The M.A.S. Newsletter
Journal of the Mauritius Astronomical Society
August 99
The Next Meeting:
...on the night of Thursday
12th to Friday 13th of August. Rendezvous next to the chapel at Cap Malheureux at 1:00 in
the morning of Friday 13th for the observation of the Perseid Meteor shower. This year the
first peak of activity is expected on the 13th at 2:00 in the morning. Still on the 13th,
two more peaks are expected at 8:00 and 16:00 but will not be observable from Mauritius.
The sky this month:
Just before sunrise on the 5th, the Moon is at about 2° of Saturn, then on the 10th, it is very close to Mercury. Venus will be at inferior conjunction (lined up with and between the Sun and the Earth) on the 20th. On the 22nd, if you have a telescope with a magnification of at least 60 times, try to have a look at Saturn whose rings are at maximum opening (21°) for 1999.
The Barn Door Mounting:
Some of you may have tried photographing the stars and constellations after reading last month's newsletter. The number one limitation of such photos is the short exposure time due to the rotation of the Earth. To overcome the problem posed by the apparent motion of the stars across the sky, you need to move the camera so that it keeps its aim among the stars. You need a tracking device which compensates for the Earth's rotation. You can build a simple, manual tracking device for an ordinary camera. The quality of your photos will then depend on how accurate you are at aligning it with the axis of the Earth and at adjusting the speed of the tracking device. Such a simple tracking mount is called a barn door mounting and is very popular among amateur astronomeurs for wide field photography. It consists of two plates (wooden or other) hinged along one edge. One is fixed and the other movable. The assembly is oriented so that the hinge is aligned with the Earth's rotation axis, i.e. pointing to the South Pole. The camera is attached to the top, movable platform, which is driven by a screw so that the camera follows the stars in their diurnal motion (about 15° per hour). When looking toward the south along the hinges, the movable platform should be made to rotate clockwise i.e. to the west (toward sunset). Being driven by hand, it has an advantage of being tranportable provided of course it is not too bulkily built. The diagram illustrates a possible design. If you wish to discuss a specific design, please contact me at the next meeting.
Serge Florens, Secretary