SCT mirror shift

[JG777]

Hi, just wondering if anyone else uses a feathtouch focuser in their SCT and if so how much mirror shift they see when they use the fine focus. Mine is a standard C9.25 about 5 years old and well looked after, it has had a starlight in for a few years and I remember shift happening with the course focuser, but not so much using the fine focus, maybe I haven't paid enough attention to it lately as I haven't used it a lot. Last night was a decent night so took the SCT out and for example with jupiter centred using a 20mm 68 ES using fine focus would shift it about halfway to the edge, maybe a bit more sometimes and that is focusing pushing the mirror up. Is this similar to other people's experiences using the feathertouch or should it all be a bit better than this? Would be interested to hear other people's views with this set up. 

Other than this it was quite a productive evening but dew killed me off about 0045 as I had forgotten my shield, plus half the sky had clouded in. 

Thanks 

 

[Davey-T]

I think anything that moves the mirror will result in mirror shift, I use an electronic focuser on my SCT, once near to focus it's very easy to get it spot on especially when imaging planets as  they tend to disappear out of the field of view using manual focus.

Dave

 

[Luna-tic]

I think image shift in a SCT must be fairly variable between scopes. The shift in my C6 is noticeable, but minor enough that it doesn't disturb me, it's mostly a visual use scope. It has the factory focuser.  My Edge HD 8" had very minimal shift with the factory focuser, and I've installed a Feathertouch on it. I see little, if any, change in shift when using it, although I can get a finer focus with less twiddling (the whole reason for adding it, IMO). For imaging, the shift hasn't bothered me, my mount will track past the meridian when imaging in that area, and if I do a flip, it's between image gathering, so I refocus anyway. The mirror locks do help, BTW.

[JG777]

Thanks for your thoughts, I have probably forgotten how bad it was with the stock focuser and possibly the minor shift when fine focusing the scope with the feather touch. Only visual anyway. 

[Merlin66]

Over the years I've used many (!!) SCT's Meade and Celestron.

The Meade focuser arrangement is poor compared with the sturdier Celestron method. Any mirror slop/ droop was always better with the Celestrons.

To minimise slop you should regularly run the mirror up and down , through the full focus travel to re-spread the grease  and maintain the gap between the mirror sled and the baffle tube.

Excess slop is usually found where the mirror is left in the same position for an extended period, and the grease film thins.

 

[JG777]

Thank you @Merlin66 I had forgotten about that tip to move the mirror the full range. Thinking about it the mirror probably does not move much from the same area for visual  so will factor your tip in the routine maintenance.