|
|
BEFORE THE EVENT
It is very important with planetary occultations to accurately identify the star involved several hours, or days before the event. A good finder and a very low power (wide field) eyepiece for the main scope help in easily locating this. Know the angular diameter of the field of view of your finder and main telescope when various eyepieces are used; the moon's approximate half degree diameter can be used for calibration. Remember that the presence of the planet may affect the appearance of the field from a previous night.
OBSERVING
A good rule of thumb is that you should monitor the star for two minutes, plus 30 times the predicted central duration both before and after the predicted time. This will help check for secondary occultations which could be caused by the presence of possible satellites of the planet. For example, if the predicted central duration is 20 seconds, the total observing time should be about 24 minutes centred on the predicted time. For very long duration events do not try to observe for longer than 30 minutes (because concentration is difficult for longer than this) - however do at least try to cover ten times the central duration before and after. Be especially alert over the five minute interval centred on the predicted time for your location.
Note that to observe these events you DO NOT need to be able to see the planet itself! You only need to see the star. If the asteroid is too faint to be seen in your scope, then at an occultation the star will simply disappear!. If on the other hand the planet is just visible but fainter than the star, you will see the star's brightness suddenly decline.
TIMING METHODS
Visual timings should be made in the same way as for grazing occultations - i.e. by using several stopwatches stopped (or started) to a radio time signal, or preferably, a tape recorder recording both the observer's comments ("off", "on", etc) and radio time signals. The short wave radio time signal station most commonly used is WWVH Hawaii on 5, 10 and 15 MHz. (Note that interference from the Japanese station JJY can sometimes be heard on some frequencies). If short wave signals are unobtainable, leave the recorder running and record the next (or preceding) hourly time pips from an AM radio station - be sure to use a C90 tape, and also make sure that you record a reputable station! Some private radio stations broadcast time pips that bear almost no relation to the true time. Within New Zealand you can call the IRL talking clock on 0900-45678 which gives you about 60 seconds worth of pips. (Calls cost 99c). In Australia you can call the Telstra Talking Clock, although continued monitoring of this service has shown that it can be highly unreliable with time pips sometimes out by many seconds. Always be sure to use fresh batteries in your tape recorder, to minimise the possibility of variation in tape speed over long intervals of time, and check that all equipment is operating satisfactorily a sufficient time before the event. More detailed information on timing methods and reporting is also available.
AFTER THE EVENT
You should ALWAYS fill in and return a report form, whether you see an occultation or not. Even observations of "no event" are potentially valuable, because they can help to determine where exactly the occultation track did in fact pass.
Howver, should you see an event, you should first determine from the tape the time at which the disappearance or reappearance occurred, to an accuracy of preferably 0.2 second. Note all the necessary details on the minor planet occultation recording form, and return it as soon as possible to the R.A.S.N.Z. Occultation Section.
A FINAL NOTE
You should not expect to see a minor planet occultation particularly often, as the chances of a shadow perhaps 100 km wide crossing your observing site can be quite small. However consistency pays off, and there are now several regular observers who have seen more than one occultation. Based on past events you might expect to have at least one success in about every twenty to thirty events monitored.