note the split time (this is the duration of the event). Stop the watch on the WWV second mark as above.

Alternative method 2: If you don't have access to a short-wave radio or a phone while in the field. Try to set your watch to WWV before you go into the field. While in the field follow Alternative method 1 but use your watch as WWV and stop the stopwatch(es) when our watch reaches an 0 seconds of a minute.

Easiest method: If you don't have time to mess with any of the above... Just find the star and see if it disappears - perhaps you can time the duration of the event with a stopwatch and note the approximate time. This is still useful data.


WWV is a shortwave radio broadcast by the US National Institute of Standards and Time (NIST).  WWV and WWVH are broadcast at 5, 10, and 15 megahertz.  Portable shortwave receivers are still avaiable from some electronics retailers.

Steve Preston

2019 Feb 5