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
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
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: