Birchardville Observatory
Site Always Under Development
80 mm short-tube telescopes are made by several companies and sold by many more. My ST-80 Telescope was purchased from Orion Telescope and Binocular in California.

Click on the picture to see a larger view, and use your browser BACK button to return to this page!

Inverted Mount Orion ST-80

The idea of mounting the telescope in an inverted position is to bring its center of gravity much closer to the tripod's elevation axis. Inverted Mount Orion ST-80

This idea was published earlier (May 2005) in Sky and Telescope magazine. I adapted it to materials I had on hand. Inverted Mount Orion ST-80

Materials included this perforated steel angle stock, a 1/4-20 bolt and two 1/4-20 nuts. Big expense! Side view. Inverted Mount Orion ST-80

Front view. Inverted Mount Orion ST-80

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:

  • 2+ pounds (1 Kg) of additional weight to haul around
  • The weights interfere with the tripod legs, restricting the ability to follow objects
  • The weights at the end of their arms adds angular moment to the telescope, causing long settling times (lots of vibration) when the telescope is moved or focused
By simply mounting the telescope upside down, it moves its center of gravity so close to the axis of elevation that counterweights are not needed. The damping or settling times also becomes much shorter.

Thanks to Yeldahtron, and his reference, a description in Sky and Telescope magazine which had inspired him.

Go Back to the Astronomy Main Page.

Contact Info:
Name:Dan Janda