How can you determine the focal length of a telescope?
Brack_871 asked:
I have a refractor telescope and I am trying to figure outs it focal length. It is an old Zeiss Telescope and I have no literature on it. Here is what I know:
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I have a refractor telescope and I am trying to figure outs it focal length. It is an old Zeiss Telescope and I have no literature on it. Here is what I know:
The diameter of the front lens is approximately 100 mm.
The approximate llength of the telescope from lens to the back is about 4 and 1/2 feet.
Anyone out there able to help me with this?
April 19th, 2010 at 2:40 am
When the Moon is back in the evening sky, point the telescope at it and measure how far the image of the Moon is from the end of the focuser. Then measure the distance from the end of the focuser to the objective lens. Adding the two together will give you the focal length. Zeiss made fine refractors with and aperture of 110mm and a focal length of 1650mm. I used one of these as a teenager, and it was one of the finest telescopes I have ever used. If this is such a telescope and in good condition, I would be interested in purchasing it. The catalog below may help you identify it. Geoff G