Prime focus

[egiesker]

Can't get focus on a Dob with a low profile focuser and short 2" adapter. I've read you can achieve focus by moving the mirror up 2". Can you accomplish focus by shortening the tubes by 2"? And if this will work will you lose the ability to manualy view thru the scope? Any help appreciated, Ernie

[theodog]

If focus is long then you'll need an extension tube, like a Barlow without the lens. I gather that your focus is short. If you do move the mirror up and shorten the tube, you may have to add mass to the bottom of the tube to keep it balanced on the pivot points. An extention tube may now be needed to use the scope visually. I have done this & had no issue.