12.5" Newtonian Astrograph for SLATO

[David Fitz-Henr]

I thought I would start chronicling the making of my next telescope, a 12.5" Newtonian Astrograph that will replace my current 200mm Newtonian. I've learned a lot from building the 200mm scope (including fibreglassing technique), and am putting that experience into building this one, which I have been targeting as my ultimate imaging scope (at least for the near future ;)). I've been waiting nearly a year for the carbon fibre sandwich panel tube which was delivered today :) I also recently took possession of the 12.5" main (conical) mirror from Deep Sky Optics (Marc Suchting).

The components are as follows:

12.5" F/5 conical mirror (standard Newtonian paraboloid) by Deep Sky Optics.
64" x 16" OD carbon fibre sandwich panel tube (and high volume filtered air system) from Dream Telescopes (see attached image).
3" Optec Crayford focuser (TCF-S3)
100mm minor axis diagonal (1/20 wave) from Antares.
3" Keller corrector from ASA.
70mm finder / widefield imager from William Optics.
My own design for the secondary support and spider - parts to be machined by Anglade Engineering. Now that I have the tube and can take precise measurements I will be able to finalise my autocad drawings.
Also my own design for the main mirror cell that I will custom build using fibreglassing / CF castings.

[DavidU]

This will be a good project to follow Dave. Thats one nice mirror you have.
Good luck with it.

[David Fitz-Henr]

I have created the raw components for the main mirror cell. They are basically carbon fibre castings, which is different to the usual fibreglass castings which typically use polyester resin and chopped strand mat to fill the mold reasonably quickly. I'm not sure how good my technique really is, but I think it would be a lot stronger for cast components. I simply lay up numerous layers of cloth / tow in the mold, wetting each layer with epoxy as I go. It is very, very tedious and time consuming, but I end up with components that have about 80 layers of carbon fibre cloth saturated with epoxy resin. I'll coin the term "lami-casting" to describe it (don't know if anyone else has done this). Anyway, the results can be seen in the attachments. I use MDF board, a router and circle cutting jig that I made to create the molds in a single process. I saw this technique on the web - I find it creates better molds than the pattern / plaster technique described in N.E. Howards book and elsewhere. Once the epoxy has set, I sand (using a belt sander) the surface of the casting flush with the top face of the mdf mold, and then separate the mold from the casting. If done carefully, the mold is also reusable, although here I ended up breaking some parts of the molds (if you don't wax everything fully the epoxy will stick mdf sections together).

The mirror cell is designed to support a 12.5" conical mirror, and comprises three parts: 1. Supporting ring, 2. Collimation Support and 3. Mirror Attachment plate. Collimation will be achieved using push/pull screws which will be set near the outer periphery, giving both finer adjustment control as well as greater stability due to wide spacing. The mirror itself will be attached to the Mirror Attachment plate via a threaded rod screwed into an embedded nut set in the back of the mirror (a la Royce style). I am also planning on allowing lateral adjustment of the Mirror Attachment plate w.r.t. the Collimation Support to enable centration of the mirror in the tube.

[strongmanmike]

Great project Dave, really hope it comes together as you imagine, the specs are close to perfect :thumbsup:

What camera are you planning to use on it? The focuser and focuser attachment are critical issues with a large heavy camera.

What is the ID of the tube?

A 100mm secondary sounds a little small..? What chip size are you illuminating?

I have purchased a (modified) Orion Optics AG12 with FLI Atlas digital focuser, so we will be imaging with very similar scopes :D

Mike

[multiweb]

Amazing stuff. Watching with interest.

[gbeal]

Looks nice Dave, I did one similar a while back, when I was in "C/F Mode". Mine took the shape of two separate triangles of foam core board, over which I layers C/F matting. Bolts stick out, and recessed metal screw in sections allowed the necessary fittings to attach.
Worked, and worked well, plus it was as light as. In the end I opted for a couple of very thin stainless triangles instead, reducing the back focus a heap, and the C/F jobbie has sat unloved ever since.
Gary

[David Fitz-Henr]

Thanks Mike. I will be using an STL-11000M (the same one I am currently using with my 200mm Newtonian) with an Optec TCF-S3. Far larger and more solid than my current 2" Moonlite - it is a crayford design and I believe is tensioned to 500lb pressure. The focuser will be attached to an adapter plate via a circular dovetail and 6 set screws, and the adpater plate will be shimmed and bolted to the flat focuser platform (seen in earler post image of the tube) at three points on a diameter of about 150mm, so should be rock solid.

The ID of the tube is approx 370mm.

The primary is f/5, and using Mel Bartel's diagonal calculator with my optical configuration gives a 100% illuminated field on a 30mm diameter field, dropping slightly to 94% in the extreme corners of the STL chip. From memory I believe the 3" Keller corrector that I have has similar vignetting so the diagonal size is really quite optimal.

Have you taken delivery of your (modified) Orion Optics AG12 yet? I assume that part of the modification is to incorporate a raised platform for the FLI focuser (due to the backfocus required by the Wynne design corrector)?




Me too!! Tell me how it turns out when I'm done - I'm too nervous to look! :screwy: Sorry - I guess that's why they call us ATM nuts !! :lol:



OK, sounds like you used sandwich core lay-ups. That's how the tube I will use is made (it has a 15mm honecomb cell core - made by Dream Telescopes in the USA). I did actually make my current 200mm scope tube in similar fashion. Trouble is, to get really good results you need to vacuum bag (to achieve a 30-40% resin to cloth ratio) and ideally use an elevated heat cure epoxy. Also, although sandwich panels are structurally quite rigid they tend to lack the mechanical strength needed when high point stresses are applied eg. the push/pull screws that I will be using in the cell. That's why I went for "lami-casting" - it's very strong but still around 40% of the weight of same dimensioned aluminium. I'm not really sure how stiff it is in comparison, but I'm assuming that with such a high number of layers of CF cloth it should be pretty good.

[strongmanmike]

Sounds like you have it covered and I saw the raised section on your Dream tube too, nice idea, I think the thicker core of the Dream tubes means this should be adequate to stop tube flex with a heavy camera and filterwheel..?



That's what I thought the AG12 tube is 350mm but it's a 12" not 12.5" I guess.



Ok fair enough, F5 is a tighter cone than F3.8 of course



Delivery is still waiting on the Atlas focuser to be built and sent to the UK

My AG12 will have the complete tube ring brace option fitted as per the attached photo, this is a similar solution to Wolfgang Prompers ASA 12 (http://www.astro-pics.com/equipment.htm) . I am also having a custom CF light/dew shield made that rigidly attaches (to hold a light box if required) but is removable, integrated secondary and tube heaters controlled via a rear mounted controller and Ultra grade optics upgrade (just cause :P).

[David Fitz-Henr]

Yes, I believe the Dream tubes are excellent quality and rock solid. They are made with thick cores, the resin/CF weight ratio is about 35% (critical for stiffness) and they use high temp epoxy resins baked in curing ovens. The focuser platform is then bonded on and it too is then post-baked on the tube. Dream has testimonials from a customer using their f/3.5 16" astrograph stating that focus did not change over an entire night even with temperature dropping a reasonable amount.

My current home made 200mm scope tube that I personally made is a hand layup / room temp epoxy / standard eglass (ie. not CF) with a 6mm honeycomb core, but even it is very strong. I can stand on the end of the tube (80kg) with little flexure (may be a pic on my website). It has reasonable thermal properties judging by the fact that I have very little focus shift ovr the course of a night.




Looks very good Mike - that should give you a stable base for your focuser. I plan on making a light/dew shield as well for my scope. I'm not sure what you mean about it holding a light box? Do you mean a light trap? I built one into my current 200mm scope tube - but they go opposite the focuser to prevent reflections from the opposite wall.

Anyway, I'm going to IISAC this Thursday - if you're there we can have a chat about our scopes :)

[strongmanmike]

I wanted a matching dew/light shield extension that was removable for transport but that attaches rigidly enough so it would support a flat fielding light box on top even when not pointing straight up (http://www.pbase.com/strongmanmike2002/image/83372124/original) - I like to take my flats during the imaging session and not at the start or end. Having said this I will be going with one of the new Astronomik Aurora flat panels (soon to be released) which requires the scope to be pointing straight up anyway, doh! I may modify the Aurora to hold on at lower angles though?

Yes I will be at IISAC, arriving Friday arvo, look forward to comparing notes :thumbsup: I will be on the upper imaging field.

Mike

[David Fitz-Henr]

Well, things are progressing slowly - I have now completed the main mirror cell (except for painting) and test-mounted the mirror. The main features can be seen in the attached images and are as follows:

Widely spaced push/pull screws as mentioned earlier.
Three Lateral positioning screws for centration of the mirror in the tube.
Multiple threaded inserts at the three attachment points to the tube. There are actually 5 threaded inserts at each tube attachment point which will allow 20mm of adjustment once the three holes are drilled (and reinforced) in the tube. I drilled and tapped M8 holes in stainless 1/2" threaded rod and cut to size (15mm long). 1/2" holes were then drilled and tapped in the CF cell and the inserts screwed / epoxied in them. The M8 holes in the inserts will then accept M8 screws through the holes in the tube. This was a LOT of work and was hard to get the positioning perfect in the CF cell such that the spacing of each matched set of three holes was identical to the others. In the end it wasn't perfect (up to 1mm out), so I offset the tapped holes in the threaded inserts to compensate.
I will aim to set the mirror in the tube such that the "middle" insert is used - that way if I am slightly out I have +/- 10mm to play with. It may also be useful in the future for different flatteners / compressors / etc with different backfocus specs.
I'll call it the "SLATO Multi-Point Cell Positioning System" (patent not pending yet :P).

[strongmanmike]

Lookin pretty schmick Dave :thumbsup:

Mike

[Moon]

David
Looking very nice - when can we expect the first light images?
James

[David Fitz-Henr]

Thanks Mike; yes I'm pleased with the way it turned out. With the mirror mounted on it (fastened through central hole with wave spring washers to apply and hold the correct tension) it appears to all hold together very rigidly so I'm hopeful that this will assist in achieving good pointing accuracy.



Picture are attached below ... oh hang on, you mean through the completed scope :lol: :P !! Well, I'm hoping to complete it in the next year before the warranty expires on the Optec focuser at least!! The big thing remaining is the secondary assembly. I've spec'ed it all in autocad and have passed the plans on to my mate who is runs a machine shop and is going to fabricate the parts. It is based on the offset vane arrangement in Texereau's book and should be quite rigid. Then there are the main tube rings and some fibreglass pads I need to make for the finder and guidescope ... but I only have weekends and I'm a slow worker (if it's worth doing, it's worth doing perfectly ;))

[marc4darkskies]

Me being someone without the skills to build what I want - I'm always very impressed with what you do David!! :thumbsup: I think a site visit by Bells Observatory would be in order!! :D

[bartman]

Hi David,
Very nice! cant wait to see the updates.
Just a question: you say " With the mirror mounted on it (fastened through central hole with wave spring washers to apply and hold the correct tension)".
How/ what????:confused2: Does the mirror have a tapped thread in the bottom?
Or a hole through the middle(couldnt see one).
Just curious:thumbsup:
Keep the pics coming!
Cheers
Bartman

[David Fitz-Henr]

Hello Bartman,
The conical mirror has a blind hole in the centre at the back (it doesn't go all the way through, so the face of the mirror is not perforated). In this hole Mark Suchting (who made the mirror for me) has set a 1/2" BSW nut in resin. The nut accepts some threaded rod which protrudes through a hole in the carbon fibre base plate that the mirror sits on. Then add some wave spring washers and another 1/2" nut to secure the assembly to the base plate with sufficient tension to hold it firmly in place. The wave spring washers then maintain this tension against expansion / contraction due to temperature fluctuations, etc. It is a simple but very effective mounting system and avoids all the usual dramas with flotation cells and edge supports. The tension on the central stud does not distort the mirror at all; a fact which Mark has confirmed by cranking up the tension well beyond that needed, with no distortion showing under his artificial star test.

[Visionoz]

David

Great read on your ATM astrograph - just a query on what's the advantage of a conical parabolical mirror design as opposed to a standard parabolical? - I'm curious to know if weight-savings is one of its advantages or are there other technical benefits?

Cheers
Bill

[David Fitz-Henr]

Well, there are a few advantages to a conical mirror:

Lower mass (approx 60% of full thickness conventional mirror) which means less weight.
Lower mass also means quicker cool-down times. I've read (I think on R.F. Royce's site) that the figure behaves itself better during cool-down period as well.
Simplicity of mounting. No need for flotation cells and edge supports, and when mounted in the way I describe below, there is less freedom for the mirror to shift during use.

[Visionoz]

Thanks David

I had suspicions those reasons you gave were the ones but being inexperienced in this department thought that asking the questions would allow me to learn more ;)

Looking forward to more and learning from your progress in this project :thumbsup:

Cheers
Bill

[David Fitz-Henr]

I have just acquired an Astro-Telescopes 102mm achromatic refractor that I intend to mount directly on the CF tube of the 12.5" astrograph to use as a guidescope.

The main resons for choosing this particular refractor are as follows:
1. Economical ($USD 499) but still having good (achromatic) optics - more than adequate to achieve good guiding.

2. Relatively long focal ratio (f/11 - remember the days when these f/ratios were common?). Not only does this give a good focal length for guiding relative to the main scope (1122mm vs 1600mm), but the longer tube permits wider spacing of the tube rings for greater stability.

3. Reasonable aperture (102mm) without being excessively heavy (just over 6kg with rings).

However, although there were good reviews for the optics and it sports a nice tube with a generous dew cap, I expected that it's Achilles heel would be the generic (made in China) crayford focuser (I had seen one review on the web where the owner had problems). Nevertheless, I thought that even without the focuser it was a good deal so I purchased it. Since it will be used for guiding, I intend to add an extension before the focuser such that the camera will be in focus near minimum drawtube extension (for stability). I took some measurements of the focus position to facilitate this (see picture showing the AT102 on a fully adjustable AltAz-Beanbag mount :P).

However, I wasn't wrong about the quality of the focuser :(

Crayford Focuser Repairs

The first thing I found was an annoying ding in the end of the 1.25" adapter. On inspecting the box again, I found a slit right through it that had escaped my notice which lined up suspiciously well with the focuser. Oh well, not the fault of the focuser and just cosmetic ...
When I turned the focuser knobs, I found that it sounded very scratchy and the fine focus knob turned in unison when I turned the coarse focus knob (not correct - the fine focus should turn 10 times faster when you turn the coars knob). I found that some of the grub screws were not tightened, but when I tighted one I found it was cross threaded and turned freely without tightening. On investigation, I found that the base of the threaded hole was not well formed and the grub screw was too short (so did not engage the upper hole/threads). I replaced this with a longer (M4) grub screw that I had and this solved the problem.
The next problem that I found was that the fine focus seemed to "grab" every third of a turn, as if there were 3 detents per rotation. Of course, this should not be, so I began investigating; Gary Hand (HandsOnOptics) said that the most likely cause was a bent pin. This is used as the centre "gear" for the planetary gear system that enables the slow motion (central pin rotates and contacts three large ball bearings which are set in a brass fitting that is attached to the main drive shaft. I found a good article on how to disassemble a similar GSO crayford focuser (see http://backyardvoyager.com/astronomy-tutorials-reviews-plans/improvements-for-your-gso-focuser), but it didn't go far enough. I needed to completely disassemble the fine focus ball bearing mechanism to get at the centre pin. Also, instaed of a hex nut holding it in, my focuser had a round nut with a slit (similar to a straight screw head), so I had to make my own tool to undo it (see attached pictures). Once I managed to disassemble it completely, I found that the central pin was perfectly straight :screwy::confused2: . It had to be something to do with the pin I reasoned, as the false "detent" matched the rotation cycle perfectly. The ball bearings are held against a groove around the circumference of the pin near one end, so I inspected the groove with an eyepiece, and found a very slight mark/depression within the groove (see the picture to get an idea of how small I am talking about - probably of the order of 10-20 microns deep!!!). This had to be it, as combined with the high mechanical pressure applied, this would mimick a detent I reasoned. I mounted the pin in my lathe (a recent acquisition - I'm no expert by any means!), and smoothed the groove using #320 wetanddry, followed by some metal polish / cloth giving a nice shiny surface (even using an eyepiece). I then re-assembled the focuser and .... voila !!! The false detent problem was gone :) :)
Just as I was feeling really pleased with myself I noticed another problem: the focuser drawtube was not square with the focuser body !!??!! I soon found that one of the 4 roller bearings was set crooked and had even badly scored the drawtube where the edge of the roller had dug in. There is an M3 set screw that hlods the roller bearing against an aluminium pad that was not screwed in square to the hole (cross threaded). After a few attempts, I managed to screw it in square; but the screw only engages the hole by about 3mm, hardly enough to ensure a good structural fitting (a screw should engage by 2 x D in soft metal like aluminium - so 6mm would be ideal in this case). I will replace all 4 screws with longer ones later.

The end result is that it seems to be performing well, and the fine focus now has a smooth feel to it :) :astron:

[Moon]

David,
Getting back to your 12.5 - have you received the focuser and corrector yet? From what I understand, the ASA Corrector has a true OD of 3 inches and the focuser also has true ID of 3 inches. Just wondering if you had to do something special to make it fit.
James

[David Fitz-Henr]

Well, my understanding is that focuser drawtubes, etc are machined with a small clearance to allow for true sized male fittings. However, I don't like nasty surprises so I asked Optec before I ordered the focuser and here is part of their reply:

"the I.D. of the TCF-S3 drawtube is 3.005 +0.001/-0.002 so we hold extremely tight tolerences. Assuming the corrector is designed to fit into a 3-inch bore, the fit should be snug."

So, about 1/8mm diameter clearance. I have had the Keller corrector and focuser for a while now, and yes I did try the Keller in the focuser and it does fit :)

[Moon]

Thanks David
In case anyone is considering something like this, I already received confirmation that it does NOT fit in the 3.0" Feather Touch.
James

[David Fitz-Henr]

That's interesting - how was it confirmed? You made me paranoid so I just went down to the observatory and measured the OD of the Keller corrector and the ID of the focuser with my new Digital Caliper (accurate to 0.03mm). The ID of the drawtube was 76.27mm and the OD of the Keller corrector was 76.18mm, leaving about 0.09mm clearance (close to the 0.125mm specified value difference given the caliper accuracy, and also the fact that an OD reading may be too large and ID too small if you are not very careful taking the reading).
The 76.18mm (+/-0.03) is virtually spot on for the theoretical 76.2mm (3") diameter of the Keller corrector. I would be very surprised if FeatherTouch did not allow some clearance. Of course, it could be a QA issue on either side.
One other thought - I have found with my 2" Moonlite Crayford that because I have the pressure adjusted quite high (to try to cope with the weight of my STL-11000M), that my 2" laser collimator will scrape the bottom of the draw tube when I push it in. When I ease off the pressure, it goes in smoothly. So the extra pressure will tend to deform the drawtube very slightly. I think a 3" Feathertouch is a rack and pinion though? So may not be the problem here as it doesn't rely on high pressure on the draw tube. :confused2:

[Moon]

I emailed Starlight Instruments to check. They said it won't fit in the 3.0" focuser, you have to get the 3.5" version (http://starlightinstruments.com/shop/product_info.php?products_id=67) with an adaptor (http://starlightinstruments.com/shop/product_info.php?products_id=292).
The adaptor looks a bit small to me, the TCF-S3 sounds like a better solution.

James

[David Fitz-Henr]

I have completed the tube counterweight assembly that will be used on the scope to provide fine balance when changing camera / eyepiece, etc. It is based on a similar principle to my 8" prototype, whereby you can slide the counterweight along the axis of the tube, but also slide the whole assembly around the tube circumference (max 120 degrees) to provide lateral balance (eg. if the guidescope, finderscope, RigRunner, etc results in excess weight on one side of the tube). To facilitate rotating the whole assembly around 120 degrees of the tube circumference I have added a clutch on each end (a small teflon "washer" that I turned that is tensioned against the CF rail via a grub screw) - this prevents the whole thing acidentally flopping when the lock screws ( two at each end) are released. Anyway, see attached pictures to get an idea of what I am talking about.

[David Fitz-Henr]

I have completed the majority of the work on the tube saddles. They are basically carbon fibre / epoxy castings that I layed up with dozens of layers of CF. I also tried something new: I made some CF plate and cut it to the shape of the tube saddles and laid it in during the wet lay-up in the mold.

Anyway, I've provided a brief sequence of images below to give everyone an idea of the process involved:

Beginning to rough cut out the mold (made out of mdf board).
Mold rough out completed (using drills and jigsaw)
Mold finished using wood router (seen here mounted on my home made circle cutting jig).
Vacuum bagging the CF plate using compressor and vacuum transducer. Note that this is simply some square flat CF plate; it's not the mold / casting. The plate was later cut to shape with a jigsaw and used as a couple of "layers" in the wet layup in the mold for each saddle.
It was a cold day so I created a warm environment to cure the plate quickly using a home made, low temp oven (ie. hair dryer and plastic cover ;))
The first saddle cured in the mold and sanded flat using a belt sander.
The first saddle demolded with the second saddle in the mold ready to be sanded prior to demolding.
I have also cut the slots in the ends since taking these images (to accept the strap ends) and made some bronze cylindrical retainers. I have also picked up the stainless straps that will be used to hold the tube on the saddles - I'll take some more pictures next weekend.

[marc4darkskies]

Very impressive David - looks great. You're certainly not skimping on detail and effort.

I'd like to place an order for one of those counterweight assemblies please! :)

Cheers, Marcus

[David Fitz-Henr]

OK, a few more pics this week. I have done a bit more on the tube saddles and made some progress on the secondary spider / hub support. The design for the latter will be based on the offset vanes outlined in Texereau's book as well as a few ideas of my own.
The images are as follows:

I have drilled, tapped and glued in the threaded stainless inserts (used to attach the saddles to the versa-plate) and cut the slots in the saddle ends. I have also sourced the stainless straps from a local supplier. You can also see a length of 12mm stainless rod that I will cut up and thread and pin to the straps, as well as some of the bronze retainers that will provide a method of tensioning the strap to the saddles.
I used some 25mm wooden handle to wrap dozens of layers of fibreglass & carbon cloth to form a cylinder, which will be cut at 90/45 degrees to be used as the spider hub. Here you can see the start of cutting using an old GMC compund saw.
Initial cutting completed - you can see the final part sitting on the saw platform, as well as leftover parts of the layup.
Here you can see the spider hub cleaned up with the wood dowel removed, next to the leftover section. If I muck it up I can use the leftover section to make another one :D
Here you can see some of the components that will be used to fabricate the spider assembly:
- the 45 degree hub
- 2 elliptical carbon fibre plates that I made (approx 6.7mm thick each). M2 will be attached to one by three dobs of silicone at the appropriate positions, and the other plate will be glued to the 45degree section of the spider hub with stainless threaded inserts to attach to an aluminium collar.
- 32.8mm titanium tube that will be used as the central rotation / translation axis of the assembly. It has an aluminium collar that I will shrink fit on one end that will be used to attach to the 45 degree hub. Note that this particular collar won't be used as I made a couple of mistakes (one was that I bored it out too much which ruined the shrink fit).
- the micro-collimator (on the titanium tube) that I made from carbon fibre / epoxy resin that will be used to provide rotational adjustment (note the 2 push screws with black thumb knobs) as well as positional control (up/down the tube).
- Not shown yet are the stainless steel vanes, 2 aluminium ends that will fit over the titanium tube (like the micro-collimator) and some 8mm titanium rod that will provide anchors for the vanes.

[thercman]

David I am absolutely amazed! First I have never seen carbon laid up like that before and now my wheels are turning. 0.o I have also never seen a sliding weight used like that either. My hats off to you for being unique. My 12.5" truss isn't quite done yet and now I may have to go back over it to see if I can use your methods in my design. :D Looking forward to following the build....

[David Fitz-Henr]

Thanks Greg. Yes I've developed some of my own ideas and techniques with fibreglass / carbon fibre - which is half the fun of ATM'ing! The thing I love about fibreglass is that you can make complex parts that would cost a lot of money to make out of metal.

I made a little more progress this weekend:

Made the stainless threaded inserts for the 45Degree fibregalss hub (used to attach it to the aluminium collar with M4 screws) and glued them in.
Completed the interference fit of the aluminium collar to the titanium tube.
FYI - an interference fit is made when one part has a smaller diameter opening than the part that it fits over. In my case, the ID of the aluminium collar was 0.075mm smaller than the OD of the titanium tube. I heated the collar to 250C in our oven (expanding it by approx 0.18mm) and put the tube in the freezer for a while. I then clamped the tube upright in a vice and (quickly) took the collar from the oven and placed it on the end of the tube (see image below). After a quick wiggle the collar fell and fully engaged the tube (I had a lip on the collar so it didn't slide all the way down!). Once it cooled the collar tightened very firmly around the tube.

[David Fitz-Henr]

Another few firsts with my lathe (still learning). The attached image shows some counterweights and attachment rods that I've made for the telescope tube. Although I have a dual sliding weight (see previous posts) that will allow variable adjustment, I realised that may not be enough to cope with a large change to the balance (eg. replacing the STL-11000 with an eyepiece for visual use).

The main points:

The threaded rods will be able to be screwed (M8 end) into some threaded stainless inserts in the tube wall.
The counterweights are centrally drilled and tapped to screw on to the M12 ends of the threaded rods (not easy tapping M12 sized holes in stainless steel!!)
The counterweights can then be snugged up on a small flat platform on the tube wall which will make them more or less self-supporting (ie. negligible cantilevering).
I made some knurled thumb nuts to lock the counterweights in place. My very first attempt at knurling - was supposed to be a diamond knurl but ended up with more of a spiral pattern as I didn't get one of the knurling wheels to "bed in" very well. Oh well, still looks OK.
The only thing left to do is drill a small hole in the M12 end of each rod that can be used to tighten it on to the tube (by inserting a small screwdriver or similar through the hole to get some leverage).
You can see a rod / weight / knurled nut screwed together in the image to give an idea of what I am talking about. I went a bit overboard with the weights (3 x 600g and 2 x 300g), but I figured better to over do it a bit rather than get caught short later!

[Mighty_oz]

Those weights look real good, how much more do u think u spent on making them vs buying off the shelf ones ?
But nothing beats doing it yourself hey :)

[David Fitz-Henr]

Well, actually much cheaper to make them myself; basically just the raw material cost of the stainless steel and threaded rod - I haven't actually paid for it all yet but I would guess approx $60. Compared to over $120 for less weights (and not stainless) from a telescope/accessories distributor. ... and yes, nothing beats doing it yourself if possible!!;)

[David Fitz-Henr]

Some more progress on the secondary assembly this weekend. I'm waiting on a couple of parts that I am having made up (ie. vanes and vane hub plates), but I have assembled the parts I have ready to give an idea of how it will work. I took a long time thinking about the whole
secondary assembly and basically designed it from the ground up. Having used Newtonians in the past (including the one I made recently), I wanted to avoid the problems inherent in typical secondary mirror spider/holders; my main objection being that they are an exercise in
frustration when it comes to collimation!:mad2::mad2::mad2:
I wanted a secondary mirror support system that holds the mirror firmly in place with no tendency to move over time, and also provides simple and precise collimation.

In thinking through the design I came up with the following basic criteria:

Since my secondary mirror has a minor axis of 100mm it is thus similar in size to a small Newtonian's main mirror, and that I should keep this in mind in order to break free of the secondary hub paradigm; viz. to avoid collimation points that act on the base of an extended 45 deg hub, where by adjusting the mirror angle it also moves the mirror laterally as well.
I realised that 3 point tip/tilt positioning is not even necessary provided that fine adjustment in rotation is provided. You only require tip/tilt in one axis (being parallel to the minor axis of the secondary mirror); this together with fine control in rotation provides full collimatable functionality.
I wanted to incorporate an offset vane configuration (described in Texereau's "How to Make a Telescope") as it provides maximum stiffness (including resistance to rotation).
I wanted to design out any residual forces that act to pull the mirror out of collimation; ie. any cantilevering forces that the mirror may exert on the assembly and rotational forces (less important) due to the off-centering of the mirror / hub design.
You can see (in the first image) a closeup of my implementation of the tip/tilt supports. Note the assembly is in mirror face down
orientation (though the mirror is not attached; it will be siliconed on to the bottom plate in the image):

There are two non-adjustable support points (seen on the left). Note that contact is made in a line between the two points to avoid stress on the bottom plate in particular.
The adjustment in the third support (on the right) is provided by the nut in combination with a wave spring washer between the plates. This provides approximately +/-2mm of adjustment. This should be more than adequate provided the vane holes, etc are set accurately in place; if more adjustment is needed this can be accomplished by inserting extra washers.
Once collimation is achieved the 2 fixed support point nuts can be tightened, which makes the whole thing extremely rigid.
The other two images show the assembly (less mirror and vanes / vane hubs). Also shown are the two titanium 8mm rods that will be
supported by the vane hubs and hold the vanes:

One of the images shows the end of the 8mm rod held in the push/push screws of the micro-collimator. This will provide fine adjustment in rotation.
Near the end of the main tube you can see a horseshoe shaped stainless steel counterweight that I made yesterday. This will bring the centre of mass of the adjustable sub-assembly (ie. mirror / plates / tube / micro-collimator) in line with the vanes, thus avoiding any tendency of the mirror to tilt due to cantilevering forces. Further, the horseshoe shape will be oriented to negate the weight bias on one side and so avoid any tendency to rotate.

Well, that's the theory anyway ...

[David Fitz-Henr]

A few pictures of my progress to date. The secondary assembly is now complete (apart from painting and siliconing the secondary mirror on), as are the tube saddles / straps.


First two images show the empty tube mounted with the completed saddles / straps. Note that the white plastic coating on the straps is just protective covering for the stainless steel face side. It will be stripped off.
The next image shows the inside tube view of the focuser platform. The tube has been reinforced at the attachment points for the focuser, with three holes drilled and tapped for the stainless inserts to be glued (One is shown fully screwed in and glued, another partially screwed in and the third not started.
The next image shows the jig I made to bend each of the vane pairs for the secondary support.
The next 2 images show the assembled secondary mounted in the jig that I used to line up and drill the holes in the tube for the offset vanes (the holes for the vanes are offset since each vane does not line up with the geometric centre of the tube). Note the lugs pinned to the end of each vane; they are designed to maintain absolute stiffness - there are three pins in each vane which prevents any tendency to "swivel" as in many commercial designs.
The last two images show the whole secondary assembly mounted in the tube (the last image shows the geometric offset of the vanes). It feels extremely stiff even when the screws are just finger tight.
Can anyone recommend the best silicone to attach the secondary mirror to the carbon fibre plate? I'm thinking maybe "Roof and Gutter" (neutral cure and good adhesion to most surfaces)?

[Satchmo]

Nice job on the secondary /spider , David. I would recommend an elliptical ring of silicon blobs at the 70% radius zone, and don't let the silicon cure any thinner than 3mm so use some spacers while it is drying. Silicon has an incredible amount of pull when it cures and is quite capable of warping a secondary mirror in a big way.

[David Fitz-Henr]

There actually isn't a lot of definitive literature on the best spacing / thickness of the silicone blobs to support the secondary. It is more complicated than a primary mirror because a) it is elliptical, and b) the 45 degree cut of the glass means that there is more mass at one end of the elliptical mirror surface than the other. So, a support point (on the back surface) at one end of the ellipse would support more weight than the other end. I don't think PLOP will model this, but I saw reference to another FEM tool that might (though I haven't tried it). I think the saving grace for many secondary mirrors is the fact that they are smaller than primary mirrors and tend to be full thickness as well.

Here is an interesting website that discusses some aspects of mounting a secondary mirror: http://www.cruxis.com/scope/scope1100_secondarymirror.htm
Note however, that this site shows a cellular secondary which doesn't have the classic 45deg cut.



My approach is along the following lines:

To have just three silicone support blobs - any more can introduce additional stresses on the mirror.
Each silicone blob to be no more than 20mm diameter (my secondary has a minor axis of 100mm).
From the Cruxis telescope site (based on his FEA), "Another important conclusion is that it's best to mount the mirror with the edge of the single support closest to the eyepiece".
This seems to make sense intuitively for a classic 45 degree cut in that this mounting point "overhangs" the actual mirror surface, and so the other 2 points (which will necessarily be closer together at the other end of the ellipse) will support a greater load (since this end of the ellipse on the back surface "underhangs" the mirror surface. I'm not really sure why this would be optimal for the cruxis secondary however.

However, I'm not sure which silicone to use?? Websites seem to only quote RTV silicone, but my understanding is that all silicones sold in the local hardware are RTV (Room Temperature Vulcanising)? I had thought a neutral cure would be best, but I see on the Protostar website they mention the acetic acid catalytic type? Dream Telescopes recommend a 1mm thickness, so I would assume that the silicone should have a low modulus (elasticity). Does anyone have an opinion here?

[Satchmo]

Looks like you have done some research! My original 8" binocular used silicone squeezed fairly thin to mount all my optics and I eventually found them all to have been seriously warped by the thinly spread silicone causing quite a bit of astigmatism , which is why I suggested a minimum of 3mm.

[David Fitz-Henr]

If you had more than three blobs of silicone or a single large area then that would constrain the glass and likely cause astigmatism. By having just three blobs this would allow any of them to shrink / swell without straining the glass (assuming the blobs are not too large as then you would get differential pressure across each blob).

No one has any suggestions for the type of silicone? Then I will probably test Selleys Glass Silicone - it appears to be low modulus (high flexibility) so I will try a thickness of 1mm for three points. My great fear is that the silicone gives out and the secondary falls off! :doh: :prey: Selleys Glass Silicone appears to provide the most secure bond for glass (and hopefully carbon fibre / epoxy resin). I had originally thought that I should use a neutral cure, but I don't think this is necessary since it is not coming into contact with metal.

[gregbradley]

I don't know about mounting a mirror but in the contracting game most use Polyurethane sealants rather than silicone. Sikaflex is a popular one that is available from Bunnings. There are various types from Sika so read the tube about what they are used for.

But overall I would use a polyurethane over a silicone.

Greg.

[David Fitz-Henr]

Thanks Greg, that's interesting. I actually picked up a Sikaflex brochure from the local hardware a few days ago but haven't really considered it as I haven't seen anyone else use it for a mirror. Do you have any specific reason why you think it would be better? Looking at the TDS on the website, I see that Sikaflex 291 has similar properties to Selleys Glass Silicone, except that it has 5% shrinkage whereas Selleys Glass Silicone indicates no shrinkage. There is a Sikaflex 291I that has 2% shrinkage (although the site says this is for "experienced professional users only" ??).

[David Fitz-Henr]

Well, time for another update; I'm getting down to the business end now ... and as far as the weather goes, it's been great for ducks and telescope building, hasn't it!?!

Attached images below:

Stainless extension shaft that I turned on my lathe. I calculated that I need it to extend the shaft on the Paramount to achieve balance (otherwise I would have to purchase a 4th counterweight which would be overkill). Cost from Bisque would have been approx $300 becuase of high shipping costs; cost of stainless stock for me to make it: $40.
Shows the extension shaft in place.
I always wanted a generous finderscope to be more than just a finder and plan to use a WO ED 70 for that rich field experience; however I became worried when I tested the scope in the WO rings I bought as I needed to tighten it quite hard to stop slip. I decided to replace the delrin tipped screws with some large nylon faced stainless inserts that I made to provide better friction but also do not rotate as you tighten the screws (non-marring). I got the idea from the Alpine Astro site.
The finder / rings attached to the tube.
I needed to drill 4 holes (2 widely separated pairs) in good alignment on the tube (which is not perfectly round) so I used this jig, which is self aligning on the tube.
I've seen a couple of horror stories regarding rodents setting up house inside a Paramount (due to the open RA shaft that has holes at the back end to permit through the mount custom wiring). Here is my solution: an end cap I made out of fibreglass that attaches to the shaft end and has 3 electrical glands that pass the wires through.
I'm currently working on the electrical / USB systems and then I will be ready for a test mount of the mirrors to determine exactly where to drill the holes for the primary. Work has been slow as I have had a lot of back pain over the last few months and looks like I will need a back operation, but hopefully I will complete the scope before that happens.
I'm also building a kit shed that I will use as an area in which I can spray paint the tube; I planted the idea with my wife that we need extra storage in order to tidy up our garage (for mower, etc) so I am not only getting brownie points for doing it but am scoring my spray painting booth as well :lol: