tonyowens_uk

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

I note you have managed to move the motors out of the enclosure (box) Gina. Are you provisioning for a heated & ventilated build chamber as a later development? If so, while a wooden structure is not particularly good at insulating it is supremely good for rapid modification e.g. holes for lights viewing windows fans and cameras! I'll be interested to see how 'square' the XYZ linear axes turn out to be relative to the bed. What are you planning for the bed anyhow? 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

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

That's very interesting material! I did not mean to diss the use of screws just to draw attention to the ease of using clip fits and one-piece alternatives to the usual multi-part assemblies. Bonding-in threaded brass inserts works well where thread wear is an issue. But I note that on one of my unfinished printed designs, for a tip-tilt guidescope mount designed to go onto a Losmandy D rail, I've used a 1mm pitch M8 polished precision threaded stainless screw straight into a slightly undersize 3D printed and reamed hole to do one of the adjustments. Creep and a little heat does the rest. The print will be CF-reinforced PC or similar.

Yes its a DLP structured light scanner not a laser one. Blue light filtered to reduce IR noise effects emanating from ambient solar lighting and with pretty mature software from a Russian developer using thermally-stable steel artefacts for calibration. A genuine metrological scanner not a toy. Its still significant work to run a scanning job but we found the fixture gave us big productivity benefits where small batches of supposedly identical parts are to be inspected. Nothing odd about your motor mount IMHO. The adaptations I had to make when designing for 3D FDM and SLA printing involved abandoning concerns about material use/thick sections and focusing more on feature access, incorporation of living hinges and flexures, deprecation of threaded fasteners, and the usual FDM printability concerns (overhang angle, scaffolding, surface texture, access for finish machining). Initially I felt like a dog with two tails but now I just want to embed and extend the process to facilitate making rapid investment castings in nonferrous alloys. I'd like to connect with anyone who has done this at a professional level BTW. Last year we invested in one of these: from Mass Portal but we are still waiting for a delivery date. Tony

Such things are better in Ireland? We get 21.1 where I live (alt. 65 m latitude 53N on the East coast 30 miles S of Dublin) i.e. Bortle 4.. The deeper Wicklow Mountains get down to 21.7 at 350m elevation with fair vehicular access (Aghavannagh area) but more oiks driving around at night Finally down in Kerry there is the Cahirdaniel area of the Dark Sky Reserve (on the Ring of Kerry) where 21.97 (Bortle 2) at 50m elevation is available when the weather cooperates - which is rare. Tony

Hopefully the technical anecdote below isn't off topic. I think there is enormous potential for rejuvenating telescope making using the better 3D FDM materials coming onto the market not to mention low cost bureau-printed SLS parts in NY 12. See below some 'gash CAD' of the guts of a special high resolution 3D scanner we built last year (for dimensional inspection of precision mouldings). Inspired obviously by a Hubble Optics Dob prototype structure I bought some years ago and which had sat in my workshop reproachfully for several years! The red bits were 3D printed parts. As part of this I decided to put the turntable on a goniometric tilting table. To make the thing backlash-free and cheap I designed a preloaded dovetail slide into the table and had the parts FDM printed in PETG. The pictures and notes say it all. That was the plan! The reality was around 1mm of flatness deviation and 0.5mm of tilt error on the top mounting surface of the assembled table. The dovetail slide surfaces needed to be 'scraped' a bit too to get an acceptable fit. Milling the mounting surface flat. I found the solid 'skin' on the plate too thin to provide a machining allowance. Should have specced it to be thicker. Live and learn. The flexures really helped kill lash in the dovetail and get the parts to conform well. A bit of Kilopoise and knurled clamp screws and the result was good - much better than I'd expected. The real payoff I discovered was the combination of prototyping speed and the intrinsic lightness and stiffness of heavy section parts modeled on traditional metal castings. This gives a high natural frequency and the structural damping of filled polymers. Very interesting for other kinds of instrument building e.g. telescopes!

It's a well researched subject Carl, with some modern twists: 1. The criterion of mounting adequacy is lowest natural frequency of the OTA with the axis clutches (if any) closed. Aim for better than 15 Hz. and better than 25 Hz if high resolution planetary imaging is a science objective. 2. start with the axis modules and make them identical. Refined worm drive or traction or direct drive torque motor transmissions - take your pick. Each has pro's and cons and there is no clear winner 3. Get rid of bending moments in your structural design i.e all unloaded metal. Minimise use of counterweights. Decide at the outset whether this is to be portable, transportable or fixed installation as the design approaches differ for each. 4. Use stressed skin structures as far as possible - they are light and stiff. Don't ignore 3D printed massive parts as a way to do this on the cheap. Replace mechanical fasteners with bonded construction. Incorporate structural damping. 5. Ignore conventional thinking in mount design and look hard at the LBT on Mt Graham USA. 6. Get the controls equipment off of the mount and put it in a nearby humidity-controlled controls enclosure (except for motors and encoders obviously). Use quality high flex robot cable and IP67 multi-pole connectors. 7. If you go 'whole hog' you need a cost effective >= 24 bit absolute encoder on each axis. The only one I know of is Dave Rowe's as used by 10Micron which is brilliant and proprietary. If there are others out there I'd like to know.... 8. You need mount modelling, for decent GOTO's and tracking accuracy, ideally a flavour of Tpoint. Have fun! This kind of work is directly applicable to robot and machine tool design so worth a bit of effort if you are an early career design engineer!

As you are based in Norn Iron, provided you are not too far from Armagh you could check in with events at the Planetarium there. http://www.armaghplanet.com/blog/category/about-us/armagh-planetarium I was a little older back in the 70's when my own parents brought me there. There were people (Pat Corvan and several others) there who cared what impression it made on the mind of a 9 year old and removed the intimidation factor. I visited on and off on observing nights to use the 16" scope there for viewing planets, courtesy of my own and other understanding parents who drove me and a friend there and back the 17 miles from my home in the then-impoverished South of Ireland. Your daughter might appreciate the star shows and some of the displayed hardware in the exhibition area. Whether the 16" is still in use I don't know. Tony Owens