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80 mm short-tube telescopes are made by several companies and sold
by many more. My ST-80 Telescope was purchased from
and Binocular in California.
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Orion Short-Tube 80 Telescope
Features and Specifications
This type of telescope is made by a number of manufacturers and marketed by many more. I bought mine from Orion Telescope and Binocular in California.
The telescope has an aperture of 80 millimeters and a focal length of 400 millimeters. This gives a focal ratio of F/5, and a fairly wide field of view. Telescopes with these characteristics are often referred to as "Rich Field" telescopes, as their view will encompass a significant area in the heavens when used with an appropriate eyepiece.
For example, a 20 mm focal length Plossl eyepiece will give a magnification of 20. Calculate: Objective focal length divided by eyepiece focal length, (400 / 20) = 20
The field of view will be about 2 degrees. Calculate: (apparent field of view of the eyepiece / magnification), (50 / 20) = 2.5
The eyepiece's exit pupil (the diameter of the light beam) is about 4 mm. Calculate: (objective aperture / magnification, (80 / 20) = 4
Young eyes with good dark adaptation have pupil sizes of about 6 to 7 mm, but I'm a bit older, have macular degeneration in one eye, and had radial keratotomy in both eyes years ago (today's Lasix is a better procedure for surgical correction of myopia), so the clear aperture of my eyes is somewhat less. In fact, if I use my 40 mm eyepiece, the exit pupil of the telescope is about 8 mm, and I see broad diffraction spikes caused by the radial keratotomy scars.
More about this telescope
Yahoo provides a facility called "Yahoo Groups" which provides a convenient way for people to share information about common interests. One of these groups, 80f5, is dedicated to telescopes of this general design.
One of the members of this group, "yeldahtron" has recently posted about the benefits of the inverted mount for this telescope when using a typical photographic camera tripod. His inspiration is behind this mounting technique.
I had previously used a system of counterweights to balance the ST-80, but it had several disadvantages:
Thanks to Yeldahtron, and his reference, a description in Sky and Telescope magazine which had inspired him.
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