Mirror dis-assembly - rookie questions

[glend (Glen)]

I pulled the primary mirror out of my GSO 12" dob yesterday to give it a clean and to install my upgraded springs. I marked the mirror on the sides of the support clamps so that I could put it all back together correctly. After getting it all back together I did a laser collimation and tested it out last night. It's great at lower power but when using the 9mm and 6mm higher powered eyepieces I am getting short astigmation tails that extend out to the 2pm position (on the image as seen in the eyepiece). I am assuming that one or more of my mirror clamps is probably to tight; is this a correct assumption? I did a star test and my donut appears to have a bite out of it on the right side (of the image). I did not attempt to fix it last night because the clamps require disassembly, and I didn't want to tackle that in the dark.

Secondly, at the back of the mirror where it sits on the articulated support arms of the support frame, the arms have what seems to be double sided tape pads on the end of each of the triangular arms, two of the three on each arm still have the protective cover on the adhesive and only one on each arm was sticking to the mirror - how important is that? Are they just there to cushion the mirror or should all of them be stuck to the mirror back - admittedly this can make it hard to remove the mirror? And does each triangluar arm have to go back into exactly the same spot, which is what I tried to do?

Finally, GSO (in their wisdom) make both the adjustment knobs and locking knobs the same thread size so it is possible to mis-align the frame in the carrier when remounting the springs. Am I correct in that the locking knobs only go through the holes where only one side is threaded and thus push against the internal mirror support frame to lock it in place; and the spring knobs screw into the internal mirror support frame? Or do I have it wrong way around?

Rookie mistakes I am sure and it's a learning experience. I will take it apart again today to fix the mirror clamp problem, so any advice is appreciated.

Cheers

[multiweb (Marc)]

Originally Posted by glend

I pulled the primary mirror out of my GSO 12" dob yesterday to give it a clean and to install my upgraded springs. I marked the mirror on the sides of the support clamps so that I could put it all back together correctly. After getting it all back together I did a laser collimation and tested it out last night. It's great at lower power but when using the 9mm and 6mm higher powered eyepieces I am getting short astigmation tails that extend out to the 2pm position (on the image as seen in the eyepiece). I am assuming that one or more of my mirror clamps is probably to tight; is this a correct assumption? I did a star test and my donut appears to have a bite out of it on the right side (of the image). I did not attempt to fix it last night because the clamps require disassembly, and I didn't want to tackle that in the dark.

Flaring is likely to be caused by one of the primary clips too tight. Astigmatism is another thing altogether and usually induced by the secondary, not the primary. It is obvious when you go in focus and out focus. The pattern of the diffraction rings will become oblong and rotate 90 degrees on each side of focus, so at focus point you won't get a pinpoint star but a somehow blurry cross. Sometime the secondary is pinched and causes that. Otherwise 90% of the time you can fix this by rotating the primary until you find a spot where it's gone.

Originally Posted by glend

Secondly, at the back of the mirror where it sits ... which is what I tried to do?

Can't help you with that. Others will chime in.

Originally Posted by glend

Finally, GSO (in their wisdom) make both the adjustment knobs and locking knobs the same thread size so it is possible to mis-align the frame in the carrier when remounting the springs. Am I correct in that the locking knobs only go through the holes where only one side is threaded and thus push against the internal mirror support frame to lock it in place; and the spring knobs screw into the internal mirror support frame? Or do I have it wrong way around?

The locking knobs are threaded on the top ring and push against the mirror cell. The collimation knobs are threaded into the primary mirror cell (the one with the clips) and are spring loaded against the top ring. Once collimated the three locking knobs just need to touch the cell to stop it from moving. No pressure is needed. Ideally locking knobs should be right next to the collimation screws for an efficient push/pull system. Not sure why GSO stuck them in the middle.

[barx1963 (Malcolm)]

Glen
I made the same mistake of over tightening. Ideally they needn't even touch the surface of the mirror.
With the double sided tape, I think it is just there to hold the triangles in place. I don't think you need worry about being super precise with the placement. These mirrors are quite thick relative to their diameter and minor misplacement of the cell elements shouldn't affect the performance. In Kreige's book he suggests making a Kydex ring and attaching the inner corner of the triangle to it to hold it's alignment.
I solved the problem of removing the double sided tape by simply removing the entire cell and washing the mirror while in its cell. The is nothing there than cannot handle a bit of moisture and while it makes the process slightly more cumbersome, reassembly is much easier!
With the locking knobs, once you have replaced the springs, they are possible the single most useless bit of kit you will own, just remove them and forget about them. I used my old GSO 12" for 4 years without them and it was SOOOO much easier. Also, you never see locking knobs on big dobs.

Malcolm

[Peter.M]

When I get astigmatism of the secondary mirror it usually manifests itself as triangle stars in focus. This is speaking from an imaging point of view, I am unsure what they would look like to the eye.

[barx1963 (Malcolm)]

When I overtightened the clips on my 12", all the stars looked like pinched triangles and could get them to focus at anything above 40x.

Malcolm

[glend (Glen)]

Making progress (of sorts). Took it all apart again today and removed the mirror clips and put them back with at least one mm between the lip and the surface, don't know how I can make them looser. Collimated it again, and it seemed perfect. I tested it tonight and it's much improved at higher power but there is still a slight tail on the stars with the 6mm eyepiece - which wasn't there before I started all this.
I did the "Star test" and the collimation seems great. Sadly, I now have a new problem, my wonderful GSO 2" 30mm Superview eyepiece will not achieve focus now. If I raise it up in the focuser about 5mm above the lip it will achieve focus, I can only assume that this is because the upgraded springs are longer and have altered the focal length slightly. Not sure how to fix that other than cutting the springs down or going back to stock ones. I don't want to get into a situation of stacking the coil in the springs. Any ideas?

Starting to wish I hadn't started messing around with something that was performing perfectly.

[barx1963 (Malcolm)]

Glen
Never feel bad about having a go at making your gear better! It is the best way to learn how a scope works and how to get the best performance out of it.
The obvious thing to do is to wind the mirror back in the tube. I did the same thing when I bought a 10mm Ethos and found it wouldn't focus. (imagine how I felt with a $700 ep!!) Anyway I had to move the primary up the tube so I just wound it out 1 turn at a time on each knob in sucession, checking the collimation and focus every 2 or 3 winds until it was ok.
As it was the opposit problem the solution was a TV High Hat adaptor. In your case, wind the mirror. If that doesn't work, try shorter springs. Another option is the springs supplied by Bob's Knobs. (see http://www.bobsknobs.com/Newt/page57/Z12.html these are for Zhumell scopes which is what GSO dobs are called in the US) These are what I used and they worked a treat. Not sure who sells them these days in OZ.

Malcolm

[erick (Eric)]

When I bought new springs from Bunnings, I just ran the hacksaw through the middle of them to get the right length. A bit of work with a file to smooth off the cut end and they seem to work fine.

But can you compress those springs somewhat more? The more compression, the firmer the mirror will be held in place. And that may pull your primary back the few more millimetres you are looking for?

[glend (Glen)]

Thanks to everyone that replied with advice and suggestions. I took it apart again today and re-engineered the new stronger springs (cut them down) to allow the mirror to return to about the same place it was with the stock springs. I also removed the mirror clips and re-installed them. Collimated again, twice with two different lasers (one of which is collimated as well) and both agreed. As a result it's all perfect once again, had it out tonight for testing and no more focus travel problems with the 30mm EP, star test fine right down to 6mm EP (250x), no flares, coma issues, etc.

Ran through a list of seven test DSOs and it's perfect.

Thanks again, I've learned alot, and am much faster at collimation now.

[barx1963 (Malcolm)]

And now you have a sense of satifaction in knowing you have a better instrument. Well done Glen!!

Malcolm