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The Aldebaran Occultation Series

Last modified on November 12, 1998 by A. Richichi

A series of lunar occultations of the bright star Aldebaran (Alpha Tau) has begun in 1996, and will last until the year 2000. Many observatories around the world will follow these events, with the purpose of obtaining accurate measurements of the angular diameter of this red giant, from the recording of the lightcurve during the disappearance (or reappearance) behind the limb of the Moon.

At the same time, it is hoped that also amateur astronomers will be able to contribute significantly to this campaign. Here you can view ground track charts for the years 1996, 1997, 1998, 1999/2000 (obtained with a PC program written by B. Stecklum). If you need help or wish to communicate your reports, please contact us at We will be glad to provide you with predictions for your site (don't forget to include the coordinates in your request), and help you with the technical details. Observers from North America may find detailed info in the IOTA homepage.

This is a short summary of results and reports of which we are aware:

  • 6 November 1998: both disappearance and reappearance recorded under very good conditions at the TIRGO observatory by A. Richichi. Both events were recorded in a narrow CVF filter at 3.55 micron, where the light is expected to come from the stellar photosphere without any contamination by atonic or molecular lines. A preliminary analysis confirms the findings of the previous event observed in February (see below). We are now waiting for the next opportunity (December 30, 1998) before starting a combined scientific evaluation of the results. You can have a look at the data of the disappearance, and at those of the reappearance of the Nov 6, 1998 event. The disappearance was recorded on the bright limb of the Moon, which is an extremely rare opportunity at visual and near-IR wavelengths because of the extreme luminosity of the Moon. In our case, this was made possible by the brightness of Aldebaran, and by the choice of a very narrow filter in order not to saturate the detector. The event was followed by the author also on the TV screen, and was by far the best "occultation" experience in his many years of experience!
  • 5 February 1998: disappearance recorded under favourable conditions at the TIRGO observatory by A. Richichi. The same event was to be observed at the Calar Alto observatory by S. Richter but was clouded out. The TIRGO event was recorded with the fast IR photometer in a narrow CVF filter centered at 2.22 micron, thus observing the continuum emission of this K5 star. The almost grazing conditions (contact angle=84 degrees) made it possible to achieve an excellent sampling of the lightcurve, which is shown here. A preliminary analysis confirms an angular diameter of about 20 milliseconds of arc as previously known, but also indicates a significative departure from a simple circular disk model.
  • 14 March 1997: the disappearance of Aldebaran is observed at the Calar Alto 3.5m telescope by G.P. Tozzi (with remote help from T. Herbst and A. Richichi, this latter clouded out at TIRGO). The instrument was the IR camera MAGIC in low-resolution spectroscopic mode: in this way, the disappearance of the full spectrum in the H and K bands was recorded. A preliminary movie can be viewed. Work is in progress to analyze the data properly.
  • 14 March 1997: L. Fabbroni and F. Lisi, using the 35cm telescope of the Arcetri Observatory equipped with a visual photomter modified for this purpose, record the disappearance of Aldebaran. Unfortunately, the data are affected by several sources of noise; some have a characteristic frequency signature and it should be possible to remove them at a later stage. At this time, only a very crude plot is available, with the points rebinned to 40ms sampling (original is 1.25ms).
  • 14 March 1997: report from E. Trunkovsky.
  • 14 March 1997: report from M. Kretlow.
  • 14 March 1997: report from W. Beisker.
  • 1 October 1996: observation under difficult conditions by E. Trunkovsky with the 70cm telescope of the Sternberg Institute in Moscow. Preliminary results can be viewed as text and PS figure (this latter available also as gzipped PS )

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