Image shift

[Andrew*]

I've just realised the other day the full meaning of this commonly heard phrase about focusers.

I was attempting to image the sun with my digital compact on a camera bracket I purchased recently (this year). The load was quite heavy (adaptor, extension tube, panoptic, camera bracket and fairly light camera). I pointed the scope at the sun in my 19mm Panoptic and set up the bracket. It was a bit fiddly but eventually I had the lens right up to the eyepiece. Re-aligned the sun in the eyepiece. No sun on the screen .

I worked out that the load on the focusser was pulling it right down so it missed the secondary mirror altogether :shock:

Seeing as I hadn't got focus yet, I hadn't tightened the focuser. When the focuser was fully tightened, it lined up fine, but obviously I couldn't focus!

What can I do?

Andrew

[Andrew*]

anyone?

[Kaptain Klevtsov]

Sorry Andrew, I missed this earlier. So whats pulling where? The focuser is a baggy fit or the secondary is flexing somehow?

BTW which 'scope is it that you are using?

Kaptain Klevtsov

[centroid]

Image shift is a term that's normally applied the SCT's, where focusing is achieved by moving the 'primary mirror' back and forth.

Because there in some engineeering 'slack' in the adjustment method, focusing can cause the image to move 'off centre'. Manufacturers try to minimise this, by keeping engineering tolerences as 'tight' as they can, in a mass produced instrument, but don't eliminate the problem.

Dave

[Andrew*]

oh I see... Basically i think the focuser's load capacity was exceeded, and the weight of the things in the focuser was dragging the drawtube down off centre from the focuser body and therefore from the secondary.

Andrew

[daz]

Andrew

Try rotating the tube so that the focuser points vertical. Its a bit more fiddly to get to the viewer, but it will stop the problem you are having!