What follows are some notes for those people intending to observe the occultation by Iolanda across northern New Zealand on the evening of Saturday March 20, 2004. They have been drawn up because the single greatest reasons for failure at events of this sort (apart from the weather) are that either the observer has equipment failure, or that s/he is not ready for the event.

On a previous night:

• Clearly identify the field. Once the field is identified practise by doing a complete dummy run for the event. Comprehensive charts are available here.
• Ensure that all your equipment works, and that radio/tape recorders have fresh batteries and will function without problem.
• The IRL talking clock (0900-45678) will give you one minute’s worth of time pips, at ten second intervals, for your 99 cents. If you’re using the IRL clock make sure that you can clearly record the time pips on your tape. If you’re using stopwatches practise so that you are able to start or stop them accurately to a known time pip.

On the night:

• The event will occur at around 09:59:00 +/- 20 secs NZDT for Hawkes Bay, and up to 40 seconds later for Whangarei.
• Leave plenty of time to acquire the field. Aim to be fully prepared with the star identified and all equipment in working order by 9.30 pm – half an hour before the event. The star magnitude is 10.4 and the asteroid 13.2 so don’t expect to see the asteroid unless you have a large scope. If the asteroid is below your limiting magnitude then the star will disappear completely when it is occulted. In a large scope the star will dim to the asteroid’s brightness for the duration of the occultation.
• Put your maximum concentration into the 30 seconds surrounding the event time for your location. When the occultation occurs it is likely to be VERY SUDDEN and – no matter how much you think you are prepared – it will surprise you. (This is the case even for experienced occcultation observers). The ‘surprise factor’ – where it takes some time for your brain to react and send signals to your mouth or finger – is your reaction time. Excessive reaction time is the greatest source of error in occultation observations.
• Timing methods are described here. While using a tape recorder and shortwave radio is best, you can also use a tape recorder and the IRL talking clock on 0900-45678 as follows:
  
  
  • First set up your equipment and acquire the field
  • Around 5-8 minutes before the event phone the talking clock and record a minute’s worth of time pips on your tape.
  • Return to the telescope (remember that the field will have drifted if you don’t have a drive!), leaving enough time to reacquire the field. Observe for several minutes either side of the predicted time.
  • After the event phone the talking clock again and record another minute’s worth of time pips.
  • Later, using a stopwatch, check the length of time from the first recording of the talking clock to the second recording. (This helps take account of any stretching in the tape). Then use the stopwatch again to determine the points between the two sets of time signals at which the events occurred, so deriving the time of the disappearance and reappearance.
  
  If you are using two stopwatches and the IRL clock:
  
  
  • Start the first watch at the disappearance
  • VERY QUICKLY swap to the second watch without taking your eye from the eyepiece. Alternatively, hold the second watch in your other hand.
  • Start the second watch at the reappearance – but BE SURE NOT TO STOP THE FIRST WATCH THAT YOU HAVE JUST STARTED!
  • After the event, stop both watches to a known time using the IRL time signals. If you have practised beforehand, by getting into a rhythm you should be able to stop the watches to within 0.1sec of the time signals.
  • Note the time signals to which you have stopped the watch, and then work out the time of the events.
  
  
• If the asteroid is circular the maximum expected duration is expected to be about 4 seconds. If the asteroid is elliptical (as is likely) and you are near the centre of the track your occultation may be longer or shorter than this depending on the orientation of the asteroid relative to the track across New Zealand.
• If you are towards one edge of the path you may experience a very short occultation, and in some cases almost a blink. (This will make timing difficult if you are using two stopwatches). For very short occultations the star may fade out and in again rather than just instantly blinking off then on. This fading may be due to Fresnel diffraction at the limb of the asteroid. If you experience this, try to gauge how long the fade in/out took.
• Following the event fill out a copy of the report form here and return it to the address indicated. You must do this whether you saw an occultation or not as negative observations can be crucially important in defining the exact limits of an occultation track. (For an example of this see the results from the 2004 Feb 25 Gudula occultation).
  
  

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