Retaining collimation - how to improve things

[osbourne one-nil]

My Sumerian Optics 10" Dob is lovely, but the one criticism I have is it doesn't retain collimation very well. I collimated the scope last night and haven't touched it since. When I checked this morning with my laser collimater, it seems to have drifted off quite a way (an inch from the centre of the primary). Now firstly, I don't know if this even matters particularly, although at f4.8 I reckon it does. That aside, the movement seems to stem entirely from the secondary as when I adjust that, the laser dot goes all over the place and even the slightest touch can send it on its way. This is, I'm sure, due to the springs. Can I "upgrade" these springs, or replace them with something like "o" rings which will remove the play that's currently affecting things? 

Sorry about the rubbish photo, but it's hopefully sufficient to get some good ideas?

[Moonshane]

could it be flex in the trusses/spider? does the collimation vary as you go from zenith to horizon?

[osbourne one-nil]

No, I don't think so, although the trusses are in two parts, so I could sacrifice a small amount of convenience by replacing the trusses with single pieces? 

It's a crude method perhaps, but as I move the scope about, the collimation seems to remain bang on. It's the secondary's alignment which does seem dodgy; if I even touch it lightly, the springs seem to shift moving the mirror. It's almost like getting something to balance and hoping it stays there. 

[Moonshane]

I am struggling to be sure where the springs are. are they between the heads of the bolts and the top square plate or between the lower square plate and the secondary stalk? either way, they could be replaced with stronger units which may help. how does the secondary remain in position on what appears to be a round cross member? i.e. what stops it rotating around the diameter of the bar?

[osbourne one-nil]

Here's a better photo - the springs are directly under the heads of the knobs, and the mirror itself has three magnetic prongs on which stick to the bolts which protrude through the metal plate. 

Does that make sense?! I'm not very good at this!

[Moonshane]

that's better. just so I understand, the black plastic parts are more like nuts than bolt heads? if so then tightening them sequentially all the way until tight and then back them off just one turn will put more stress into the springs and probably hold better. stronger springs will help too.

another matter - don't the heaters project into the light path creating diffraction spikes?

[osbourne one-nil]

They're like a "Bob's Knob" knob...just a knurled handle with a bolt attached I think. Good idea about tightening them all the way - let me try that, but next time I'm passing a shop like the Two Ronnies Fork Handles shop, I'll get some better springs. 

The heaters...they do, but I've never noticed any additional spikes at  the eyepiece, but then I'm generally looking at deep fuzzies, so they might not manifest themselves at times like that? To be honest, I hardly ever use them anyway, but then I hardly get to use the telescope this winter!

[Moonshane]

good point about the heaters. yes it matters less (not at all) on faint stuff.

see how you go with tightening them first. might work. hope so

[osbourne one-nil]

Well I've just tightened them almost all the way and already I can adjust them much more smoothly; before it felt like it was moving in clicks...in stages, but not now. Let's see how that goes.

Thanks - much appreciated!

[Moonshane]

maybe a washer between the spring and the head might help too?

[osbourne one-nil]

Another good point - thanks!

[StarryEyed]

 Moonshane suggested washers and beat me to it. 

[albedo0.39]

I had this on my dob but then I modified the screws

1. Make all 3 into points using a simple drill sander

2. Using a big drill (creating a shallow angle) make a "crater" into which the 3 adjuster screws sit

3. Use a spring over the thread to keep the whole set up under tension

This stopped the screws from "wandering" and thus changing collimation.

On the primary:-

1. Screw the mirror hard down on all 3 springs.

2. Back off 1 to 1.5 turns - all there the same.

3. Adjust collimation on 2 from 3 screws only.

In this way, the mirror cell is held better then normally would be the case and the collimation also varies less with change in altitude.

Finally, always collimate the primary at 45degrees to horizontal thus ensuring that irrespective of pointing at zenith or horizon (both unlikely) the collimation will be close.

Good luck !

[osbourne one-nil]

Thanks for that - interestingly, my primary simply sits on three nylon threaded bolts which rest against the back of the mirror. No springs, nothing. Odd?