[masraum]

Originally Posted by Pazuzu

What measurement is 10 inches? The tube itself, or the mirror? Did you check?

Based on the era (1970s), that would be an 8 inch f/6 or 8 inch f/8. Probably not a 10 inch, but it's possible. If so, 10 inch f/6. How long is the tube (most Newt tubes are very close in length to the focal length, the secondary is set back from the edge the same amount as the tube diameter. You only need to be within 2 inches or correct to know.

The secondary is small, which means a tightly shaped light cone, which means an f/8 system. I bet it's 8 inch f/8, with a tube diameter of 10 inches, and a length right around 60-65 inches.

Mike's the man!

I too noticed that the tube seemed long and the secondary seemed small (based on my experience with an 8" f5.9).