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 http://lunar-occultations.com/iota/iotandx.htm. Or, contact me via email and I'll help you if I can.

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

(From IOTA's web page)

Accurate time signals are needed for timing occultations. The time provided by the telephone company in most North American cities is not accurate enough; you should not use it. In the U.S.A., accurate time signals, from the U. S. Naval Observatory master clock, can be obtained by calling 900-410-8463; the call should be placed via AT&T to ensure use of land lines, which will give an accuracy of a few hundredths of a second, more than sufficient for visual timings. If the call is not made via AT&T, or if the National Bureau of Standards' WWV 303-area-code number are used, the call might (or might not, you would not know) be routed through a geosynchronous satellite, causing a quarter-second delay, which is unacceptable, even for visual timings. In the Washington, DC area, the USNO master clock can be reached with a local call to 202-762-1401, but those outside of the DC toll-free area should not use that number, because it might go through a satellite. Some other nations, such as the U.K., also have accurate time signals available by telephone.

Often more convenient than telephone time, especially for field use such as grazing occultations, are short-wave radio time signals, such as WWV and WWVH at 5, 10, and 15 megahertz. Radio Shack used to sell a convenient receiver, the "Weatheradio-Timekube", for these frequencies for about $40, but they are no longer available. Some grazing occultation expedition leaders have extra Timekubes that they may be able to loan. A more expensive alternative (kit, $59; wired board, $99) that requires some assembly is the Hamtronics' WWV Receiver, described on pages 48-49 of the January 1996 issue of SKY AND TELESCOPE. It gives a much more reliable signal than similarly-priced general-purpose short-wave receivers. If you buy a general-purpose short-wave radio, try to get one that at least covers 5 and 10 megahertz, the best nighttime frequencies. Also, one with digital tuning is highly recommended; many observations have been lost while observers tried and failed to find a weak time signal with a dial (analog) tuner. Reception with Timekubes and general-purpose receivers can be improved by connecting a copper wire, at least 50 feet long, to the receiver's antenna, and stringing it out approximately perpendicular to the direction of the transmitter (WWV is in Ft. Collins, Colorado, and WWVH is on Kaui, Hawaii). The wire should be suspended above the ground and attached to a tree or a wooden (or other non-electrically-conducting) post. Time signals are also broadcast at 5, 10, and 15 megahertz from Japan, China, and Russia, so WWV receivers are also useful in much of the rest of the world.

NOTE: if you don't have access to a portable short-wave receiver but you know someone that has a short-wave receiver, ask them to use an audio tape recorder to record WWV along with a strong AM radio station. Then you record the AM radio station in the field and correlate back to WWV later.

Steve Preston
 

Nov 5, 2007