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Astrophotography - Following on from last month's article on beginners astrophotography Michael A. Covington now goes one stage further and explain about telescopes and how to use them to view stars and planets.
If you don't already own a telescope here's a quick rundown of the specifications and what to look for in a telescope.
The diameter of the telescope is all-important. Larger-diameter models give brighter images because they allow more light through. The images are also sharper with less diffraction. Smaller telescopes are easier to transport.
Models with a diameter of 2inch to 5inch are ideal for beginning amateurs (or anyone requiring portability). 5inch to 12inch models are more suited to the advanced amateur and 16inch to 30inch are used by small observatories, such as the University of Georgia.
This is the power of a telescope and you can get any magnification with any telescope by changing eyepieces, but you might not want to.
The magnification = focal length of telescope divided by focal length of eyepiece.
Use low power almost all the time. You can't magnify detail that isn't there, so when you make the image bigger, it does not necessarily show any more detail. A magnification of 20 to 40 power is fine and will generally be achieved using a 20- or 25-mm eyepiece.
With larger amateur telescopes (5-inch to 8-inch), you can go over 100x. The University of Georgia's 24-inch telescope usually operates at 200x.
Eyepieces vary in quality. Those shorter than 8mm are usually very hard to use (your eye needs to be impossibly close). Most telescopes can be improved by adding high-quality eyepieces. Everybody needs an eyepiece somewhere in the 20mm to 32mm range. A Barlow lens doubles the magnification while preserving comfortable eye distance.
Steady air is necessary for good telescope performance. You will get sharper images if you set up the telescope outdoors at least 30 minutes before you plan to use it, so that temperatures can equalize. Observe over grass if possible; avoid hot asphalt, even after dark. Never observe through a window.
What to look at
The Moon is a rewarding object for any telescope. It is lit from a different angle every day, and you can watch it for a long time without seeing the exact same view twice. Rkl's Atlas is very helpful.
The Planets are rewarding once you train your eye. Telescopic views from earth will not look like Hubble photos, but with care, you can learn to distinguish stripes and the Red Spot on Jupiter, the rings of Saturn, etc. Planets are bright and can be observed even in the presence of moonlight and city lights.
How to find things
Learning the sky takes practice. You must learn the constellations and learn to use a star map. (If you can't point your finger at the Andromeda Galaxy, you can't point a telescope at it either.) A computerized telescope helps a great deal, but you must still be able to identify a few bright stars in order to initialise it at the beginning of each session. Anyhow, learning the sky was why you got into astronomy, wasn't it? Monthly maps in Sky and Telescope and Astronomy Magazines are very helpful.
To take a picture of the stars at night, try this: Using ISO200 or ISO400 speed colour slide film, place the camera on a steady tripod, aim it at the starry sky, and expose 20 seconds at f/1.8. This technique also works for photographing bright comets. Color print film is OK if you can get good prints made (most minilabs will print the pictures too light or too dark).
Michael A. Covington is a Ph.D. (Linguistics) and Associate Research Scientist at The University of Georgia. He is also Senior Member of Institute of Electrical and Electronics Engineers and contributing editor of Poptronics Magazine.
Michael is author of Astrophotography for the Amateur, published by Cambridge University Press ISBN 0-521-64133-0 (hardback), 0-521-62740-0 (paperback)
Available from Amazon.Com and other fine booksellers and co-author with Jay M. Pasachoff of The Cambridge Eclipse Photography Guide, Cambridge University Press.
Go to http://www.covingtoninnovations.com/telescope