In my first impressions I made a few assumptions based on other peoples experiences, one thing I needed to check was if the retaining clips were contacting that mirror and causing the stress on the mirror. I disassembled the telescope and removed the retaining rings completely. The telescope was reassembled and retested - triangular stars still apparent in images 1,2 &3
Knowing that R/C's are sensitive to collimation issues I took a series of 10 images as close to focus as I could visually get and ran these through CCD Inspector - they showed that the collimation I was able to get with the Takahashi Collimating Scope was close. Camera used was SBIG ST10XME 6.8 micron pixels.
I was not surprised by this outcome. The design of the mirror retaining cell was the primary cause. Most of the articles I have read concerning telescope mirrors and telescope building say that the mirror should not be firmly anchored in the cell, yet here is a mirror that is SOLIDLY fixed within the cell - I was unable to move it. The mirror has been fixed by injecting a silicon like substance through 3 points 120 degrees apart. It has been injected under pressure and this pressure has been further increased to the point where excess silicon in the cell has been forced up and become crowned.
The increase in pressure is due to the contraction of the cell at a greater rate than the shrinkage of the mirror. The coefficient of expansion for plate glass is 5.0 - its 7.3 for steel of 12.3 if the frame is aluminium. The result is just like placing you finger in a vice and as the temperature drops just keep on winding. Telescope was assembled in Taiwan and now resides in South Otago - a diference in ambient temperature of about 20 C - The crowning of the silicon sealer is an indication of how much pressure is being transmitted to the mirror and deformation of the surface is inevitable - I just hope that the mirror has not become permanently stressed. Another thing is that this major stress point is directly above the connection/adjustment screw to the main body of the telescope so and stress caused by that connection is also transmiltted to the mirror.
This matter has been bought to the attention of GSO - what action is taken we can only wait and see.
Have you checked the tension on the secondary 3 collimation screws?
Yup - tested them after I took the images with live camera feed on focus - plus a few other tests as well - this website http://www.loptics.com/ has a good article "Why arn't my stars round" gives good information into why the malformation of stars occurs - he should have a reasonable idea on what he he talking about - he builds the custom mirrors for the big SDM Dob's.
Well at least you can see what is causing the issue. If it was not the clips this was the next thing to think of. My mirror set does not appear to have this silicon in it. I don't know why but I had heard it was being used in the scopes. When I stripped mine I could not see any silicon. It might be there but it would certainly affect the performance of the mirror. I would suggest slicing this but as you say get GSO to work out what to do next. I don't know why they have put it in these scopes. The baffle tube holds in the scope and the mirror clips are just there to prevent the mirror coming out if the baffle is not secure.
I am glad you pulled the scope apart. Jim needs to know these issues. Your collimation looks similar to mine.
Hi Paul - I'm also interested to find out why the silicon is there - I personally don't think that it serves any practical purpose - I intend to go up to the observatory today and to try and cut the silicon out - I will then wait a couple of days for the optics to settle and hopefully see some improvement in image quality.
The quality of the mirror is not in dispute, to give a 4 pixle star in that sort of situation, with a quick manual focus is better than OK.
Geoffrey
I sincerely hope that the silicon is the problem behind the triangular stars and if so you are to be commended for getting to the probable cause.
Watching this with interest and the best of luck.
Allan
The use of what appears to be silicon (but which could be something like a polyurethane) is a common practice by GSO. GSO has been using this substance on its large 16" dobs for at least a couple of years. It is on my 16" dob between the mirror carrier and the glass, three blobs spaced 120 degrees apart (and equi-distant between the clips. It has been my view that the purpose is to further support the heavy thick mirror and prevent shifting in the carrier or pinching against one of the clips, which can be a problem during transport or at low angles of operation. It does not affect my mirror performance in anyway that I can see as the substance is compressible and will yield with any expansion or contraction of the carrier or the glass. My collimation stays perfect through changes in temperature experienced at Bretti (where its not unusual to have hot days followed by cold nights). I have not trianglular starts. This substance has been noticed by the 16" dob owners on Cloudy Nights and in reviews without any negative performance credited to its presence.
Some people have cut it out (using a wire saw) mostly so that they can remove the mirror for cleaning, but most don't bother as there is no link with an negative effects. I suspect GSO would have tested that before they used it.
It is a good idea to ask GSO why its there, something I have not done. I'd suggest not removing it until you get a reason from GSO as to why its there. You can email them here: service@gsotelescope.com
Thanks for that info Glen - I presume that you are using your Dob visually - in the beginning I checked the R/C with an eyepiece and I did not see the triangular stars only what looked like a slight trace of coma and that did not concern me unduly as I knew that the telescope needed further adjustment to reach collimation. It wasn't till I put the camera in that I discovered the problem of triangular stars.
I removed the silicon earlier this morning - further star tests will tell if my guess was correct or not.
Any triangulation you see at prime focus should be seen easily visually at high power but close to focus - don't rack a star out of focus to fill 1/3 of the field at high power and expect to see triangulation - only 5 waves or so of defocus will show the triangulation visually ( 5 dark Fresnel rings in the out of focus star ) ...
Its possible of course that the triangular pinch appearance is actually in the polished surface , but silicone has incredible power to warp optics if not done properly so we would hope it is the silicone.
Thanks Mark - I very rarely look through a telescope and I wouldnt know what to look for. My main thrust is astrophotography. I only own 2 eyepieces, a super special ($49) 2" OPT 50mm and a 1.25 40mm Celestron. A pretty distressing admission for someone who owns 3 Taks, one a 250S Mewlon.
I have been warned that the stress could have been polished in but I am keeping my fingers crossed. GSO does have a reasonable reputation for good mirrors - one reason I went for the 12" R/C in the first place.
I can't see how a blob of silicon can pinch a mirror. It is not applied under any more pressure than that which pushes it out of a tube with a caulking gun. Once applied to the carrier it does not expand rather oozes into the vacant space between the mirror and carrier ring. If applied before the mirror the weight of the mirror will push it into filling the gap space. Silicon does not expand when curing, unlike some polyurethanes. Cured silicon will have much greater ability to compress than the rubber under the clips, it can only be there as a bumper in my opinion. Large mirrors have been supported by slings, posts with rollers and pads for decades. Get an answer from GSO.
I can't see how a blob of silicon can pinch a mirror. It is not applied under any more pressure than that which pushes it out of a tube with a caulking gun. Once applied to the carrier it does not expand rather oozes into the vacant space between the mirror and carrier ring. If applied before the mirror the weight of the mirror will push it into filling the gap space. Silicon does not expand when curing, unlike some polyurethanes. Cured silicon will have much greater ability to compress than the rubber under the clips, it can only be there as a bumper in my opinion. Large mirrors have been supported by slings, posts with rollers and pads for decades. Get an answer from GSO.
no it doesn't expand Glen, most varieties of the sodding stuff shrink by a couple of percent on curing. Putting it into a gap like that seems to me to be asking for trouble - the mirror will be pulled three ways.
I wish I had a dollar for every time that debonding a siliconed secondary mirror in a Newt has cured severe astigmatism in the star test . It certainly does have the power to warp optics - particularly if applied and then cured at a thickness below 3mm .
I wish I had a dollar for every time that debonding a siliconed secondary mirror in a Newt has cured severe astigmatism in the star test . It certainly does have the power to warp optics - particularly if applied and then cured at a thickness below 3mm .
This was the problem that plagued my infamous ASA N12...after much heartache and failed attempts at rectification, unfortunately this was only discovered after I had returned it to Austria for a refund
I wish you the best tracking down your issues Geoffrey
I've had a lot of experience with silicone. Its actually a very strong adhesive and tough. Horrible stuff really. Has the ability to get on everything.
That seems like a crude way of mounting a secondary - cheap and nasty. No wonder it causes problems.
I've had a lot of experience with silicone. Its actually a very strong adhesive and tough. Horrible stuff really. Has the ability to get on everything.
That seems like a crude way of mounting a secondary - cheap and nasty. No wonder it causes problems.
Greg.
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.
Hmmmm.
I have had a 10" f5 optical set from the mid to late 90's, bought two of them in from the US, I suspect they were the beginning of the GSO line-up, but am not sure.
I have had the optics in a slew of tube reincarnations, the latest being a self-built C/F tube.
When I first started I used a solid U/O primary mirror cell, heavier than the mirror itself (and that is 5 kilos). After a while I sold this cell, and built a couple of others, one being a foam core covered with C/F, the other being two triangular "slabs" of 6mm S/S.
In both instances, (C/F or S/S) I use three small blobs of silicone adhesive to attach the primary to the "cell".
At no stage have I had deformed stars, or what I'd call pinched optics. Those that have looked through it are constantly amazed at the quality.
The 10" has sat unloved for a few years, but of late is my primary solar/lunar and DSO imaging scope, from DSLR through the PGR Grasshopper.
I'm not saying you are wrong with the condemnation of silicone, but I am saying I haven't had any issues. As our American friends would say "YMMV".
Gary
Hi,
Silicone rubber has a very large coefficient of thermal expansion, about 250-300 ppm/deg C, the mirror itself is on the order of 5 ppm/degC. So putting silicone on the outside rim is about the worst place it could be. It will pull the mirror outwards from three points as the system cools throughout the night - causing trefoil astigmatism (triangular stars), what is observed!
Putting Silicone on the back where the support points are will only pull the mirror down into the cell more, and so if nothing else "is in the way" pressing against the mirror, no force is transmitted to the glass, no distortion, no problem.
These are simple engineering principles and fairly basic telescope building know-how. I get the feeling GSO's design philosophy is heavily weighted to trial and error, which would be fine if it never left the alpha testers backyard, but to be shipping mass produced stuff, well you get what you pay for, and product development is not cheap.
Hi - It would not have been so bad if there was only a small amount of silocon used but if you look at the image the mirror was well and truely stuck in - it was approx 20 mm square and 5 mm thick.
I have since last post cut the silicon out. The astronomy gods have responded by clouding the skies and we have had rain, hail and snow. So it seems like I may have made the right decision - will test as soon as the skies clear again.
In terms of mounting a largeish secondary mirror (in my case 100mm minor axis / 142mm major axis and 25.4mm thick) I'm not aware of a better method of mounting, although you need to apply some solid basic principles. If you were to use retaining clips on the periphery to mount the secondary that will introduce a larger wavefront error (though still within acceptable limits I believe, ~1/20 wave PV ballpark) as well as diffraction effects.
Principles that I have applied:
Use only 3 silicone blobs supporting roughly equal areas (taking into account the 45 degree cut angle) at around 70% radius, being ~18mm diameter. More than three can result in astigmatism if the supporting cell deforms due thermal effects, etc; whereas three will act more or less the same as a standard three point support cell. I'm amazed when I hear about commercial manufacturers using 4 or more blobs, or worse, rings of silicone around the whole mirror - a recipe for astigmatism.
The support "cell" should have a similar cte as the mirror substrate and be extremely rigid. In my case I have a pyrex secondary (cte ~3.3 x 10-6 / deg C) and I made a solid carbon laminate plate ~7.6mm thick which would have a cte closer to zero. FYI - aluminium has a much higher cte ~ 23 x 10-6 / deg C.
Note that the silicone "blobs" that I have formed are only ~ 0.3mm thick and I do not notice any distortion in my star images (CCD); though I have not performed any high-magnification visual star tests.
