Filter Wheel Issues

[gaseous (Patrick)]

Hi all,

I've recently purchased a 9-slot 1.25" filter wheel, and have struck the problem of not being able to focus it without using a Barlow. It will "just" focus with a 40mm EP without a barlow, but no other EP will come close. I've since read of people having this problem with dobs (I really should have done a little research first), but haven't really come up with a solution. Apologies if this has been discussed somewhere on this forum - I couldn't find it anywhere and would be grateful to be pointed in the right direction. I've got the wheel right against the focuser opening with a type of ring adapter, so I'm at the bare minimum distance EP to focus tube, but it's still too far. Will I need to buy some sort of low power barlow, or put it down to experience and try to on-sell it? Cheers.

[ZeroID (Brent)]

You will need to move your primary mirror up the tube a bit. You're outside the focal point. The solution is to move the mirror up maybe 20 mm or so. Is this a solid tube DOB ?
A low profile focusser might help instead.
This situation is often seen on DOBs when folks attempt to do DSLR photography. The Focal plane of the camera cannot be brought in close enough to reach focus.
On my original truss scope build I just chopped 25mm off the struts. May sometimes mean you need an extender for some EPs and if the secondary is too small you may lose a little light but not a problem for visual.