TheSkySearchers

Hi,
I've owned my 14" Skywatcher Dob for a year and a half. I have started to have trouble achieving focus visually. If I add my 35 mm extension tube, no problem. When I first got this scope I only needed to use the extension tube when I used a barlow and a higher magnification eyepiece. Now it seems I have to use it all the time. I think it may have something to do with the calumniation of the primary mirror, tightening/loosening the screws for the primary mirror. This would move the mirror closer or further away from the focal point. I never had this problem with my 10" Meade Starfinder. Has anyone else experienced this or has an explanation/solution. Thanks, Lance.

I have heard of this happening as well as the upper ring not being in the proper position.
Check that it is actually locked into the proper position for visual and has not started to slip.

Hi Lance,

It is highly unlikely to be due to movement of the mirror for collimation, as this only moves it a tiny amount (generally 1/10's of a millimetre at a time).

I agree with Gabby that most likely it is the seating of the collapsible tubes. Make sure they are fully extended when you set up, or if you leave it set up (especially in a vertical position) then check they haven't slipped down at the mirror box end: this would explain the need to put the extension tube in.
If you do leave it set up in future is is probably better to leave it in a horizontal position: this has the bonus of being less likely to get muck on the mirror too.

All the best,

Dean

I have been very careful to assure that the upper tube is extended completely and locked into position and generally check it a couple of times during observing sessions. I have never seen any slippage. Using an extension tube is all that much of a problem but then I don't think it's suppose to be that way. Next time I'm out I will take extra care to verify that the tube is extended and tightened. Thank you, Lance