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Choosing a Telescope: Is bigger better?

A common comment for visual astronomy is that bigger is always better. The bigger the telescope, the more light it gathers so you can see fainter objects and finer details better. Another comment is to buy the scope you'll most likely use. You can buy a big monster scope but if it's too heavy or too much of a chore to move and use, you will tend not to use it and it will collect dust. But for video astronomy bigger isn't necessarily better. A bigger telescope can create tracking problems.


As telescopes get bigger, their focal length gets longer. That can make it more difficult to track an object through the sky without distorting the image. For visual, it's not usually a problem but with a camera, you want as steady of an object as possible. A camera exposes for a certain length of time and you want it to be steadily tracking that object otherwise it will distort the image. If your mount doesn't track well, you'll get elongated stars that start to look like eggs or dashes.


Also, bigger scopes require beefier mounts, which increases their cost. For astrophotography, the rule is generally keep your telescope size to about 50% of the mounts stated capacity. So for example, if your mount is capable of supporting 30 lbs of weight, you shouldn't put a telescope greater than 15lbs on that mount (a 30 lbs scope is fine on that mount for visual only).


Video astronomy isn't as stringent as astrophotography but you should try to keep the scope size to about 70% of the mount's capacity. So if you had a monster newtonian tube that weighed 40 lbs, you'd need a mount that could support 60 lbs of gear (they typically sell 30lbs, 45 lbs, 60 lbs). As the weight capacity of the mount goes up, the cost also goes up.


Until you gain experience, try to keep the size of the tube to 8" or less. The smaller the telescope, the more forgiving it will be with tracking. A great starter size is an 80mm refractor telescope or a 5" to 6" newtonian. For a cassegrain, I'd probably tend to stick with an 8" since that's probably the biggest seller and the size with the most available accessories (e.g. focal reducers).


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