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Observing Techniques - Needed Equipment

As the magnitudes of the occulted stars are usually far below 10 observations of grazing occultations do not require large telescopes. Even a 6 cm refractor or an 11 cm reflector do this job for most graze events. A stable telescope mount and a good tracking device are much more important than large apertures. A good instrument for stellar occultations by the moon must be transportable and easy to set up someplace in the countryside.

The biggest technical challenge is the recording of the event. For visual observations the use of a simple stop watch is not sufficient since that way only the first of probably several dis- and reappearances can be recorded. Special stop watches with a memory device allow the recording of maybe up to 10 time stamps. Any kind of voice recorder nowadays availably in almost every mobile telephone can meet this purpose as well when a time signal is recorded simultaneously. The most simple way to record a sufficient time signal is by some beep or knock that is being recorded every 10 seconds by an assistant observer always watching a clock that was set to UTC shortly before the event. Also a calibration of the recording device through an acoustic time stamp at the beginning and one at the end of the recording would be sufficient if no assistant is available. The voice recording also allows spoken comments while the event is going on. These comments can be immediate time corrections due to a longer reaction time of the observer at the eyepiece or remarks about possible uncertainties of noticed dis- or reappearances, for example resulting from bad seeing or partial disappearances of a double star system.

Video recordings allow a much higher timing precision but also require much more and rather expensive electronic equipment. A sensitive video device must be connected at the telescope’s focal plane increasing the necessity for a stable telescope mount and good tracking. A recording device is necessary, either the classical analog or digital tape recording or by use of an A/D-converter-software when feeding the analog video signal via a USB-port directly to a laptop next to the telescope. The latter technique must be fast enough to guarantee that not a single one of the recorded video frames is lost. Also the hard disc must be fast and empty enough to cope with the high amount of video data to be saved.

Timing along with the video recording presents the next problem to be solved. Video time inserters are electronic devices that feed the time signal directly into the video frame. In those countries where a radio time signal is distributed an appropriate receiver is included in the time inserter. Recently video time inserters have become popular that also connect to a GPS-antenna thus receiving the precise and universally available GPS-time and displaying it along with the measured geographical coordinates. These devices are especially well suited in countries with no radio time signal. In Iran Aria Sabouri has developed a system to feed the laptop’s system time into the video recording.

No matter which technical effort is chosen to approach observations of grazing occultations anyone curious in these phenomena will not be disappointed. IOTA will offer any help to support this work. Predictions of the best events are published regularly in the different IOTA newsletters for different regions in the world. Anyone interested to find out still more about upcoming events in his area can obtain IOTA’s GRAZPREP software (via the IOTA/ES homepage) that assists on any step towards a successful observation. IOTA also explains the forwarding of occultation reports in order to have all data collected and reduced to utilize and evaluate successful timings. The author can also be addressed for any questions that arise.

Dr. Eberhard Riedel, IOTA/ES

Here you can download the article as a pdf file.

Page 1 (Introduction ... The scientific purpose)

Page 2 (Circumstances at the lunar limb; Observing precision requirements)