I’ve been having a problem over the past few months of the focus in my scope drifting unacceptably out of focus when imaging targets around the sky. Even when staying on same target for extended periods of time there was a noticeable shift. In some extreme cases the collimation also appears to be way off. This seemed to be a rather severe case of mirror shifting that larger SCT’s suffer from. The solution to which is to securely fix the primary mirror within the cell, usually by threading bolts into the mirror from the back of the scope. This seemed to fit the symptoms I was seeing, though some people did mention that I shouldn’t bee seeing this sever an effect with a relatively small scope. So yesterday I decided to disassemble the scope and see what the back of the primary was like and if it would be possible to secure it. It didn’t take me long to discover a more likely cause.

First thing to do was to take the the auto-guider rail and counterweight rail off. Once that was done I noticed that there was a little bit of play between the OTA and the fork. I’d noticed this before but put it down to the mount design and that pushing against the corrector plate probably wasn’t a fair thing to do! However today I noticed that the tube (and hence the corrector plate) were able to move independent of the rear cell by several mm. Here’s a video to show what I mean:

Time to take things apart.

Getting the OTA off a CPC fork isn’t too difficult. Only 4 screws hold the OTA in place, two on each fork. However you need to take the panel off one fork and then loosen the bolts that secure the fork arm to the base (these are really tight – ‘I think I’m going to break my Allen key trying to turn this with a pliers’ kind of tight) to get enough slack to get the OTA off once you have the four supporting bolts off.

I thought I could then just unscrew the screws holding the rear cell onto the tube. Oh no. For a start, the four screws are held in with nuts so there would be no way to put it back together. Secondly, there were all loose. All of them.

The scope is second hand and I bought it from someone here. The previous owner hadn’t done any modifications or cleaning so I can only assume the scope was like this when he bought it. Though perhaps it has loosened a little over time. The support it got from the metal bars for the guide-scope and counterweight masked how obvious the problem really was.

So how to get at these screws. The corrector ‘cell’ is held in by three screws and nuts so you can’t take those off either. What you need to do is remove the corrector plate, by taking the six screws on the front out. Then you can reach down with a pliers and tighten the screws. Here’s a pic of three of them.

I decided not to mess with the mirror after that. I’ll see how much better things are now before taking a drill to the rear cell. So the next clear night will be spent getting the polar alignment correct and nailing the collimation and testing how well we keep focus and collimation while imaging targets over a range of altitudes and azimuths.

Here’s a video of the lack of movement now:


  1. Hi I just bought the same telescope used and have the same problem. Although i noticed that there is a moisture or oil like substance around each hex nut. My question is. Did yours have this and is my scope no good because of it? What can I do to get rid os the oil or moisture?

  2. Sorry for the delay.

    Oil around the hex nuts might just be to help prevent them corroding. if the scope is going to be out in dew a lot some of the metal nuts starting to rust can be a problem and a little oil can help stop that. Only a little though, and not where it can contact the optics!

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