Mirror Cell question

[Tommohawk]

Hi and Happy New Year to all!

I'm looking at various DIY Newtonian designs - key objective to be superlight- and I notice that TS optics do a "conic" mirror with a dedicated mount. It's not cheap but it looks good. See here

However, in my research I see that the cell design, especially the way the mirror is supported , is critical to avoid mirror flexure/warping.  Typically multipoint floating systems are favoured.

But this cell only seem to  support the lens centrally via the threaded mount. Does anyone have any experience or views or thoughts on this mirror / cell combination?

Thanks!

[Peter Drew]

Similar mounting systems for catadioptric primary mirrors or any thin edge/thick centre profile mirror.    ?

[Tommohawk]

Hi Peter - well that's a fair point! … and a pretty obvious one I suppose on reflection!

I guess partly I've become sensitized to  this because my current experimental newt - a Quattro 10" - seems incredibly sensitive to how the mirror is held in the cell. It's difficult to do consistent tests but it doesn't seem to like any pressure at all on the mirror. 

I guess the TS people know what there doing!

 

[mapstar]

On 02/01/2019 at 21:13, Tommohawk said:

Hi Peter - well that's a fair point! … and a pretty obvious one I suppose on reflection!

I guess partly I've become sensitized to  this because my current experimental newt - a Quattro 10" - seems incredibly sensitive to how the mirror is held in the cell. It's difficult to do consistent tests but it doesn't seem to like any pressure at all on the mirror. 

I guess the TS people know what there doing!

 

Are the clips on it over tightened? It shouldn't noticibly distort with a mirror of that size/weight? What use are you intending? Visual or A.P.? 

I guess for 18" with a cell its about right price wise looking at other mirror makers and then adding a cell.  A skywatcher stargate 18" is about the same for a full scope though. Not sure how they compare with the spec of the optics? 

[tonyowens_uk]

Hub mounting is the professional way to mount small to medium first-surface mirrors. Provided the mirror is given a conical or elliptical profile to the rear surface carefully designed to minimise gravity sag errors at the optical surface, this style of mount works better than almost any alternative up to around 0.6 m aperture. The big benefits apart from providing sag-free support are absence of any slop or lost motion between mirror and cell which helps collimation stability and the very open mirror cell structure which helps the rear of the optic to freely exchange radiation with the telescope's surrounding and equilibrate quickly.

A problem can arise however where the perforation through the mirror is bonded with RTV onto a metal peg or tube. Unless this joint is carefully designed the mismatch of thermal expansion rate between metal, mirror substrate and elastomer can impose enough stress on the inside of the mirror perforation to deform the surface at low temperatures. I have no idea whether this was addressed in the design of the TS Optics line of ribbed conical primaries.

As far as the TS Optics mirror cell is concerned, in my opinion it is overpriced for what it appears to be. If you are planning a scope build it might be better to buy the mirror first then design your own tip/tilt mirror cell to suit the metal hub and either make it yourself or contract others to do so.

A final thought. TS Optics has an arrangement with a Wolfgang Rohr who has a reputation in Germany for reliable independent quality testing and interferometric certification of optics. http://r2.astro-foren.com/index.php/de/ On enquiry they should be able to organise this for you, and you may be able to specify a minimum quality optic you are willing to accept.

 

Tony Owens

[Tommohawk]

On ‎04‎/‎01‎/‎2019 at 12:40, mapstar said:

Are the clips on it over tightened? It shouldn't noticibly distort with a mirror of that size/weight? What use are you intending? Visual or A.P.? 

I guess for 18" with a cell its about right price wise looking at other mirror makers and then adding a cell.  A skywatcher stargate 18" is about the same for a full scope though. Not sure how they compare with the spec of the optics? 

Thanks for the input - nope, the clips are loose. But the mirror on the Quattro doesn't have a multipoint mount so not sure if that's an issue. TBH its difficult to  be sure quite what's going on because there's so little time for testing. Either the seeing is good in which case I'm imaging or its rubbish in which case I'm indoors! Maybe its tube currents that's the problem.

On ‎04‎/‎01‎/‎2019 at 13:10, tonyowens_uk said:

Hub mounting is the professional way to mount small to medium first-surface mirrors. Provided the mirror is given a conical or elliptical profile to the rear surface carefully designed to minimise gravity sag errors at the optical surface, this style of mount works better than almost any alternative up to around 0.6 m aperture. The big benefits apart from providing sag-free support are absence of any slop or lost motion between mirror and cell which helps collimation stability and the very open mirror cell structure which helps the rear of the optic to freely exchange radiation with the telescope's surrounding and equilibrate quickly.

A problem can arise however where the perforation through the mirror is bonded with RTV onto a metal peg or tube. Unless this joint is carefully designed the mismatch of thermal expansion rate between metal, mirror substrate and elastomer can impose enough stress on the inside of the mirror perforation to deform the surface at low temperatures. I have no idea whether this was addressed in the design of the TS Optics line of ribbed conical primaries.

As far as the TS Optics mirror cell is concerned, in my opinion it is overpriced for what it appears to be. If you are planning a scope build it might be better to buy the mirror first then design your own tip/tilt mirror cell to suit the metal hub and either make it yourself or contract others to do so.

A final thought. TS Optics has an arrangement with a Wolfgang Rohr who has a reputation in Germany for reliable independent quality testing and interferometric certification of optics. http://r2.astro-foren.com/index.php/de/ On enquiry they should be able to organise this for you, and you may be able to specify a minimum quality optic you are willing to accept.

 

Tony Owens

Thanks for those comments! Is the mirror perforated with a newt hub mount? I assume it was just bonded - though I guess this can still cause a problem. I've certainly read of this issue with bonded secondaries.

Interesting you feel the cell is overpriced and I did consider fabricating my own - in fact this would be better in that I have a non-standard truss tube in mind. I don't have a machine shop so mustn't get too ambitious with my design. The more off the shelf stuff I can get the better.

Re the certification, yes I saw this on TS website. But.... is it me or does it seem a bit steep to have to pay for certification?? I guess everything has its price, but do they test the individual mirrors at all? If so, shouldn't this be enough? Maybe they don't?

 

Regards

Tom

 

[tonyowens_uk]

10 minutes ago, Tommohawk said:

Thanks for the input - nope, the clips are loose. But the mirror on the Quattro doesn't have a multipoint mount so not sure if that's an issue. TBH its difficult to  be sure quite what's going on because there's so little time for testing. Either the seeing is good in which case I'm imaging or its rubbish in which case I'm indoors! Maybe its tube currents that's the problem.

Thanks for those comments! Is the mirror perforated with a newt hub mount? I assume it was just bonded - though I guess this can still cause a problem. I've certainly read of this issue with bonded secondaries.

Interesting you feel the cell is overpriced and I did consider fabricating my own - in fact this would be better in that I have a non-standard truss tube in mind. I don't have a machine shop so mustn't get too ambitious with my design. The more off the shelf stuff I can get the better.

Re the certification, yes I saw this on TS website. But.... is it me or does it seem a bit steep to have to pay for certification?? I guess everything has its price, but do they test the individual mirrors at all? If so, shouldn't this be enough? Maybe they don't?

 

Regards

Tom

 

Re the issue of mirror perforation Tom, more or less all conical backed primaries are either partially or fully perforated. For Newts full perforation is an inconvenience as there is nowhere to stick a collimation donut. I've dealt with that problem in a 14" F4 conical Newt by providing an accurately centered machined plug with a donut-like target machined into its centre and using a barlowed laser for basic collimation. Fine collimation can be done in the usual way (correcting the shape of defocused star images at the eyepiece using the collimation adjustments.) Robert Royce in the US is a well-regarded large optic optician who has popularised conical primaries up to 16" aperture, although he had to do some development to achieve a properly athermalised hub mount arrangement so his customers don't have to deal with the issue. My advice would be not to worry about the bonding of the hub into the TS Optics 20" conical. In the unlikely event that it causes problems it isn't difficult or costly to remove it and replace it with a better design. I did that some years ago with a 10" F6 conical and have been very satisfied with the results.

If you decide to build a cell rather than buy, consider getting the parts (be they birch plywood or 6082 T6 aluminium plate) waterjet cut including holes. This should be a fraction of the price TS Optics are charging. The central hub can be turned and milled. Contact me off-list if you need a recommendation for low cost machining suppliers willing to consider R&D quantities.

I cant answer your question about certification. The fact is that this is a 3k euro piece of glass made by an undisclosed Chinese optician, supplied by a German distributor with no objective performance certification and no clear undertaking that I could see as to its optical quality. Non-trivial optical parts are normally certified against a relevant performance standard in order to meet the expectations of their buyer as well as to have an enduring asset value. If you are happy with TS Optics proposed terms of sale and customer acceptance, good luck to you! If it was me I'd insist it was tested by a reputable optician such as Es Reid or Hr. Rohr and I'd be happy to pay for that service.

 

Best  Tony

[Tommohawk]

22 hours ago, tonyowens_uk said:

Re the issue of mirror perforation Tom, more or less all conical backed primaries are either partially or fully perforated. For Newts full perforation is an inconvenience as there is nowhere to stick a collimation donut. I've dealt with that problem in a 14" F4 conical Newt by providing an accurately centered machined plug with a donut-like target machined into its centre and using a barlowed laser for basic collimation. Fine collimation can be done in the usual way (correcting the shape of defocused star images at the eyepiece using the collimation adjustments.) Robert Royce in the US is a well-regarded large optic optician who has popularised conical primaries up to 16" aperture, although he had to do some development to achieve a properly athermalised hub mount arrangement so his customers don't have to deal with the issue. My advice would be not to worry about the bonding of the hub into the TS Optics 20" conical. In the unlikely event that it causes problems it isn't difficult or costly to remove it and replace it with a better design. I did that some years ago with a 10" F6 conical and have been very satisfied with the results.

If you decide to build a cell rather than buy, consider getting the parts (be they birch plywood or 6082 T6 aluminium plate) waterjet cut including holes. This should be a fraction of the price TS Optics are charging. The central hub can be turned and milled. Contact me off-list if you need a recommendation for low cost machining suppliers willing to consider R&D quantities.

I cant answer your question about certification. The fact is that this is a 3k euro piece of glass made by an undisclosed Chinese optician, supplied by a German distributor with no objective performance certification and no clear undertaking that I could see as to its optical quality. Non-trivial optical parts are normally certified against a relevant performance standard in order to meet the expectations of their buyer as well as to have an enduring asset value. If you are happy with TS Optics proposed terms of sale and customer acceptance, good luck to you! If it was me I'd insist it was tested by a reputable optician such as Es Reid or Hr. Rohr and I'd be happy to pay for that service.

 

Best  Tony

Thanks for that Tony - all very helpful.

Interesting you mentioned 6082 ally as the material for the cell, and waterjet cut with holes. It seemed to me looking at on online supplier that this could be a cost effective solution, so reassuring to have it confirmed by someone with more experience. What thickness do you think I should go for? 

BTW I think the link I sent might have been for the 18" mirror - that was a bit random just to show the conic version - I actually would only be working with a 12" version, for a prototype anyhow.

I'm not sure if TS optics open to haggling, but maybe as you say I'd be better to fork out for testing. Maybe with Brexit just around the corner I can expect a surge in the value of the £ ?

Probably not!

[tonyowens_uk]

My suggestion would be to forget about making anything until you have a design. To have that, you need to start with some ideas about your goals. Is this primarily for looking through or for imaging? If its an imager, is it for hi-resolution lunar/planetary or widefield and optically-fast deepsky? Does it even need an eyepiece or can it be prime focus?

Once that is clearer, you will need info about the mounting interfaces to bought-in parts like the hub of whatever mirror you want to use., before you can detail a design On occasion it is easiest just to buy the part and measure it. Some people like to use paper and pencil, some use 2D CAD, some use 3D CAD. Whatever works. But when you have a concept you a re happy with, prepare working drawings of the various bits to be made. Before laying out money for a set of waterjet-cut aluminium plates (or anything!) consider making a partial or complete mockup using furniture grade plywood which is easy to cut and modify and can be pinned screwed and glued easily. It is also a very decent natural composite that in many cases is as good as aluminium or CFRP depending on what you are trying to do.

Stay away from heavy plates in your design. The goal should be to create a structure in which every piece of material is loaded in tension or compression, but not in bending. You also need to make sure that the various 'hardpoints' - places into which heavy loads are punched - are strong enough.  I dont know how much research you have done already but lightweight telescope structures are an internet meme. There is a huge amount to be learned from the work of other people. Check out Martin Lewis at www.skyinspector.co.uk, Mel Bartels at www.bbastrodesigns.com/NewtDesigner.html

Take your time and enjoy the journey!

 

Tony

[Tommohawk]

Thanks Tony for that further info and link.

I have a pretty clear design in mind and I'd like to make a prototype version. I'm confident about the optics, its some of the mechanical fittings that bother me. The problem is that most of this has to do with the way the mirror cell and struts are mounted and there isn't any easy way of doing a cheap trial. Using ply/timber for the cell is a great idea, but the fittings would be so different to those used in the final version that it wouldnt really serve as a prototype. 

It doesn't help that I always seem to  have a million things on the go! One thing is for sure - I won't be rushing into anything!

 

 

 

[davejlec]

If your prototype is for a 12" mirror, you may want to check out a double plate mirror cell design made from a high-grade plywood. A 6-point PLOP designed cell would be more than adequate to provide support, (esp. if the mirror isn't full thickness) Basically described, a top plate with 6 contact "points" pivots on a (20 mm or 3/4") steel ball bearing that is sandwiched between the top and bottom plates, recessed washers hold the bearing in position .  The "points" can be short pieces of dowel that are screwed and glued to the top plate. The mirror is attached to the dowels with silicone- it's important to use removable spacers between the mirror and top plate until the silicone fully cures - this prevents any torsional stresses to the mirror and applies them only to the plate. The design also includes 2 edge support points for the mirror.   

12.5 6point cell (1).pdf

[Tommohawk]

Thanks for that, it looks useful for sure, though I think I'm committed to a hub mount - that said I guess the design could be adapted. 

Tony - does the threaded hub on the mirror typically screw into a threaded plate, or does it fit through a plain hole with a nut on the other side? Or maybe both- a threaded plate and a lock nut? 

[planetman83]

Hello guys, I would like to build a 20-24" dobsonian and wondering about the quality of these conical mirrors. Is there any optician in Europe that can build a conical mirror of good quality?

[tonyowens_uk]

Yes of course. The issue have less to do with the conical back than to do with the hassle factor of altering established polishing and testing setups to suit a large optic with a conical rear. You need to look out for a small highly capable firm used to jobbing precision optics and capable of making one that large. Know what profile of conical rear you need and have a proper enginering drawing ready before you make an approach, or you may look like a time-waster. If you dont have the FEA capability to design a hub-mounted blank with the right profile to give you deformations low enough to meet your use requirements, you should find someone with that experience too.

The optician is unlikely to generate your required conical profile in-house. Diamond milling of glasses and optical ceramics is generally contracted to the optical glass stockist. I had a 14" F4 parabolic mirror to my own design produced by Oldham Optics UK years ago, later refigured by Es Reid. I believe others in the EU such as Toscan Optics in Italy can also supply the semi-pro as well as the professional market for custom optics, as can Norman Fullam in Quebec.

Good luck!