SCT with loose secondary mirror

[Jaybeeaitch]

Hi, some advice from a more experienced user would be appreciated. I have a Celestron C6 XLT. After purchasing my first additional eyepiece(Baader Hyperion 10mm) I realised the collimation of the scope wasn't up to the additional magnification over the standard 25mm plossl. My first go at collimation was a disaster, but after purchasing Bob's knobs it's much easier.  However, as I fitted the new knurled screws, I noticed that the secondary mirror was moving - the securing ring was loose.  As I tightened that, I realised that the whole secondary mount was loose. Despite that, I have achieved a reasonable collimation, though looking at Jupiter last night I hope i can do better!  My question is, should I worry about the loose secondary mount? I think it can only be fixed by removing the corrector and holding inside and outside parts and tightening.  I am not ham fisted, but I am not sure I want to do that!  Should I consider looking for  proffesional repair?

thanks, John

[PeterCPC]

Firstly, when you fitted the Bobs Knobs did you replace them one at a time? If so everything should be ok. You tightened the securing ring and yet you say that the mount was loose. This seems contradictory to me. Can you explain more what you think is loose after tightening the securing ring.

Peter

[Peter Drew]

As PeterCPC says, the situation needs clarifying. You could only tighten the retaining ring by removing the corrector to access this, unless of course the latest design is different. If removal of the corrector is necessary and as you are not hamfisted, the procedure is straightforward, the main point is to mark the relevant position of the secondary and corrector for reassembly.

[Dave In Vermont]

I first question, as PeterCPC asked, is quite relevant: Did you remove more than one screw at-a-time when you installed Bob's Knobs? If yes - you thereby loosend the entire secondary-mirror assembly. But don't jump out of your shoes. Tell us what happened in full. Then we can walk you through it.

In the meanwhile, keep the scope level. This way if that assembly comes loose fully - it won't fall inside the tube and strike the primary-mirror below.

Hang in there,

Dave

[Jaybeeaitch]

Thanks all.  I am not sure of the accepted naming convention for the parts, so I will describe as best I can. Bob's knobs were successfully replaced one at a time in accordance with the instructions.  I would describe this as Bob's knobs pass through a mounting plate into the backing plate of the secondary mirror.  This makes what I will call for now a secondary mirror assembly. The secondary mirror assembly is clearly designed to be removable (something to with fitting Fastar maybe?). It is held into what I will call the secondary mount by a locking ring.  Undo the locking ring, and the secondary mirror assembly comes out, which is no problem because it is keyed into the secondary mount for accurate replacement.  The secondary mount itself is attached to the corrector.  Presumably the secondary mount consists of two parts that screw together with the corrector in between them.  I guess if they are screwed together tightly enough you have a nice tight friction fit to the corrector and the secondary mount holds nicely in place.

 

what I noticed as soon as I touched the secondary mirror assembly with a screwdriver is that it could move in and out very slightly.  This was because the locking ring wasn't even finger tight.  As I tightened the locking ring, I noticed that as it got finger tight, the whole secondary mount would spin in the corrector.  More importantly, the secondary mount can move side to side in the corrector.

current situation is that I used a steel rule to check the secondary mount is reasonably central in the corrector, and did the initial collimation looking into the corrector as described in the instructions.  That went ok, so later I went outside and did the next stage of collimation with a defocussed star.  Had no problems getting the concentric rings described in the various on line articles.  Viewing was ok at 150x, but I think it could be improved with more careful collimation (airy patterns maybe the next step?)

my concern is that with the secondary mount not secure in the corrector, I won't get perfect collimation because the mirror has moved from its central position, or is rotated with respect to the factory set position.  I don't know if these latter points are important.  Also I don't think the collimation will hold for vey long at all!

With luck, we may have clear sky tonight, so I will see how the collimation holds after a short car journey to a dark site.

[PeterCPC]

I may be wrong, somebody please correct me if I am, but it sounds as if the Bobs Knobs have not been tightened down.

 

Peter

[Dave In Vermont]

8 hours ago, PeterCPC said:

I may be wrong, somebody please correct me if I am, but it sounds as if the Bobs Knobs have not been tightened down.

 

Peter

That is what it's sounding like alright. So do try snugging them down a bit - one by one. Then see where we are.

Dave

[Jaybeeaitch]

Hi all,

i don't think I am explaining the problem vey well.  I'll try attaching a couple of photos.  If I have done that correctly, the fists the photo is the secondary mirror assembly removed from the telescope, complete with Bob's knobs.  The second is the scope.  In the centre of the corrector is what I am calling the secondary mount.  It is this part that is loose, and I think I need to remove themcorrector, centralise the secondary mounting, and tighten everything up.

hope that's clearer.

thanks John

[knobby]

Mines a bit loose, but not as yours... Sounds like the front needs to come off, just mark everything with a sharpie.

[Peter Drew]

The collimation of the optical components is very sensitive due to the nature of the design. For starters, the primary is F2 and although essentially spherical it needs to be well centred and fixed, this is usually bonded into position at the factory and is non adjustable. The secondary mirror imparts a 5x amplification compared to the 0x amplification of a Newtonian, very small adjustments have a large effect on the collimation. Centralising the secondary mirror is also important. Retaining the factory set orientation of the corrector and secondary mirror certainly used to be important but is currently a topic of debate. I don't think you have done anything wrong with your procedure so far but it is looking like the corrector will need removing to tighten the secondary holder.

[Jaybeeaitch]

Thanks everyone.  I guess I will have a try at removing the corrector after carefully marking the alignment.  I will report back next weekend!

[Waldemar]

Do not forget to apply antislip rubber gaskets to prevent things to move again after you tightened it.

[Dave In Vermont]

A clean sheet spread over the work area, and some fine cotton-gloves would make for good gear for this operation.

Be gentle, please. And I look foreward to your writing up an account of your adventure for future SCT-owners who find themselves in the same boat.

Dave

[Jaybeeaitch]

Hi everyone. I have been away for a couple of weeks, so I have only just had chance to get back to my loose secondary. Not sure if I have done everything correctly, but the results seem better. 

I will describe the procedure I followed, but anyone reading this post should check follow ups from the experts in case I made a silly mistake.

Taking a careful look at my Celectron C6, it seemed as though there were two approaches possible to gain entry to the interior of the scope.  Firstly, the corrector could be removed from its mounting onto the optical tube, or secondly, the whole corrector mount could be removed from the optical tube.  In my very inexpert opinion, it seemed as though the second method would be easiest.  Following is the approach I took:

1)     Buy a couple of things.  Being a posidriv man, I actually had to buy a Phillips screwdriver before I could tackle the work. I already had Bob’s Knobs, but I also bought a Hubble 5 star collimation aid.  The thought of being able to practice collimation in the warm made this an easy decision.

2) I decided to make a template that would help centre the mirror assembly in the corrector.  I carefully measured the i.d. of the corrector holder, and the o.d. of the secondary mirror assembly, and used a CAD program to draw it and print a template. A bit of work with scissors, and I had a paper template that would fit into the recess of the corrector holder and help centre the mirror (you will see from the photo that I actually made a Duncan mask as a possible collimation aid – more on that another time)

3) I chose a clean work area, donned some cotton gloves and marked the position of the corrector assembly (including the bottom of the scope to make sure I could eventually position the secondary mirror with the Celestron logo horizontal).

4) I removed the four screws holding the dovetail, and the four screws holding the corrector mounting.  The corrector mounting slid easily off the tube (see photo)

5) I completely loosened the secondary mirror holder using finger pressure, no tools required.  I centred it using the template, and tightened by hand.  The outer part of the secondary mirror mount was a bit difficult to hold.  At this point I realised that a modified C spanner to hook the keyway in the mount would have been a help, but I made do without it.  There was already a rubber gasket in place, so I am not sure how anything came loose in the first place.

6) Re-assembly was the reverse of disassembly. There were no problems. 

7) I did some rough collimation looking into the front of the OTA. This is fully described on the Bob's Knobs pages 

8) I set up the Hubble 5 star in the garden at dusk, with the scope in the house pointing through the French doors (just to get some distance for the collimation procedure). I followed Thiery Legault’s collimation procedure (stage 1 and stage 2) at x150 magnification. Stage 3 at x 300 (or higher) will have to wait until I have bought a barlow or new eyepiece.  With Bob's knobs and the Hubble device this process was very easy.  I know I will have to finish collimation using a real star, but being able to practice in the warm and light is a huge plus.

9) I took a look at Jupiter and gave a sigh of relief.  The results were better than when I started, so I probably haven't broken anything!  At x150, it was easy to make out the two main bands on Jupiter (previously there was only the suggestion of banding).  My wife claims she could see more detail than that.  I suspect there is still more to come as I achieve a better collimation

All in all, this was well worth the time and minor effort.  And whilst I would not suggest that anyone should dismantle a scope in good order, I think its proof that even beginners should have a go at collimation if they own an SCT

John

[Peter Drew]

Interesting that the front cell can be replaced easily after initial removal. On many versions there are internal nuts that fall off if the attachment screws are removed meaning that the corrector lens has to be removed after all in order to replace them. The same usually goes for the rear cell.

[Jaybeeaitch]

That was a concern Peter.  I was able to use a torch to check inside the tube before starting, and was pretty convinced that internally I could see a screw in a tapped hole, not a nut.  Removed the first one with no rattling, so went for it..  I guess I'm lucky with the type of scope I bought.

[Rambrosi]

On 4/17/2016 at 01:42, Jaybeeaitch said:

Hi, some advice from a more experienced user would be appreciated. I have a Celestron C6 XLT. After purchasing my first additional eyepiece(Baader Hyperion 10mm) I realised the collimation of the scope wasn't up to the additional magnification over the standard 25mm plossl. My first go at collimation was a disaster, but after purchasing Bob's knobs it's much easier.  However, as I fitted the new knurled screws, I noticed that the secondary mirror was moving - the securing ring was loose.  As I tightened that, I realised that the whole secondary mount was loose. Despite that, I have achieved a reasonable collimation, though looking at Jupiter last night I hope i can do better!  My question is, should I worry about the loose secondary mount? I think it can only be fixed by removing the corrector and holding inside and outside parts and tightening.  I am not ham fisted, but I am not sure I want to do that!  Should I consider looking for  proffesional repair?

thanks, John

Hi John,

This has nothing to do with Bob's knobs. This is a known issue with some Celestron OTA's. I have the same issue. Send to Dean at Starizona and he will put a gasket in to stop the secondary from turning.