It's really a practical matter. The field of view for a camera is going to depend upon the focal length of the telescope. The smaller the telescope, the larger the field of view can be. The f/number is the ratio of focal length to telescope objective diameter. Typically, this can range from around F/4 to around F/14 for typical telescopes commercially available. This value also corresponds to how 'fast' the telescope tends to be. A schmidt camera style telescope might be around F/2 to F/2.2 which is faster than an F/4. Some planetary type telescopes might be somewhat higher F/numbers. Going one way or the other in F/number usually results in lots of optical problems (or practical problems).
A 100mm telescope is small compared to most professional models where light gathering is often the problem. It's even quite small for the serious amateur. It will provide for maximum field of view for a given camera.
Since the advent of the ccd cooled astronomy camera as a sensor, the need for large telescopes has been reduced. The incredible sensitivity of the ccd (and linearity). An amateur in mediocre skies can roughly duplicate the results of very large telescopes with film.
This exoplanet hunting is a major numbers game. Having fairly large field of views for imaging permits more stars to be viewed at one time and provides for easier robotic pointing and more reference stars.
As a simple example, I have a 1/3" diagonal camera - definitely small but it works and I can afford it. The 1/3" really refers to the size of the outer diameter of a vidicon imaging tube barrel that this might have replaced once upon a time. Actual sensitive diagonal distance of this ccd chip is more like 1/4" but that is the way they do things with ccds. I have an 80mm F/6.7 telescope and an 11" F/10 telescope that the smaller one rides on. I also tend to use what is called a focal reducer which effectively reduces the focal length of the telescope by a factor of 1/2, which doubles the field of view in each direction so I get 4 times the sky area. I almost always use the focal reducer, even with the small telescope. If I didn't, images with the 11" telescope would not contain complete objects for many of the more popular viewing objects as they are too large to fit. For example, even with the focal reducer on the 11" telescope, I couldn't image the moon in one frame or objects anywhere close to that size.
As for the exoplanet searth, using proper technique, an amateur with decent amateur equipment is fully capable of detecting one of these transit planets.
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07-August-2007, 02:41 PM
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