Silicone is horrifc on mirrors. I experimented with the damned stuff in the 90s on a full thickness 10" Mirror. The deformation was horrific.
Back decades ago cheap small Newtonains used a single vane spider for the secondary, often little more than a bit of bent piano wire. The manufacturers used a blob of Araldite to hold the secondary. That was bad enough.
Any manufacturer in this day and age using Silicone anywhere near a mirror is simply cutting corners IMHO. I for one would never buy a telescope that was using silicone anywhere near the optical train, regardless of manufacturer.
I talked to Rick Hendrick one of the owners of Planewave instruments at AAIC on the Gold Coast last year. I asked him if 1/4 wave diffraction limited was good enough or is 1/10th noticeably better.
He told me its one thing to have a mirror at say 1/4 wave or higher on a bench but mounting mirrors and retaining that 1/4 wave or better is another and how much distortion can easily set in.
Planewave scopes have the primary glued ( not sure what with) and have a nice mirror mounting support system. I have heard of any issues with them.
Greg - I have have made a few enquiries on that very subject and have been told that if you know what you are doing and use the correct adhesive that the primary and the secondary mirrors can be glued to a suported glass substrate for extra support and isolation.
Re the matter of the telescope it is only fair to say that the manufacturer and the NZ agent have stood behind the product and I have been offered a replacement. At this point in time I am retaining the original in an effort to hopefully isolate the problem it is just that the weather is not co-operating. The observatory is snowed in at the moment with more snow predicted for tomorrow night.
The silicon does not make any difference. I had to remove the mirror from my steel tube several months back and reinstall that into the new truss. I found that the cause of the triangular shaped stars is the tightness of the baffle tube against the primary. You need to make this just touching only. It should be just barely touching and nothing more. Keep backing if off until you get good shaped stars. Be also aware that the primary mirror locks need to be snug but not over tightened too. The mirror clips should not be touching the mirror at all.
Hi Gert - I was not able to solve the problem with the stars - I returned the telescope to the dealer several months ago for GSO to fix the problem, this never happened and I recieved a full refund recently. I am now in the process of ordering a Planewave CDK to replace it.
Hi Gert - I was not able to solve the problem with the stars - I returned the telescope to the dealer several months ago for GSO to fix the problem, this never happened and I recieved a full refund recently. I am now in the process of ordering a Planewave CDK to replace it.
Cheers
Geof
Too bad you had to make that decision. Hopefully I dont have to do the same. I already contacted OPT (dont have a local dealer) and told them wat was happening with my scope.
Hi Geert - Over the intervening time I have done a fair bit of research into both types of telescopes, I know that the CDK is much more expensive but I think the improvement in image quality is there and the obvious improvement in build quality is worth the extra money. I will write a report once it arrives.
The silicon does not make any difference. I had to remove the mirror from my steel tube several months back and reinstall that into the new truss. I found that the cause of the triangular shaped stars is the tightness of the baffle tube against the primary. You need to make this just touching only. It should be just barely touching and nothing more. Keep backing if off until you get good shaped stars. Be also aware that the primary mirror locks need to be snug but not over tightened too. The mirror clips should not be touching the mirror at all.
Interesting... These are tempting instruments, but the occasional campfire horror stories are off-putting: Will be keen to see how often the solution turns out to be this simple.
Interesting... These are tempting instruments, but the occasional campfire horror stories are off-putting: Will be keen to see how often the solution turns out to be this simple.
Collimation of these scopes is not a simple matter and pitched optics can occur easily, getting the configuration right takes some time and knowledge. The optics do require quite a bit of care to get best results but once on a mount and collimated they hardly ever require work. I am still waiting to tweak my collimation just a little but for the most part I am happy with the results so far. I have had my mirrors completely out twice now and have resiliconed mine back in. The link below shows the results so far.
I had no success loosening the central baffle. I could even feel that the mirror had almost free movement in the cell.
Tried three different colimation procedures and still have triangular stars.
I even replaced the whole imaging train (Camera, Filter wheel, OAG and Foccuser and put a lodestar in place in order minimize weigth and to mimic as best as possible a single ocular, but results where still the same.
What else can I try?
As far I can see, the the mirror is resting on only three visible cork pads in the cell, may be adding more cork pads could solve the issue, but for that I have remove the silicone to lift the mirror, and doing that may void the waranty?
Have you checked the mirror clips and the rear collimation screws Geert? The causes of pinched optics in these scopes can be the central baffle tightness, the mirror clips touching the primary and over tightening of the rear collimation screws. I have it on good authority that silicone cannot cause pinched optics because it is pliable and does not act as a brace. It is not stiff enough to cause problems.
So check your rear screws too. Those should be firm but not over tightened.
[QUOTE=Paul Haese;1155916 I have it on good authority that silicone cannot cause pinched optics because it is pliable and does not act as a brace. It is not stiff enough to cause problems.[/QUOTE]
I have to correct that one Paul from my own direct experience - Silicone is absolutely capable of warping optics -it is only warping on a sub -micron level we are talking here . This is my own practical experience of testing optics on and off silicone mounts in a highly sensitive bench star test . Diagonals are also susceptible to warping if not mounted just right if silicon used . I would always recommend optics to be free floating both from the edge and back for the best possible level of support without possibility of induced strain at different angles of altitude .
It is certainly possible to pull off a silicon mounting but it willl rarely be as good as a free standing optic and can certainly end up much worse if it is not done right.
Have you checked the mirror clips and the rear collimation screws Geert? The causes of pinched optics in these scopes can be the central baffle tightness, the mirror clips touching the primary and over tightening of the rear collimation screws. I have it on good authority that silicone cannot cause pinched optics because it is pliable and does not act as a brace. It is not stiff enough to cause problems.
So check your rear screws too. Those should be firm but not over tightened.
Hello Paul.
I left a lot of space between the mirror an the clips 1/2mm or so. I was able to move the mirror in the cell. (so dont think silicone is the cause).
Also the rear screews were only fastened enough to hold the cell in position.
I am thinking my mirror was pinched during factoring...
I have not disassembled de secondary mirror cell. this mirror is retained in its cell with a screwed metal ring. Any experience with this?.
So Mark does the type of silicone matter? Some are slightly stiffer than the other but in this case I know from Jim that the silicone is only used to steady the mirror in the housing to prevent slop. I can see how a secondary might be affected but a 2" thick mirror is not like a thin secondary.
Quote:
Originally Posted by gvanhau
Hello Paul.
I left a lot of space between the mirror an the clips 1/2mm or so. I was able to move the mirror in the cell. (so dont think silicone is the cause).
Also the rear screews were only fastened enough to hold the cell in position.
I am thinking my mirror was pinched during factoring...
I have not disassembled de secondary mirror cell. this mirror is retained in its cell with a screwed metal ring. Any experience with this?.
Regards
Geert
I don't any experience with the secondary disassembly Geert but I am sure this could be another place to check too.
Hello
I think I'm seeing the light at the end of the tunnel.
Last night I colimated again the scope from scratch.
I first completely backed the primary, then removed the secondary and adjusting only the focuser colimation ring I ensured that the HG lasser beam passed exactly through the center of the hole.
Then attached the secondary again, but put it flat, touching the spiders without fastening the colimation screws.
Inserted the concentric circles attachment and projected it to a flat surface.
The result was perfectly concentric circles.
Then loosened the central screw (3 turns as it was originally) and adjusted the secondary with the colimation screws until I obtained the same concentric circles.
After that I did a visual test to a bright star, getting a perfect circular defocus donut.
I then attached the lodestar and using CCDinsector did some minimal tweaks on the secondary until i got less than 1 arc second colimation error.
Then focused the star.
What I could see, is that I got difraction spikes from the secondary holder vanes, but also got spikes from the primary holder clips.
The combination of both gave me the sense of a triangular shape....
After that I did some tests on the omega centaury core getting reasonable round stars.
My next step is to remove the primary holder clips and do a test on brighter stars (or augmenting exposure times).
Same with your current setup too 
good to see.
