Occultations

Introduction

Occultation observations are the observations of astronomical objects that are obscured by other astronomical objects.  Examples (and the traditional occultation events) are the observations of stars that disappear and/or re-appear from the limb of the moon (moons edge).  Example 2 is known as a Grazing occultation where a star 'grazes' the dark limb of the moon, disappearing and re-appearing as it is obscured by features on the moons surface.  The third example is an Asteroidal Occultation where an asteroid passes in front of a star.

These observations have traditionally been undertaken visually with a reasonably powerful telescope (8" is a good average) and the use of a Shortwave Time signal (from Hawaii now that our Australian borne signal was shut down a couple of years ago).  Ideally the user will record the observations on tape (that also records the time signal) calling the disappearing and re-appearing event(s).  The best observers can really only record events to +/- 0.2 seconds.  Not bad when one considers the precision this actually represents (in some cases representing astrometric positions within milliarcseconds - suprising many professional astronomers)

But, there are inherent problems with this manual observation.  Sky conditions, limiting magnitudes, the temperature and fatigue can make the observers life hell.  The star may be barely observable, the temperature may be so cold that they eyes water or the observer blinks at the point of disappearance. (All these details are required as part of the detailed observation report for each event)  Crowded star fields can make it difficult to pick the right star in time for the occultation or confuse the observer if they look away for a moment.

Typical setup - the old days Occultation setups are portable.  They are portable because some events, typically grazing and asteroidal occultation's are pretty few and far between and will almost never occur over ones own home.  As such, the observer needs to pack up shop and drive to a location that is predicted for the event to occur.  Most observers are happy to respond to events that occur within 150-200km. This image shows my original setup.  A Computer controlled Telescope that makes it easier to find the target star within a short period of time, a Shortwave Radio for the WWV Timesignal and a tape recorder to record the time signal and my occultation calls.

Today

Things have moved on in recent years as observers have sought more precision.  To this end observers have experimented with and adopted some new higher precision techniques.  The first is a method to actually visually record the event.   The beauty of recording the event is to ensure that the event is not missed.  Digitally recording the event also allows for very accurate measurement.   Methods of visually recording the event started with low light video cameras.  These were further modified to allow manual adjustment of the signal gain thus improving the cameras limiting magnitude.  A second method was developed to use cooled astro ccd cameras.  These astro CCD's are still cameras so to use them required the scope drive to be turned off while the shutter was left open.  This caused the star to trail across the CCD chip.  A bright trail that would be interupted when the star was occulted.  Measuring this gap in the star trail based on the time of start and end of the star trail produced highly accurate results and these cameras cane see events that are far fainter that the eye or video camera can see.

The second part of the recording is catching the time signal as well.  The problem for us here in Australia is the loss of our own time signal service.  If the airwaves are not kind it can be impossible to get the signal from Hawaii.  Some ingenious observers have come up with numerous solutions.  The first is a beeper box.  This is a box that beeps the same time signals as WWV synchronised to a time signal at some stage.  Whilst it is left on it maintains its signal to a few milliseconds and provides both an audible and visible time signal.  The downside is that without a 1pps GPS trigger, the box must be manually synchronised so the timing accuracy may still only be +/- 0.2 seconds - but this is a fixed error rather than a variable error.

The next is a 1pps (pulse per second) GPS feed.  This is a very accurate time signal source but reasonably expensive but combined with Video recording (and time values fed directly into the video feed) makes extremely accurate observations possible.

I have used the CCD drift method but my camera lacked a shutter so it had to be simulated with a matt black board over the face of the telescope.  The 'shutter' was manually opened and closed resulting in errors of around 0.2 seconds in the timing.  I now have a modified low Light CCD Video camera and beeper box.  These low light cameras are very small indead (as can be seen in the image below.  The C mount fits inside the eyepiece adapter of any telescope)

So, what has changed.  Well the occultation kit is still portable.  I still use a computer controlled Telescope, the small low light CCD Video camera to record the event connected to a 5" B/W monitor then connected to a video recorder (run off a 12 v Inverter) or have a handheld video Camera recording the event via the monitor screen.  I still take my SW radio in case I could not synch my beeper box and a tape recorder as backup.  A GPS is also necessary to accurately record my location in the field.

This kit, all up, is pretty expensive but the advantage is - it will last a very long time.

● Email Me: higginsdj at bigpond dot com ●