When I first collimated the gs12 I found the collimation varied significantly depending on the angle of the scope-horizontal,45degs,vertical.
A quick inspection revealed the mirror floating around on the 3 springs supplied on the adjusting screws. I collimate at 45degs for convenience but moving the scope to vertical then horizontal for a final check. When positioned at vertical the springs supplied compress too easily under the full weight of the mirror upsetting the collimation .
An interim fix was to turn all screws in so the springs were 'nearly' fully compressed and collimating by turning out only the 2 screws required. This gave better support and reduced the variation in collimation at different angles.
A quick pm to another member confirmed the problem and agreed the springs are too soft.
A quick run around this morning and 3 new springs,slightly longer with a thicker diameter wire were found and fitted. I was only able to find coated steel springs,but will chase up some s/steel springs later.
The mirror now stays put and the collimation doesn't change at all.
I know collimation is hard enough to come to grips with at first, let alone having a mirror floating around.
Hope this post helps to remove some of the frustrations with collimation on these scopes.
Hi MiG
The 3 'locking' screws you mentioned ,unless you've a different setup I presume your talking about the 3 that hold the dustcap on, are'nt.
I'm sure you could screw these in to take up the movement at vertical (if they could be locked) but in the process you've created a situation where you now have 6 screws to adjust for collimating. 3 to adjust to collimate and the other 3 to hold the weight at vertical. Any pressure on these last 3 will change the collimation again. Too loose and they'll eventually fall out.
3 screws holding the dust cap on??? huh! Never heard of that set up.
I have the same scope as yours and there is a thread in here somewhere about how to set the lock screws on the GS 12". I will go and look for it and then let you know where.
hi slice, they are talking about the locking screws for you primary mirror. at the bottom of you OTA there should be 6 screws. 3 of these are for adjusting the angle of the primary mirror during collimation and the other 3 are to lock it in place when finished. undo the 3 locking screws on the bottom of the OTA before collimating and do them up when finished to lock the mirror in place.
ps, dont do the locking screws up too tight.
The theory behind the lock screws on the GS 12" was covered in another thread which I have been unable to find.
Now, the idea is to do as David said above. Geoff and one of our overseas members went into great detail to help me get mine just right.
<b>Contraversy Time!</b>
I did it this way for a week or two and found that it still went out of collimation easily. The cell did not wobble, rattle or move. It just kept going out! I decided to do the wrong thing and yet it works for me. I get collimation as close as I can using the spring screws, then I gently tighten the lock screws (as above) then I break all the rules: I use a screwdriver to adjust the lock screws to get collimation perfect. Since doing it this way I have not had to re-adjust collimation (1 month so far) and I get a much crisper focus on objects than I had previously.
What I am doing is against everything I have been taught and should slightly distort the image but it doesn't. (Maybe because I have the mirror clamps quite loose)
I don't recommend this method one bit, but it works a treat for me.
Ken, that's pretty similar to what I try to do with my 8" and makes sense. I get the collimation right with the spring loaded screws, and then gradually tighten the three locking screws while keeping an eye on collimation to make sure it stays good. I do only give them a firm finger tightening though.
Originally posted by ballaratdragons
The theory behind the lock screws on the GS 12" was covered in another thread which I have been unable to find.
Now, the idea is to do as David said above. Geoff and one of our overseas members went into great detail to help me get mine just right.
I just had a search for that thread as well without success. I searched on keywords I knew I used but the thread seems to have dissapeared. Its a shame too as Ken made some nice diagrams.
Quote:
What I am doing is against everything I have been taught and should slightly distort the image but it doesn't. (Maybe because I have the mirror clamps quite loose)
I don't recommend this method one bit, but it works a treat for me.
Distortion of the cell may not affect the mirror depending how tight the cork pads are I imagine. If it works for you well and good Ken
Quote:
The 3 'locking' screws you mentioned ,unless you've a different setup I presume your talking about the 3 that hold the dustcap on, are'nt.
By dustcap do you mean the metal plate covering the rear of the mirror cell ? Yes the lockscrews work against the surface of the metal plate.
The spring loaded collimation screws ( the larger ones ) adjust the angle of the mirror cell, and the lockscrews push against the underside of the mirror cell via the metal back plate.
Technique
Adjust the three larger collimation screws and then very lightly seat the locking screws against the metal plate. If you then gradually tighten each lock screw in turn a little at a time working around in a circle, checking collimation as you go, you'll find that you can get it right and stay there.
I just had a search for that thread as well without success. I searched on keywords I knew I used but the thread seems to have dissapeared. Its a shame too as Ken made some nice diagrams
I think I know where it went, Ken may remember too he just needs to cast his mind back a bit further Self inflicted missing thread unfortunately
Anyway it sounds like the locking screws can work well for most people.. sometimes I forget to do them up at all.. But only a finger tight when I do adjust them.
Might have to try really locking them in place, see if it keeps collimation in place for longer, though I find that it doesn't need adjusting very regularly as it is.
Originally posted by iceman I think I know where it went, Ken may remember too he just needs to cast his mind back a bit further Self inflicted missing thread unfortunately
OOPS! Must have been one of the threads I wiped when I went on a deleting frenzy. Thanks Mike, Sorry about that everyone.
Luckily I saved the artwork in Paintshop. This is the Bum end of the GS 12" showing the collimating screws.
Use the method explained in the above posts.
After each finger tightening (or tighter) re-check collimation.
That is a strange sort of setup - and a bit counter intuitive if you ask me - no wonder there is confusion about this. Whenever I have read / talked about collimation of a mirror cell it has been about "push and pull" screws. I guess in your case the push is referring to the locking screw, while the pull component refers to the spring screw. I say the setup is strange as most scopes I have seen have a push / pull grouping of screws right next to each other at three locations. This makes it easier to imagine how they operate. Looking at this layout I can imagine that collimation is slightly more painfull having to loosen off two push screws to adjust a pull screw. If the screws were paired together it would be slightly easier. I can see why you would loosen all the push screws and just adjust the pull screws, then quickly do up the pull screws and hope nothing moves. As I said, this is an unusual setup - wonder why the desingers went this layout? Is it for more support? What does the documentation that came with the scope say about collimation technique?
What documentation? Do you mean the microscopic assembly instruction sheet.
You are right though, actually you have to undo all 3 lock screws to move the spring screws or the pressure on 1 becomes too much. It's a real Dopey-Dorah design.
I can see a bit of mirror distortion being introduced if the lock screws are tooo tight. I wouldn't rely on the thickness of the 'cork' packing to provide the expansion/grip req'd. As Andrew has just mentioned, seems like a bit of redesigning might have helped.
Still, If you do your collimation correctly, it shouldn't change by simply going from 45 deg. to vertical!
The lock screws should never be too tight or used to adjust the collimation.
Use heavier springs or *bottom them out* & then ease each adjuster till you are collimated, then just *NIP* the lock screws.
My 2cents. L.
As Astro_south has pointed out the normal configuration is 3 push-pull screws .Each of these are fitted with springs capable of supporting the mirror. The springs also add enough pressure to keep the adjusting screws as set.
Why would anyone want to complicate a simple ,tried and proven system with another 3 screws?
With a simple 3 screw system I know I can tweak collimation in the dark if need be. It's a simple excercise. Why confuse the situation with 3 more screws?
Fact........The fault lies in the factory fitted springs which aren't capable of supporting the mirrors. Probably the same springs for a 6".
Solution.........Replace the springs.
Simple logic....Find the problem and fix it,dont patch it.
Each to their own but the $1.50 I spent on springs means I can maintain collimation easily.
Slice
I did a search for this problem first but couldn't find anything.
Now I know why.
Self inflicted missing thread unfortunately 
