tonyowens_uk

(Courage x Brains x Hard Work) x (recursions)^0.5= Quality of Result!

In this case, looking at that 90 sec image the QoR speaks for itself. Stunning Gina.

3 hours ago, Gina said:

Having worked out a way of pressurising the ASC enclosure with dry air and sorted out the sealing, I'm having second thoughts about having the RPi etc. up in the ASC casing.  A few things really, perhaps the worst is that having the RPi+HAT in the ASC casing means that any alterations to either wiring or firmware would entail taking the whole rig indoors and breaking the main seal to gain access.  Cooling the RPi and other electronics to cope with a possible summer heatwave, with the unit in the full heat of the sun, meant adding a heatsink to cool the air and hence the electronics, to keep it working.  Also, I would like to keep the volume of air inside the enclosure to a minimum.

The reason for having the RPi and other electronics in the ASC casing was to reduce the wiring etc. passing through the sealed bulkhead, particularly the awkward USB cable with a plug on each end, as well as all the other wiring (apart from power and pressure switch).  I guess the problem of the USB cable could be got round by cutting the cable and rejoining it after passing through a grommet in the base - otherwise some printed parts to clamp together round the cable like I had before.

I would appreciate any thoughts on this.

Good result on the leaktesting and snagging of the leaks I/D'ed by that Gina! Its not altogether reassuring that the silicone didn't stick well to the PETG, but provided the joints are lightly compressed all may be well when they see rain and condensate. Given the work it took to address environmental sealing, you have earned the karma for leak-related problems to recede!

Re location of the electronics: you are right. The proper place for electronics, given a choice,  is in controls enclosures. Which implies properly sealed field wiring to sensors and devices is needed. The machine-build world has lots of high cost elegant solutions to this which I'm sure you are well familiar with.  One idea you could consider is a 5m USB 3 cable run up the pylon to the ZWO camera  (you might want the extra bandwidth in a later ASC incarnation) and seal this using a tight-fitting sleeved rubber grommet lubricated with Hellerine (or olive oil!) and stretched over the connector at one end of the cable. Maybe this one: https://ie.rs-online.com/web/p/rubber-grommets/1366294/ which is polychloroprene rubber and is stretchy with good recovery. A cable tie around the sleeve to seal off the cable entry. Print the ASC enclosure base to take the (expanded) grommet diameter once this can be measured. No need to cut anything. How much force to stretch this I dont know. Warming it would help. There is an electronic technician's loom-making tool for stretching grommets and heat shrink sleeves but I dont know if it would be strong enough...

Tony

2 hours ago, Horwig said:

Thanks for taking the time to give such full answers both, lots to consider.

Firstly, the mount is stripped back, the OTA removed, and the fork removed from the polar axis for ease of measurement. So all tests are done off load.

Initial results this morning. The worm bearings are plain ball, but preloaded. Measurement just now shows end float of about 20 microns when changing directions on the motor, on a 21 cm dia. wheel I think that equates to about 40 seconds of arc. Could be the principal source of error.

Also the pillow blocks show appreciable slop, resulting in about 100 microns displacement at the worm when push/pulling the trunion hard by hand, this also needs addressing, but is only going to give problems under heavy load.

Engineers blue was applied to the drive, nice even pattern, no bottoming out exhibited.

Bearing research starts now.

Much appreciated both of you.

 

Huw

Sounds like you have nailed a big part of your issue!

1 hour ago, Horwig said:

An update, the preload was not as tight as I'd thought, tightened it, backlash is now closer to 16 arc seconds, getting there I think.

I am still going to change the worm block bearings to angular contact, these are what I've been looking at :  https://www.bearingboys.co.uk/SKF--FAG/7201BEP-SKF-Single-Row-Angular-Contact-Ball-Bearing-22415-p

Also, the pillow block pack on the driven trunion is going to be replaced with a pair of taper roller bearings, these look possible:  https://www.bearingboys.co.uk/Imperial-Taper-Roller-Bearings/HM-801346-X2310QVQ523-SKF-Imperial-Taper-Roller-Bearing-82351-p

Any thoughts from anybody?

Huw

1. The angular bearings are fine Huw. Loads of preloading capacity - they will be nice and stiff and still run with low breakaway friction. They are unsealed I believe so lube with fluorocarbon thickened grease e.g. DuPont Krytox then keep them clean and forget about them.
2. Those are are man-sized roller bearings Huw. I assume you are happy to deal with the overconstraint issue that will arise if you run the driven Dec trunnion shaft in those and keep the other idler trunnion shaft just 'propped' in a ballbearing pillow block! (either the fork or the OTA will flex a bit as the OTA is moved through its range of motion, which can be undesirable if significant). You could either align the constrained trunnion accurately with the 'prop' one, or use a flex-plate interposed between the drive trunnion and the OTA to decouple the tip/tilt nonlinearities. Or you could just do it and take your chances!
3. I assume your existing trunnions are imperial shaft sizes. Otherwise you would find that 40mm ID taper bearings are cheaper and stiffer than those HM-801346-X2310 units. You can avoid the problem of sourcing precision grade taper bearings (usually P5 or P6) to deal with the 20 micron run-out tolerance on their inner races if you can make the trunnion shaft hollow and optically align it with the opposing 'prop' trunnion!
4. As for the basic notion that the driven trunnion shaft should be run in a bearing design that constrains it like a machine tool spindle - basically you are correct! And taper roller bearings in popular sizes (as used for auto wheel bearings) are the cheapest, stiffest solution. Unfortiunately not the lightest and most compact.

Tony

20 minutes ago, Gina said:

Trying a different 3D print layer arrangement to avoid grooves along the tube connections.

Nice!

I'm still new to 3D FDM printing and would not have thought of that!

Huw I looked over your previous thread on your R&D work on worm gearing so I think I've got a picture of your drive designs. Having refined some of the rough edges that were previous there I'd make a few suggestions for taking things further:

1. As you've discovered there are a great many variables involved in getting instrument gearing to run well. So focus on one problem at a time. Work on one drive at a time. Remove as many variables as you can from that drive before making changes so you can clearly see which are important and which are not. So in that vein get rid of heavy grease lube and use a semi-grease like engine assembly lube on the worm mesh.

2. You need to be able to measure the effects of your changes. As you know for imaging use (backlash less than 500 millisecs/7.5 arc-seconds) the tangential tooth clearance on worm gearing is only a few microns. That is not reliably measurable using most DTI's let alone lever arms, 'feel' etc. So either use the sky and PHD2 for measurements or buy/borrow a laser displacement sensor and do the work on the bench. If you can get a 1/4" bending beam torque wrench or a Tohnichi gauge in the correct torque range that will help you set bearing preload on the worm shaft in a scientific way.

3. The gold standard in instrument adjustments is use of flexures. Try not to use bearings where possible. The conical tipped preloaded setscrews may work for a while (subject to differential thermal expansion and wear) but flexures are the way to go. I can help with standard designs for doing different adjustments and as you have a mill and are not aesthetically obsessive you should be able to make anything needed!

4. Remove the payload and balance the mount as well as possible before performing mesh setting.

5. Your RA and in particular Dec drives will have fairly narrow torque limits within which they exhibit good tracking and backlash control behaviour. If there is no 'design' as such in your drives and you dont have these limits, I'd suggest you choose conservative values like 5 Nm for RA and 2 Nm for Dec and balance and operate your mount to within those limits. It should be possible to jury-rig means of assessing your out of balance in each axis. 

6. Remove as much friction from the main bearings as possible. If you are using a pair of simple ball bearing pillow blocks on each axis ensure they are properly aligned and preloaded enough to remove clearance. but not enough to raise starting torque to more than 1 Nm or so.

7. Your wormshaft bearings ideally should be angular contact bearings and must be square to each other and axially preloaded. Again use starting torque as the criterion for correct preload. But here a much lower figure in the range 0.-02 - 0.1 Nm should be used, depending on the type of bearings and the detailed design of your mount.

8. Dont use Rep-rap commercial-grade timing pulleys and belts if you can avoid it. Use quality kit. Try Transmission Developments in Poole or Misumi for quality kit. For your mount Gates GT3 type in 9 or 15mm width would be about right. Only one pulley should be flanged not both. The belt needs to be tensioned significantly to work properly but don't overload or obsess about this.

9. All instrument gearing for imaging needs to be lapped then burnished. Never use aggressive grinding media for this, especially with a soft metal wormwheel unless you want to turn it into a permanent grinding wheel! The abrasive gets embedded into the soft surface and can be impossible to remove. Aluminium gears should be hardcoat anodised to BS 2536:1995 to 20 um thickness. Burnishing should be done with oil or light grease, minimal torque load, low speed and lots of direction reversals. 

10. As you've discovered the mesh preload used should be as low as possible especially in thin wormwheels to minimise nonlinearity from wheel deflection out of its own plane. Astro imaging is desperately sensitive to the slightest imperfections and flexure and friction enforce very conservative torque limits.

11. Keep drive adjustments independent of each other. Your mesh clearance adjuster for example imposes a parasitic lateral displacement of worm to wheel when it makes a radial clearance adjustment. This changes the mesh line. If the worm radial runout is significant there will be a visible nonlinearity from that effect for example

12. You use a gearbox on your servomotor. Look hard at that for quality standard. Most such things don't come up to the mark for transmission linearity and backlash. Backlash can be measured directly down to a degree or so using hand tools and a protractor on the bench. Anything more can be measured by using the Guiding Assistant feature in PHD2, with at least 40 minutes of 1 second guide camera exposures. Doing an FFT on this data will provide a very revealing analysis about the mechanics in your drive, including the gearbox.

Have a think about these points if you want to improve your tracking capabilities. I can give you a hand with some of this if you want to contact me off-list.

Tony Owens

I look forward to your repeat leak tests with interest Gina. I have notions about using 3D printed enclosure components myself on one of my own projects, with integral flanges and sealing cord grooves, that cannot be moulded conventionally. I will be guided by what you discover!

BTW - did you find the clear RTV sealant peeled easily off the PETG printed parts, or not? I am thinking about adhesion and possible issues in that area. Obviously the RTV evolves acetic acid and water during cure, but neither have any effect on PET so with some cure time should have no effect. On the other hand the so-called PETG filament is likely to be a proprietary blend with a few unspecified substances in the blend to improve lubricity, provide colour or whatever. There might be an issue there...

Best of luck when the cure is complete and you retest!

Tony

3 minutes ago, Gina said:

Testing - lower pressure switch - ON at 5" and OFF at 4" approx.  Need a longed tube for the higher pressure switch.

Typical switch and hysteresis values as I recall Gina. I remember getting into trouble when I adjusted the screws - the behaviour was not predictable as I recall as there was cross-coupling between the adjustments. I sure I forgot to mark them before fiddling!

Tony

Exactly. Its a bit of work to wire all that and more to adjust the setpoint and hysteresis for each of the two switches, but the result would be more informative.

My thought about having a volume at the control room end was to have enough dry air to feed the leak. But we have little idea of the magnitude of the leak that needs feeding. On the positive side, this method of dehumidification actually requires there to be a leak in the camera volume. Otherwise there would be no nett inflow of dried air, and no therapeutic effect, and some form of twin-tubed air exchange mechanism or controlled-leak would be required, with more complexity.

1 hour ago, Gina said:

I can get one like your photo from Amazon - £6.37 with delivery Tuesday.  Might try that.

CDA Tecnik Washing Machine Pressure Switch. Genuine Part Number 49579500

 

I envy you the fast cheap delivery. We dont see a lot of that in Ireland.

Two sets of contacts = DPDT? I think one screw controls contact closure pressure and the other controls opening pressure. It would be interesting to know what sort of leakrate you will be looking at.

Re use of water manometers. The vapour pressure of water is what it is, given a closed system pressure and temperature. A gradient can exist along the capillary, though it will equilibrate quickly enough. Use a dessicant in the camera, and the water column in the manometer will shorten. But as reading these is differential (LHS column height - RHS column height), that would not affect accuracy. But it will kill the dessicant needlessly. To maintain non-condensing conditions exist the camera volume during cooling periods probably best not to use a water manometer anywhere (except for initial switch calibration perhaps). One thought would be to pump air using a bicycle pump into a reservoir 2L PET water bottle with screwcap) a quarter-filled with dessicant, and to connected this to the camera via a 4mm soft PU pneumatic tube. The pressure switch would be teed off this line, near the bottle.  And when the switch resets (or the PET bottle gets a bit flaccid) just give it a bit more air. With some determined initial purging, that might actually work, depending on the leak rate!

Tony Owens

Just a thought Gina if you are leaning towards overpressure monitoring.

Don't overlook washing machine pressure switches. Cheap, reliable, switching pressure generally in the range 3" - 7.5" H2O. Many are adjustable. Most are a bit large but certainly not all. All the diaphragm and microswt stuff taken care of. Best to go get one from a local appliance repair business.

Tony Owens

56 minutes ago, Gina said:

The focus ring turns quite easily, particularly at a radius of about 40mm.  The focus quadrant should be a push fit on the focus ring as the hole is the same size as the earlier focussing setup which worked fine.  I agree that a longer contact with the focus ring might be better or I might bring the motor further up though it's about the same as before.  The stepper motor is the ubiquitous 28BYJ-48 with about 64:1 internal gearbox and the pinion directly drives the quadrant.

I hadn't seen that motor Gina - with the offset output shaft. It will fit perfectly as-is and no more transmission needed!
 

18 minutes ago, Gina said:

The ASC assembly less main enclosure, camera, cooling parts and RPi etc.  Focus motor will be fitted under the camera support with the shaft projecting above and with a small 9t pinion on.  This will engage with the focus quadrant gear attached to the focus ring of the lens.  The gear ratio is 8:1.

Nice and minimalist!
With a pinch clamp to fix the quadrant to the focus ring and a bit of axial length to that clamp to react the radial component of pinion contact pressure, through the quadrant, and into the focus ring. Alternatively, provide a lower bearing land for the lower skirt of the quadrant to ride on. Pinion/quadrant mesh clearance adjustment via slightly oversized camera fixing screws. Use of Nye Lubricants PTFE damping lubricant to kill any stick-slip in the gearing and pinion bearing.

I assume you are connecting the motor to the pinion shaft via a timing belt given the offset position of the shaft?

Tony Owens

16 minutes ago, Gina said:

Been watching a science programme on BBC4 which mentioned the Challenger Shuttle accident and the cause, which turned out to be freezing cold weather affecting rubber O rings.  Now I wonder if frost was the reason my ASC seals failed.  Bathroom sealant is used at room temperature and not designed for large temperature range, particularly cold.

I don't think so Gina. Remember that low outgassing grades of silicone adhesive and potting (not RTV types admittedly) are extensively used in aerospace for mounting large refractive optics. Silicones are fine at low as well as high temperatures. If the adhesion of the sealant is not in question, I'd look carefully at permeability of the printed parts. Layer adhesion (a function of internal voiding) of PETG is purported very good - less so some other filaments, and its vapour barrier properties are well-known from food packaging uses in PET foil form.

1. Would it be practical for you to reverse-pressurise and waterbath/bubble test the enclosure you require to be hermetic (without the camera inside obviously)? I'm thinking about how to simply prove the seal integrity and identifying cracks/porosity if any

2. Crude and a cop-out perhaps - but would a 3mm plastic tube run into the enclosure and used either to positively pressurise it to a couple of inches of water head, OR a flow/return pair of such tubes solve the problem permanently? The air pump could be a small high head DC fan and the dessicant for the recirculating system could live in the vessel along with the fan somewhere indoors near the pylon...

3. If disassembly is not a requirement, there are alternatives to RTV silicone sealant and encapsulant which may work better for you. RS Components for a quick review, and Easycomposites UK for a more focused assessment of some of them.

Keep going!

Tony

On 15/04/2019 at 21:52, Velvet said:

Hello as anyone got this mount working successful on eq mod  was trying one and when i connect using Mount Options  auto detect i get the message Overflow 

if i change  to  custom then AZEQ5 it connects  so my question is as any one got the right settings for the AZ-GTI mount

the manual says Gear ratio 6480 resolution 2073600 counts /rev

 

thanks in advance

 

 

A mate of mine from Melbourne has helped one of his pals who has an AZGTi to get connected to EQMOD.

As you mention the Custom option needs to be used. You are correct about both gear ratio and motor encoder resolution. In addition, as there is no PPEC function on this mount the mount controller returns zero for the microsteps per RA/Az worm rev, causing a divide by zero error. Apparently it is possible to manually override this in EQMOD. Our own AZGTi is still laid up and awaiting a spare part so I cant confirm what to do exactly.

Give it a lash and let us know you get on!

Tony Owens

I think the principles of their design are spot-on.

1. No counterweights: These are an Edwardian engineering anachronism that have no other function than to allow badly-designed worm gearing to deliver acceptably smooth operation and wear life. Sadly the GEM with counterweights has become something of a meme. Industrial robots have managed to combine good accuracy and repeatability for many years without resorting to them, as have professional scope mounts.

2. Compact and lightweight. The value of this to end-users is more substantial is commonly realised by equipment manufacturers. The success of giant-killing small tracking AltAz mounts from iOptron and Skywatcher is evidence of that. With the shift away from long exposure CCD technology to CMOS stacked subframes, and the growing awareness that high resolution (>24 bit) axis encoders and direct drive provide no compelling advantages over an autoguided open-loop mount outside a permanent observatory, I expect to see fewer cost-optimised 'toy' tracking AltAz mounts in the future and more 'authentic' mounts based on industrial automation technology, like this one.

3. Billet-machined construction. This offers no functional advantages whatsoever compared with a mount made from machined castings (and sometimes serious stiffness disadvantages), but in the minds of many buyers it connotes superior performance

4. Freedom from routine mechanical maintenance. This is increasingly important as imaging equipment proliferates, and it characterises harmonic-type reducers as are used here.

The pricing is going to be an issue I suspect.

The controls environment (ASCOM, smartphone interfaces, homing, cable-wrap, backlash detection and compensation, sky modelling) is generally a greater challenge than the mechanical engineering. It will be interesting to see how well evolved this is.

All credit to the developers for bringing this to market!

Tony Owens

Gina
Based on what we read your approach to developing this camera to be as simple as possible (but no simpler) and how you've allocated your time to testing is exemplary.

Its a bit of a pain to have to put the remote focusing back in but youve already developed this module if I understand your previous sky camera efforts so hopefully it will be 'just' a packaging problem.

And while that is being designed and built I'm sure I'm not the only one curious about the hermeticity and environmental resistance of heavy duty 3D printed structures like those you've used. Being able to monitor the dessicant condition visually from afar is a nice touch. how that changes in the face of daily thermal hilo's and rain and vibration will be interesting.

Thanks for the filament material reference. I'm only getting started myself.

Tony

Gina what was that translucent filament you made the exterior casing and dome retainer ring from?

Also have you had any problems with outgassing (acetic acid and moisture) from the RTV sealer on these types of projects involving optical windows?

Thanks

Tony Owens

FWIW Gina I have an iPhone app I very occasionally use called VibSensor which enables vibration and tip/tilt logging using the inbuilt accelerometers. It operates across a decent frequency range and does power spectrum analysis, automatic resonant frequency ID'ing etc. Sensitivity (with recent iPhone) is quite good and span with most smartphones will be 0-2G. By comparison of power spectrum before/after treatments such as the torsional damper, this might help you get to the bottom of any resonance issues with your Z motor mounting and coupling, as well as help you quantify bending stiffness of your printbed and the print carriage on its linear ways. If you decide to try this, I'd recommend fixing the accelerometers (smartphone) to the structure you are interested in using 3 bits of Blu-tac.

Those larger frame size steppers can be installation-sensitive. Other treatments I've used myself to try and tame resonant behaviour and loss of steps include clamping a heavy steel annulus around the motor body (round motors obviously) and mounting the motor flange using purpose-made hard elastomer motor mounts like these: https://www.ebay.co.uk/itm/Nema-23-Stepper-Motor-Damper-Pad-Anti-Vibration-CNC-3D-Printer-Mill-/151856260838

Tony Owens

PS: Credit where credit is due: The tip about coupling the accelerometers to the structure was from Es Reid in a different engineering context but it works well for this too!

That figure sounds fine to me Rob. Its easy to add a front baffle later to kill off-axis lighting during critical lunar imaging or similar.

Tony

OK I see. Well there is no real rule for tube extension length beyond the focuser that I'm aware of apart from the matter of adequately baffling the background visible at the eyepiece around the secondary mirror. There are two extremes.

1. Dematerialised to just a potato chip-shaped flocked disk opposite the focuser like in many ultralight truss Dobs

2. At the other extreme (Intes Micro Mak-Newt's) it can be a set of close-pitched knife-edged baffle rings stationed along the tube directly opposite the focuser hole, and the provision of an external internally-baffled tube extension (dewcap/baffle tube) of around a 1.5 times the scope aperture.

Your choice will reflect convenience, weight, hassle and the quality of baffling you need which depends on what you want to do with the scope!

MN86 from Intes Micro. Note huge dewcap/baffle

...with internal knife edge baffles

...and yet more baffles behind the secondary. The scope is pointed a few degrees away from the Sun (light entering from the LHS) but the off-axis light is fairly well suppressed even thought this is a Newt

This is the tube and optical layout for an unmodified MN86 Mak-Newt (no dewcap/baffle in the model). It is almost identical to an 8" F5.9 regular Newt in baffling and layout.

Food for thought perhaps?

Tony Owens

Robert re tube design and baffling see here:

http://www.download82.com/download/windows/newt/

It gives internal baffles and their effects too. It does assume you have an accurate figure for the FL of the primary...

Tony

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

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

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