Lockwood's article says nothing about the secondary distance, it is not exact. I would not use it.

In my post, I also used 11" as the secondary to focus distance.

Marcus, what is that distance on your telescope? That distance is critical.

The formula that I use is this one: s = (__D - a) x d __ + a

f

s = ?, the minor axis of the secondary

D = 12", diameter of primary

d = 11", distance of secondary to the image plane

f = 58.8", focal length of primary

a = .52". this is from using the diameter of the Moon at the image plane, f x tan31', or 58.8" x .009 = 52"

For these parameters, the secondary size is 2.67", __if__ the it's distance to the focus is 11".

For fun, what is the diameter of the cone of light at 11" inside the focus? A simple ratio:

12"/58.8" = x/11" , x = 2.24", your secondary should be larger than that.

If your secondary distance is 12", then your secondary size should be 2.86".

So what is the distance from the center of your tube to where the focus is?

**Edited by Garyth64, 13 September 2019 - 07:23 AM.**