This is why people usually only use small telescopes as finders on giant dobsonians where the overall scope weight makes the finder's weight insignificant.
- Even with large Dobsonians, the finder weight is significant, often more significant than with a small scope. The tracking force will be about the same in a large scope as it is in a small scope so it will need to be counterbalanced. But with a large scope, the center of balance, the altitude trunnion center, is about the same height as a small scope but the scope is much taller so more counterweight is needed.
10 inch GSO: Pivot = 23", Pivot to focuser= 24", pivot to bottom of tube = 16 inches. For every pound added at the focuser, 24/16= 1.5 pounds will be needed as a counterweight.
13.1 inch F/5.5 Starplitter: Pivot = 21", Pivot to focuser ~ 48 inches, Pivot to bottom of mirror box = 16 inches. For every pound added at the focuser 48/16 = 3.0 pounds will be needed as a counterweight.
The 22 inch requires about 5.5 pounds of counterweight for each pound added near the focuser.
Peter's solution of mounting the finder on the mirror box would mean I would not be able to work back and forth between the finder and the main eyepiece. For me, that defeats the purpose of having a finder.
- I have a 70mm finder and a 80mm finder. Ready to mount, with the eyepiece, the 70mm weighs 40 ounces, the 80 mm weighs about 52 ounces. With the 13.1 inch, they would require about 7.5 lbs and 10 lbs of counterweights..
Both these finders work at full aperture, I had to rework both of them, the 70mm was about 63-65mm, the 80mm was only about 60mm. Many finders do not operate at full aperture..
- I find a 50mm finder the most effective, The 50mm SV you mention, that's what I use. As I stated previously, I use standard eyepieces and add my own cross hairs. I never expect to see the object in the finder no matter how large the finder, its only for star hopping. The wider field of view possible with 50mm finder is a big plus for me.
I did try an APM 60mm but returned it. I measured it at 55mm clear aperture, the 50mm SV measured 50mm so there wasn't much gain there. I have never measured the 60mm SV but in conversations here, another member did measure it and found it to be about 55mm.
(I am using a modified flashlight test using a 4.8mm Nagler which is focused at infinity. A laser is projected through the eyepiece and the diameter of the projected beam is measured. It's accurate to less than a 1mm.)