2nd Impressions of a GSO 12" Truss R/C

[loc46south]

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 :mad2:

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.

cheers
Geof Wingham

[multiweb]

Have you checked the tension on the secondary 3 collimation screws?

[loc46south]

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.

Cheers
Geof Wingham

[Paul Haese]

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.

[loc46south]

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.

Cheers
Geof Wingham

[allan gould]

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

[glend]

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

[loc46south]

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.

Cheers
Geof wingham

[Satchmo]

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.

[loc46south]

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.

Cheers
Geof Wingham

[glend]

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.

[Shiraz]

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.

Hope you get it sorted Geoffrey.

[Satchmo]

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 .

[strongmanmike]

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 :sadeyes:

I wish you the best tracking down your issues Geoffrey

Mike

[gregbradley]

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.

[IanW]

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.

[gbeal]

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

[ericwbenson]

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.

Regards,
EB

[loc46south]

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.

[David Fitz-Henr]

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.
David

[gregbradley]

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

[loc46south]

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.

Cheers
Geof Wingham

[ericwbenson]

Ha! I had to giggle, it reminds me so much of back home:
http://www.faintgalaxy.com/podundersnow.htm

[Joshua Bunn]

Hey Eric, If those two scopes are on a G11, you certainly get the most out of your mounts :thumbsup: Same with your current setup too :) good to see.

[DaveNZ]

Great to see the observatory v2 is holding up in the snow.

Is it inland from Milton or over on the hills towards Taieri Mouth?

[loc46south]

It's inland - trying to get away from the dreaded wet easterly fogs

[gvanhau]

Hello Geoffrey

I was wondering if you or anybody else had success removing the silicone pads.

I am having the same triangular stars in my 12" TPO RC and until now had no working solution for it.

Regards
Geert

[Paul Haese]

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.

[loc46south]

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

[gvanhau]

Thanks Paul, I will try loosening the Baffle.
I thought that the baffle tube had to be tight since it was holding the mirror....

Regards
Geert

[gvanhau]

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.

Regards
Geert

[loc46south]

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.

Geof

[BPO]

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.

[Paul Haese]

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.

http://paulhaese.net/NGC1097.html

[gvanhau]

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?

Geert

[Paul Haese]

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.

[Satchmo]

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.

[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

[Paul Haese]

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.




I don't any experience with the secondary disassembly Geert but I am sure this could be another place to check too.

[gvanhau]

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).

Regards
Geert

[Scheimfluge]

I have the same scope Astro-Tech 12" RC Serrurier Truss. A few days ago I did notice that my secondary mirror rattled so I tightened up the retainer ring.