Last updated 22 Apr 2017

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Minor Planet Occultations

If a faint minor planet happens to pass directly in front of a brighter star then for a short time the starlight will be blocked out and the observer will just see the fainter light from the minor planet as the shadow from the minor planet passes over the observer.

Measurement of an occultation involves recording the times when the starlight disappears and then re-appears again. This gives a lower bound on how big the minor planet is, but if several observers in different places manage to record the event it allows a representation of the shape of the minor planet to be constructed.

For this to be done well, the start and end times must be measured very precisely and this is probably best achieved by amateurs by using video equipment, taking many frames per second and with the frames timed against a reliable standard such as a GPS receiver.

Without any video equipment available, a different technique called drift-scanning is used at Great Shefford. This only requires a telescope & CCD camera but still relies on accurate exposure timing for good results.

The telescope is positioned at a point slightly west of the star to be occulted, then the equatorial drive is turned off. The stars then appear to drift west due to the Earth's rotation. An exposure is started slightly before the time the occultation is predicted to happen and is timed to end just afterwards. If the occultation is captured then the trail of the target star will show a break where the minor planet obscured it. Measurement of the start and end of the star trail and the start and end of when the star was obscured allows an accurate determination of the start and end of the occultation.

The technique is described in detail by John Broughton, Reedy Creek Observatory, Australia here and he includes links to software  programs ScanTracker and Scanalyzer he developed and makes available free of charge. Scanalyzer is used at Great Shefford Observatory to determine occultation start and end times.

Positive results obtained from Great Shefford are listed below:

(130) Elektra on 20 February 2010


(80) Sappho on 04 June 2010


(105) Artemis on 18 August 2010


(589) Croatia on 20 December 2015


(268) Adorea on 09 October 2016


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