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.
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.
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
Great project Dave, really hope it comes together as you imagine, the specs are close to perfect
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
Mike
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)?
Quote:
Originally Posted by multiweb
Amazing stuff. Watching with interest.
Me too!! Tell me how it turns out when I'm done - I'm too nervous to look! Sorry - I guess that's why they call us ATM nuts !!
Quote:
Originally Posted by 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
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.
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.
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..?
Quote:
The ID of the tube is approx 370mm.
That's what I thought the AG12 tube is 350mm but it's a 12" not 12.5" I guess.
Quote:
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.
Ok fair enough, F5 is a tighter cone than F3.8 of course
Quote:
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)?
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 . 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 ).
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..?
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.
Quote:
Originally Posted by strongmanmike
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 . 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 ).
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
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
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 - 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 I will be on the upper imaging field.
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 ).
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.
Quote:
Originally Posted by Moon
David
Looking very nice - when can we expect the first light images?
James
Picture are attached below ... oh hang on, you mean through the completed scope !! 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 )
Me being someone without the skills to build what I want - I'm always very impressed with what you do David!! I think a site visit by Bells Observatory would be in order!!
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???? Does the mirror have a tapped thread in the bottom?
Or a hole through the middle(couldnt see one).
Just curious
Keep the pics coming!
Cheers
Bartman
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???? Does the mirror have a tapped thread in the bottom?
Or a hole through the middle(couldnt see one).
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.
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?
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
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.
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
). I've been waiting nearly a year for the carbon fibre sandwich panel 
I also recently took possession of the 12.5" main (conical) mirror from Deep Sky Optics (Marc Suchting).
Sorry - I guess that's why they call us ATM nuts !! 
).
Does the mirror have a tapped thread in the bottom?
