Mirror retention and holding collimation

[osbourne one-nil]

My Sumerian Optics 10" mirror is held in place with three metal clips which have a sticky-foam pad on. However, these pads seem to have been squished and have lost their elasticity resulting in there being a little bit of play between the mirror and the three bolts it rests on. Does anyone know what I could replace them with? Something that has a memory I guess, like neoprene?

Also, is it a feature of compact truss Dobs such as mine that the adjustment is all taken up within these pads, rather than through a frame on which the mirror sits?

I realise it would be far easier if I took a photo, but perhaps I'm making some sense anyway?

[Merlin66]

Certainly a photo would make it easier to understand the issue..

[osbourne one-nil]

I'll get one sorted tonight!

[John]

There should be a little space between the mirror clips and the top surface of the mirror - they used to say that it was about the thickness of a business card, in the days when people carried such things about. If the mirror clips press too hard on the mirror they can warp it slightly which distorts the images.

[Merlin66]

need photos. I don't know if the OP is taking about rear axial positioning screws, radial locating screws, or front safety stops.......

[osbourne one-nil]

Thanks John,

There's certainly a bit more than a business card's width between the clip and the mirror face, although based on what you say, I think the collimation bolts (three of them) aren't quite long enough to provide sufficient adjustment for reliable collimation. It's often been the case that all three are done up as tight as they'll go before collimation is reached. It might be that I could "extend" these by placing some packing on the back of the mirror where they hit, or by getting slightly longer bolts.

A photo will make everything clear....possibly.

[Merlin66]

Why do you need these bolts screwed in all the way?

Is this to move the focus further out in the focuser???

You should be able to collimate by winding ALL the bolts back, say 1/2 way their travel and by adjusting one or max two of the bolts achieve collimation??

[osbourne one-nil]

Because when the mirror's sitting flat against whatever it sits on, the collimation bolts aren't actually reaching its underside so it proves nigh-on impossible to adjust it securely.

Until a photo later this evening, does this little cross-section help?

[Merlin66]

Hmmm,

Then I'd go for longer bolts!

You can "tin" the ends ( lead solder) or at least gently smooth and round then with a file....that's about it.

I don't recommend packing unless its a rigid plastic or even a cork gasket.

[osbourne one-nil]

Thanks Merlin - as soon as I'd drawn the problem, I came to the same conclusion pretty quickly! Right then....some long M4 nylon bolts with a nice knurled head on them.

[Merlin66]

"fools seldom differ"???

My mates at Model Fixings can supply M4 thumbscrews:

Nylon Screws

but .....

I don't recommend them if you need a support screw over say 10mm long...nylon is not rigid enough at an M4 size....maybe a brass screw??

[osbourne one-nil]

Thanks again. They're nylon at the moment and probably already 25mm long, but could well be an M6. I'll wait to see what Michael at Sumerian says regarding their dimensions.

[John]

That's quite an unusual design, or at least one I've not see before, - having the collimation bolts directly bearing on the rear of the mirror. Usually the mirror sits in a frame and the collimation bolts are attached to that, or bear on that, tilting the frame / mirror as a unit.

[johninderby]

Try Epsilon. They can make whatever size of nylon tipped thumbscrews you want. Friendly and helpful as well.

http://www.epsilon-telescopes.co.uk/products/thumbscrews.htm

John

[osbourne one-nil]

Thanks John!

And the other John (!) I guess it's all part of the minimalist approach these scopes have. Can't be easy making a 10" dobsonian which weighs under 20kg!

I'm not annoyed by this at all, by the way - it very, very nearly works perfectly. Another few mm and it would. Could always be me doing something wrong too!

[osbourne one-nil]

Here are the promised photos, which might just clarify what we've already discussed?

Leaving all that aside though, the views last night.....wow.

[Merlin66]

Hmmm

The screws are pretty long at the moment....

I think the real issue is you're trying to use them not for collimating but to move the focus point further from the tube (yes?)

You could remove the mirror and add three "plates" inline with the screws...1/4" -3/8"

thick - this would allow you to "back-off" the screws and get more adjustment.

Maybe a brass dome headed nut on each screw would get you there??

[osbourne one-nil]

Good idea about the plates - that's the sort of thing I was wondering with my "packing it out" idea. I'm not, however, trying to move the focus point further - it's just how the scope came, but really am just simply using them to align everything.

[osbourne one-nil]

Just had a reply from Michael at Sumerian, and he says (as some of you have) is that the clips are there simply to stop the mirror falling out. He says some longer screws would work and suggests some springs on them between the head and the mirror box....which sounds a good idea and is something my old Orion dobsonian had.

[Merlin66]

Well, if you dn't need the extra distance for focus, you should be able to back each screw off by 3-4mm and then recollimate from there...that you give plenty of movement...

[osbourne one-nil]

Alas no, because the mirror then simply sits on the red hatched bits I've sketched above.

5mm longer bolts will be perfect - and brass ones too I reckon.

[Merlin66]

Bumma! Then longer screws it is.....