How to focus finder scope on GSO Dob?

[maxwolfie (Greg)]

Hi all,

I have recently acquired a GSO 8" dob, however I am not sure how to adjust the focus on the finder scope?

There is a front section, which has the primary in it, then behind that there is a ring (that does not seem to do much), then the body of the finder scope. Moving the primary forwards or backwards does not actually seem to bring anything into focus - I think I may have to move the primary closer to the eyepiece but I can't, as it's at it's maximum now.

Any ideas?

[omegacrux (David)]

Hi Greg
They don't need focusing
Unless someone has been mucking around with it

David

[maxwolfie (Greg)]

Mines way out of whack, I'm sure I read in the (very crappy) manual that you twist something at the front to focus. I figure that's the primary however it doesn't seem to be having any effect, like I said I think I need to move it back rather than forward, but I can't move it any further back?

[Shiraz (Ray)]

a couple of mine have needed adjusting. The lens holder screws onto the barrel for focusing. The locking ring will stop the lens from adjusting fully inwards, so get it out of the way by screwing it as far as it will go towards the eyepiece end. You should then be able to adjust the lens to focus - and then screw the locking ring back out until it locks against the lens assembly. The lock ring tension will throw the focus out slightly, so you will need to play with the lens setting a bit.

[mental4astro (Alexander)]

Ray, by "lens" you mean the objective? That's the lens that these 50mm finders are focused by. The the objective is loosened and the locking ring is then adjusted to focus. The lens is then retightened to fix the focus.

I remember a little instruction leaflet on this, I just can't remember where I saw it.

I found this set of 'instructions' on astronomyforum.net. It shows what I mean.

Mental.

[Shiraz (Ray)]

Originally Posted by mental4astro

Ray, by "lens" you mean the objective?
Mental.

Hi Alexander - yes, the objective lens is adjusted - the eyepiece is not adjustable. You can't adjust the objective lens inwards unless the locking ring is also rotated inwards to provide free thread. While trying for initial focus, it's easiest to screw the locking ring all the way in and get it out of the way.

[maxwolfie (Greg)]

I think I've got it, the primary was a bit sticky and it turns out it can go way further back. Go away sun!