what are the focal lengths of the lenses used to make a descent telescope?

4 Comments

1. fortitudinousskeptic says:

   May 6, 2010 at 4:38 am

   There are few hard and fast rules with the focal lengths on this. It’s a matter of preference.
   I’m assuming that you wish to design a homemade refractor telescope. I’ve done this before. You will need a long tube painted flat black inside and two lenses.
   The lens farthest from your eye, the objective lens, needs to have a fairly long focal length, its focal length will be almost as long as the tube. That lens should be a few inches in diameter to be big enough to collect a lot of light. Also, if it has a long focal length, it won’t shrink the first image so much, the one that is magnified by the second lens.
   The second lens, the ocular lens near your eye, would need to have a much shorter focal length. This allows you to get a very magnified virtual image from the real image made by the first lens.
   As long as the ocular lens has a shorter focal length than the objective lens, you can make a working telescope, but it will work pretty well if the ocular lens has a focal length of maybe 3cm or so and the objective lens has a focal length of half a meter or so.
2. ftm821 says:

   May 6, 2010 at 5:19 am

   the most important factor used to determine a “descent” telescope is it’s light gathering power. i.e. the objective diameter.
3. minuteblue says:

   May 6, 2010 at 6:14 am

   The actual focal length is proportional to the aperture of the lens or mirror so focal ratio is more appropriate to talk about.

   Telescopes generally don’t get “faster” than f/4.5 and “slower” than f/12

   The f/4.5 would typically be a very large aperture telescope, probably a dobsonian, or a smaller telescope which is made specifically for richest field viewing or astrophography. The f/12 would probably be a folded design, such as a schmidt cassegrain, or a very long refractor. People with Schmidt Cassegrains usually do do alot of astrophotography but they sometimes use a focal reducer or other methods to make up for the long focal length.

   For the moon, you’d want a medium focal length. Too short and you get distortion and perhaps more chromatic aberration if you don’t have an apochromatic lens system, and too long and it doesn’t fit in the field of view.

   f/4.5 or f/12 is the number you get when you divide the focal length by the diameter of the lens or mirror. For example, a telescope that has a 114mm mirror or lens, and a focal length of 900mm is 900/114 = 7.89 and so f/7.89