Odd Question....

[Chris Stacey]

Wasn't sure where to post this....

Is it possible to have two focuser's on a Dob, one either side......has anybody ever come up with a secondary to enable this to work.....or is it completely a no-go??....my gut feeling tells me it would be impossible, but I really didn't know the answer when asked the question last night.

Well I did warn you it was an odd question!!

[Llamanaut]

probably not... if you have another focussor (2 in total) you'd also need another secondary mirror and this would interfere with the light train to the primary mirror below, resulting in rubbish images. You could have one on the other side to normal, but only one...

[Merlin66]

I used to have two focuser on a 18" scope. A low profile one for imaging and a larger one for visual. The secondary was rotated depending on the focuser being used.

Ken

[Bizibilder]

It would just be a lot of faffing (nice word!) about with re-collimation every time OR a very expensive rotatable secondary.

[FraserClarke]

Perfectly possible (rotating secondary), but it adds another constraint to your collimation; the optical axis of the primary would need to be co-aligned with the rotation point (not axis, I don't think) of the secondary. Would just take you longer to get it collimated, and you might need some more adjustments on the focusers to get them all co-aligned with the secondary.

Tricky, but nothing fundamentally impossible.

Indeed you could have as many focusers as you could fit around the top end.

[Merlin66]

Easier than that... the secondary is off set ( as necessary) in the holder relative to the rotation axis (ie the stem into the spider).

The when you rotate you maintain the correct secondary off set etc. If the focusers are set up co-linear ( ir same distance from the primary and squared etc)...there's no big deal is swapping between a camera (which can be left focused and in position) and your favourite eyepiece....I did it all the time for almost ten years!

[Chris Stacey]

Thanks, I had a feeling it wouldn't be possible by using some magic (never been heard of) secondary mirror......I like Merlins idea though, just been having a look at the secondary and wondering about removing the centre screw, and maybe using some kind of shaft that can be rotated with something like a skywatcher electronic focuser unit.........i'll have to play about and experiment I think...

[Peter Drew]

I made an 18" and 20" with 4 focusers as the tubes were truss type and couldn't be rotated. 3 were for visual and the 4th dedicated to photography. The optics were collimated to the 1st visual and the other 3 had their focusers adjusted to suit. The secondary mirror shaft rotated against click stops. A bit time consuming to set up initially, but once done remained reliable.