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800mm Telescope Project


Michele Scotti
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4 hours ago, Cosmic Geoff said:

Excuse my ignorance, but I have still not grasped the justification for this project. We imaging newbies are constantly told that one does not need a large telescope for deep-sky imaging, that an 80mm aperture APO will do just fine.   Even the massive WASP sky-survey looking for exoplanets used arrays of small optical elements.   I saw a spare element on display at the Open University and it looked small enough to put under my arm.

Apparently when a University department wants a research telescope or two, the stock response is to order up a 16" fork-mounted Meade and a dome. The OU has one.

Yes I know that giant optical telescopes are used for probing the furthest reaches of the universe, but these are mounted on mountain-tops far from light pollution and far from persistent cloud cover, or even in space. 

Very good recurrent question. Not easy to answer properly.
From a group pov we managed to build many telescopes up to 500mm - with 2 optics being sold. 
Of course you can do deep-sky with a small diameter and you may find that the major hurdle is tracking. 
I'm sure some guys here on CN can list out -better than I can do- the potential of a 800mm optics used as an imager.

Somebody said that with 800mm you can see the colour of some DSO in visual. And that's what motivates some guys in our group.

I like the challenge of project itself. I'm not an imager per se.

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It's now time to ""explain"" the intended optical system: for simplicity and familiarity with its optics we choose a reflecting Newtonian configuration. Given the generous primary diameter it has to be fast to contain the overall scope dimensions due to focal length. Although the original project was born as f/3.75 to target 3000mm focal length we are changing our mind due course - CAD model update in progress.
f/3.3 gives out a focal length of 2640mm with few benefits:

- bigger field of view

- lower sensitivity to tracking errors

- the overall telescope height will be shorter with effect to weight, ease of transport, height of the eyepiece and higher modal frequency i.e. stiffer mount

- last but not least at 2640mm the focal length is matching the FL of our ready available 500mm mirror allowing testing the mount capability before the 800mm mirror is coming to fruition

Of course the mirror will be a bit more difficult to parabolize and the secondary mirror will be a bit bigger (180mm vs 170mm give or take).

Depending on the usage and the imaging sensor size we might need to adopt a Wynne corrector - a pretty beefy one could set us off £1000 though - we'll evaluate that at a later stage.

 

I'd appreciate any comment on the optical layout - specifically on the back focus. Please consider that we'll have a filter wheel and OAG. How would it change with a Wynne corrector?

 

I guess what I'm looking for is the on-axis PM to secondary distance and the secondary to focal plane. Also I never quite developed the full travel of the focuser - when it comes to DSLR or CCDs I can figure that out but with the most common eyepieces I don't know exactly what would be the range. I suppose I need to consider 2" eyepieces?

 

I've attached a couple of NewtWeb screenshots for reference.

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Newt2.JPG

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The mirror box is simplified into two main symmetrical parts for ease of manufacture - we started calling them 'pacman'. This design integrates the mirror box itself with the ‘bearing’.

The target stiffness is achieved with 3 connecting members to minimize weight and complexity.
In the lower part an aluminium H structure supports the mirror cell as well as contributing to the mirror box stiffness.
In front, two crossing aluminium bars while in the back a single cross bar, bigger than the front ones as it carries the load from the upper cage through the beams.

The material of the 'pacman' is crucial in this kind application. This is where the combination of lightweight, stiffness and low cost is coming together. A sandwich of plywood and foam provides remarkable stiffness to weight ratio - it is how cardboard boxes get their strength from, isn’t it? A Finnish amateur astronomer had a very nice implementation - https://uuki.kapsi.fi/cf16in.html - balsa&carbon/foam/honeycomb, pretty fascinating.
The foam is an inexpensive 30mm sheet of polyurethane whereas the plywood is simply upgraded to marine grade to be less sensitive to long term deformation.
All the material is available from a good warehouse - that's essential for the spirit of the project. The whole sandwich is then wrapped in fiberglass – more as a mean of protection to the environment than anything else.

On the outer rim, a stainless steel strip will be glued to the pacman perimeter so that there are no bumps due to screws. After that the rails will be ground in place with a special homemade jig – hold on and I’ll shortly post something about that.

Finally, the assembly is crucial to have everything squared and will touch based at a later stage – needs to be embedded in the design, not an afterthought.

Pictures of the sandwich being made – foam glue to the first plywood, cut-outs for wood inserts, inserts in place. After that drum sanding machine to get everything flat for gluing the last plywood layer.

 

pacman.JPG

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I should us a fixed electric router attached to a sturdy centre support to ensure an accurate circumference.
Move the "Pacmen" against the cutter. Not with the cutter.
Any play [at all] between the router base and the geometric centre will make the exercise worthless.
A simple plank between the two may not offer enough stiffness. A thick and stiff triangle is much safer.
I speak as one who has struggled with cutting large, accurate curves in laminated birch plywood while building my dome.
The router cutter [bit] will "dig in" if you give it half a chance. Causing a rippled surface at the very least.
Small depths of cut, increasing steadily in fixed steps, using a sharp cutter.
Some swear by up-twist solid carbide bits. I only had access to straight cut, normal carbide bits.

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2 hours ago, Rusted said:

I should us a fixed electric router attached to a sturdy centre support to ensure an accurate circumference.
Move the "Pacmen" against the cutter. Not with the cutter.
Any play [at all] between the router base and the geometric centre will make the exercise worthless.
A simple plank between the two may not offer enough stiffness. A thick and stiff triangle is much safer.
I speak as one who has struggled with cutting large, accurate curves in laminated birch plywood while building my dome.
The router cutter [bit] will "dig in" if you give it half a chance. Causing a rippled surface at the very least.
Small depths of cut, increasing steadily in fixed steps, using a sharp cutter.
Some swear by up-twist solid carbide bits. I only had access to straight cut, normal carbide bits.

Thanks for the suggestion - you are spot on. The overall thickness is 54mm so it's pretty tricky....

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Follow-up on 'pacmans' (or 'pacmen'??): we are lucky enough to have a friend with a CNC router for wood furniture. Popped in some quick coordinates and had the whole thing done. It took quite some time as the thickness was borderline with the maximum allowed by the milling bit so we did several slightly overdimensioned  passages (by 1 or 2mm) and a final one with full depth.

The holes are drilled on the same clamping position to preserve the overall accuracy and consistency - will be useful at the assembly stage.

 

Next step: wrapping with glass fiber and lining with veneer on the sides to protect exposed foam. Main surfaces seems pretty flat - we might just give them a go in a drum sander after few days of settling.

 

Haven't measured properly but each pacman weight it at around 10Kg - that's pretty much in line with the expectation.

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Follow-up#2 on pacmen: after lining the side with some veneer to protect the foam we dropped a layer of CF on the outer side whereas the inner side in glass fiber. No vacuum bagging - which takes time - just standard lay-up. There's really no need for anything more complicated than that.

I'm pretty sure the wrap is not adding anything to the parts' stiffness but it definitely protects against humidity. We happened to have CF around, but enough only for 2 sides....so we went for the look!

Downside? Have to find out the holes drilled with the CNC, laying under the CF - most of them blind

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This is the focuser board to house a generous focuser/derotator, connect the upper and lower hexagons and provide attachment for one of the 4 spider vanes.
Being part of the upper assembly this component has to be as light as possible. However we could not adopt a foam sandwich as there are too many cut-outs to expose the foam. The board has some depth to properly house the focuser outer diameter and to contribute to the overall assembly stiffness - can't be just a plate of few mm of aluminium.
So a sandwich of plywood and paulownia is wrapped in carbon fiber - the latter can be glassfiber for protection against humidity. The plank itself (400x350mm) came out at 2Kg - exceeding the density assumed in the FEA by 20% - uhm..not looking on target!
Cut-outs are carried-out on a CNC although it could be easily done with a router and some jigs. We just happened to have a good friend with a CNC - who doens't hav a friend with a CNC if you look hard enough! JK.

The weight is now 1.2Kg, still room for improvements....

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Now about the UTA brackets: truss beams connections to the upper cage and the pacmans are done with a variety of brackets. The 3 upper brackets are wacky-shaped L with an angle that ruled out a standard L-profile. 

They come from a slab of 8mm thick CF -leftover bought from eBay at a cheap price- that gives the nice opportunity to bond the 2 parts together. Again the material used was chiefly a matter of availability and convenience while the FEA is carried out assuming these parts being Aluminium. In hindsight I would bond Aluminum as well to avoid the warps of welding and the need for a milling operation afterward. Also, 8mm thickness is a total overkill - 6mm would be totally OK weigh some weight reduction.

Each of the brackets weighs in at 153g which keep the forecast for the overall UTA at less than 15Kg.

CAD snapshots:

Picture1.png.7d4cdc0e8824e825273ef597f8cfa3cc.png  Picture2.png.aea7e1c963ac694b6d3c99cb19b52704.png  Picture3.jpg.37e0229071783845a8c6f11d141a27de.jpg

Al the sub-parts cut squared together:

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Finished parts:

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Now on the Azimuth table: that's the last big subsystem to introduce is the bottom part of the structure. The original design comprised of 2 platforms - one to support the mirror box and the other -under the first one- to carry the the metal track for the azimuth movement on rollers.

 

This area was identified from the very beginning as crucial to achieve the telescope modal performance - shouldn't be a surprise as it's the foundation of the overall structure.

An exacerbated view of the two plates deformation proves that a serious improvement of the rigidity of such system was much needed. The following were intended to be GIFs to show the structural vibration modes:

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Without going into heavy or expensive solutions the way forward is to increase thickness. So from 40mm to almost 84mm by means of layers of plywood/foam/plywood/foam/plywood.

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With such an effort on the rotating table we wanted to simplify the manufacture of this system - big boards are a pain to move around!- so the track is moved to the bottom of the azimuth table and the rollers are placed on 3 legs of a sort of a flat tripod. Such tripod doesn't carry much of the load (bending momentum-wise) as long as the rollers and the ground feet are close enough.

Here is the azimuth table with some wooden inserts for attaching parts later on and for reinforcement. Ready for height calibration through a drum sander:

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Back from the shop for a final retouch with the belt sanding machine to have the face as parallel as possible

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Wrap of carbon fiber - no need for vac-bagging, only wet lay-up. More for protection than anything else - could be glass fiber

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Finding the CNC holes under the carbon skin and inserting rivnuts bonded in place. Holes are for the altitude bearing brackets

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Next step on the pac-men build.

Some 2mm stainless steel was laser cut at a nearby shop. Total for two cut and rolled 1800mm strips + 3 arches is 125Euros - I felt it wasn't much of a rip-off... still one of the highest amount of money spent on parts so far.

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My line of thought is the following - the joining needs to be stay in place for as long as I can think of and should have a minimal settling along with not creating bumps. hence no way I'm screwing this to the pac-men.Also tensioning by the ends with springs or other mechanism - which look cool- I feel they don't provide the stability we need.

 

We resorted once again to bonding with epoxy. Tricky bit is always exerting some sort of clamping pressure - truck-style ratchet band 50mm wide can do the trick. The overall operation was remarkably straightforward after sanding the surfaces and loading the epoxy with some filler to achieve higher viscosity.

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