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800mm Telescope Project


Michele Scotti
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Quite some time from the last post on this.

Hoever not being able to physically progress the telescope we used some time to update the cad model and run some FEA – this is actually the second run after modifying a number of ‘details’.

Modal analysis, deformation and strain energy were carried out.

Here is a check on the deformation of the telescope at 45deg -just to get the feeling at an arbitrary altitude. A mass of 2.5Kg is applied on the focuser axis 200mm outboard to simulate camera/filter-wheel/etc…

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Specifically, I was interested in re-evaluating the deflection of the light beams compared to the focuser center-line.

To properly evaluate the light beam I’m evaluating where the primary is now pointing and where the secondary is aiming at.

The secondary concerned me as it’s an overhanging mass held in place by tenso-cross. And it wasn't extensivly analysed in the previous FEA run. Not really a big idea how would it react at 45deg. Would it twist? Twist and translate?? Twist, rotate and translate???

Here is the primary mirror:

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Predictably, it rotates.   >>>> Disclaimer here is that I didn’t include any whiffle-tree or lateral constraint – more on that later on <<<<

So with a bit of math, the rotation is 0.017deg. Now where the primary is pointing now? At the distance of the secondary mirror, it means that the beam is now landing 0.55mm lower with negligible skew on (my) Z-axis.

 

How about the secondary? Looking at 3 axis it translates by 0.29mm on the altitude plane and again negligibly on the Z-axis.

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Overall: the primary tilts by 0.017deg, the secondary translates with the UTA by 0.29mm and doesn’t (appreciably) sag relative to the UTA (that’s good and slightly surprising). So effectively half of the deviation of the beam due to the primary rotation is taken by the secondary that sags with the UTA. This leaves an off-center light path of ca. 0.26mm but (more importantly) paralell to the focuser axis.

To me it's OK  although -important remark - all of this is what I can do now waiting for the build to complete and correlate this analysis.

 

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While it is fun/instructive to design the telescope structure and analyse it's performance on a computer, I would have thought that the fundamental question in this project is "can you make a mirror to the specifications desired?"

Personally that is the first thing that I would do.....make the mirror. If it works out fine then it will wait for the rest of the structure to be made. If you make the structure first, or alongside the mirror making, and the mirror is not up to your specs then it could be a big disappointment.

Nigel

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Hi Nigel, good question – it is probably the most popular one.

The few telescopes I made in the past indeed hinged around the optics so the mirror was the first one to focus on and to complete. Then the rest of the telescope. I agree that this is the most difficult part to sort out. With a caveat – the size. And the purpose.

This project aims at a robotized imager eventually been remotely controlled in a small observatory.

Let’s start with a consideration – if you pool the amateur telescope in the 700mm+ category there are very few scopes being used as imagers – actually, none as far as I know at a purely amateur level. And for good reasons – the engineering of the mounts gets more complicated. And focal lengths are getting bigger, making tracking more challenging thus demanding more sophisticated mounts.

That’s why we started with the mount. If the mirror is not up to the task it will be disappointing. But it will be even more if the mount is not capable to be an imager – it would fail the entire project, leaving us with a big light bucket. Still enjoyable but not serving the ultimate purpose of this project

And if the quality of the first mirror is not adequate we’ll keep trying as we did in the past.

Lastly, we have a 500mm mirror that can be adapted to mount as it has a similar FL – not the perfect situation but it will serve to debug the mount while the mirror progresses.

Not trying to convince anybody this is the "right" approach however I agree that we are adopting the same principle here which is: start witht the most critical part of the project.

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  • 2 weeks later...

Had the chance to do a couple of solid sessions on the hardware. It clearly was about time....

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Anyway, focusing on the Lower Tube Assembly I've finished installing all the stud-screws. They are all pre-drilled and epoxied in place. This is a crucial element, just like any joint, the connections work with friction. And friction is proportional to the amount of load that you can torque the nut with. That is  quite a big pull the stud-screws have to sustain.  I've used M8 and M10

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To assemble the H- frame I wanted to have all holes aligned on both sides. They whee pre-drilled on the CNC but in the meantime I had to change the position of one of those. Anyway - the 2 pac-men are aligned on the circumference and bolted together. Only then I drilled the hole - 3 per side., through holes. 

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Here is how the Aluminium H-frame is going to be secured to the internal side on the pacman - which is covered in fiberglass whereas the outer one is CF covered. Layed up on the pacman you can see the M8 bolts - I actually going to use shanked ones, a big washer and another plate made of 3mm Al to spread the contact pressure even more and they fit snug inside the H-frame side elements. So the screw heads, washer/plate is inside and in close contact with the inner wall.   This is a design decision to maximise the joint strength. 

ig31.jpg.bcfae0fec4b185554f5a69e1a1dd00bc.jpg

 

Last but not least, with the help of a specific cutting wheel for Al - it really does a good job!- I'm preparing all the beams and crossing elements to complete the LTA.

ig5.JPG.a036a73c940010fbd6f90b60b0c8bc89.JPG

 

Edited by Michele Scotti
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LTA assembly dry-run.

IMG_7543.jpg.a201a4d14158ae7e375eea382ee1f08f.jpg

 

After trimming some beams we pulled together the entire sub-assembly. That was mainly to check the robustness of all joints but also for the sake of seeing how it looks like.

The structure feels sound and light enough to be carried by a single individual if you stay inside the structure - otherwise it's very bulky. Overall it's should be around 28kg/60lbs.

 

IMG_7600.JPG.59616ada52536f7ede15486a6be8c7c7.JPG   IMG_7584.jpg.36d36b44e041b4b6a74777eb288fc874.jpg

 

Next up is grinding the Altitude bearing surfaces.

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Despite the SS strips are bonded to a CNC machined surface and there are no screws or discontinuities, they do not provide at all the accuracy required by this project.

During grinding I'll take some before/after run-outs measurements.

 

What's following from that is a specific assembly procedure This is critical to ensure that the two rolling surfaces sit on an ideal cylinder.

 

At this point we have all the 4 main assemblies that took shape: UTA, "tripod", Azimith table and now the Lower "Tube" Assembly. 

Still a lot to do though.

Cheers 

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  • 1 month later...

Small update - small parts.

With bigger elements starting to come together it's coming the time for smaller parts needed for the final telescope assembly.

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The truss heads are found at the trusses/beams ends. There are 6 beams hence we need 12 heads that will slide into the beams end and eventually secured with glue.

These end bits are made of aluminium and they have been turned into shape and hollowed on a lathe. The hollow end is tapered to make the parts as light as possible.

 

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The following step is to cut and mill the element of the head that is mating with the flanges on the upper and lower sub-assemblies - that's an 8mm flat.

The cutting operation, which is meant to alliviate the milling effort -given that we have a mini-mill- is fairly painstaking....3 out of 12 are completed so far. 

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Are you aware that woodcutting circular saws, with carbide tipped teeth, can cut through aluminium like butter?
The bench type machines and table mitre saws can save hours of tedious work reducing large aluminium sections.
Downside it it makes a hell of a mess in the workshop! Swarf collection should be attended to.
It also needs very firm work-holding to avoid accidents and potentially serious injury. Plywood jigs?
Use odour free, lamp oil to reduce saw tooth clogging. It really helps.

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On 16/08/2020 at 06:00, Rusted said:

Are you aware that woodcutting circular saws, with carbide tipped teeth, can cut through aluminium like butter?
The bench type machines and table mitre saws can save hours of tedious work reducing large aluminium sections.
Downside it it makes a hell of a mess in the workshop! Swarf collection should be attended to.
It also needs very firm work-holding to avoid accidents and potentially serious injury. Plywood jigs?
Use odour free, lamp oil to reduce saw tooth clogging. It really helps.

I might actually ahve a suitable cutting disc with inserts. I'm a little bit unsure about the jig holding the piece. As of know I'm using ALuminum specific thin abrasive disks for angle grinders. The pieces are getting sooooooo hot I ned to keep spraying water on it. it's a solid 30/45 min work per piece 

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  • 1 month later...

Before going into the next phase i.e.grinding the pac-men rails I ran a quick&dirty vibration analysis check using readily available and easily accessible techniques. With the Lower Telescope Assembly pulled together we were curious to check the ability to record and analyze the frequency spectrum of the physical modal response.

We downloaded AnaHertz free app on Apple store and double-sided tape the phone directly to the assembly in the area that feels more flimsy.

The excitation is provided by a suitable....hammer.

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By no means this wants to be an exact analysis but rather and attempt to see if some info can be extracted with a tuned-down methodology that usually costs few grands.

That part of the structure - the upper part in the back- is at 5Hz which by itself doesn't meet the project target. however, this is just a warmer while the test that counts will be conducted on the whole telescope.

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The front upper area returned a slightly higher 9Hz wheres the bottom is much more constrained and felt more rigid.
Also, this is possible as the mass of the phone is more than a magnitude less than the probed structure

Amazing to see what you can do today with te MEMS in your phone and a free app!.

Here's the full video:

https://www.youtube....h?v=Uy-H-QXyRwc

 

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  • 1 month later...
On 16/08/2020 at 06:00, Rusted said:

Are you aware that woodcutting circular saws, with carbide tipped teeth, can cut through aluminium like butter?
The bench type machines and table mitre saws can save hours of tedious work reducing large aluminium sections.
Downside it it makes a hell of a mess in the workshop! Swarf collection should be attended to.
It also needs very firm work-holding to avoid accidents and potentially serious injury. Plywood jigs?
Use odour free, lamp oil to reduce saw tooth clogging. It really helps.

Hi Rusted, I previously said I was giving it a go. Game changer! 

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Time went down from 20min to 5min - without the piece to get hot thus the benefit of spraying water to cool it down...and producing a nasty swarf. 

 

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Preparation work for grinding the Altitude rails and achieving correct precision. 
Stacking up the 2 pacmen and fastening them together. 

I'm using two steel beams to overhang the pacmen so that the swiveling 'apparatus' can go around the circumference with ease. 

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To start with the run-out (departure from ideal circumference) is checked using a digital gauge that can sense microns. 

The dial starts from 0.000 at one end of the pacmen and climbs up to almost 2mm. However that seems to be attributed to the hinge point being off-center by interpreting the plotted measures. Ideal situation is to have only a few tenths of mm of stock removal to grind off.

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Next up: drilling a slightly bigger hole for the swivel center to allow recentering and setting up the grinding apparatus.

I'm wondering how many things can go wrong...

Edited by Michele Scotti
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  • 1 month later...

The SiTech controller accepts 5v pulses as 'telescope encoder' so that gives an easy way to directly wire industrial encoders to it.

In the last few months I managed to salvage parts of a Renishaw encoding system - now 'obsolete'.  I suppose they are coming from some CNC machinery. 

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Here I'm testing one of the 3 read-heads which has a 1micron resolution although the final system needs 0.1micron resolution. This equals ca. 30 pulses each arcsecond - hopefully it should be plenty for tracking accuracy.

This is a very crude test just to see if the salvaged hardware is working properly. The grating ruler is 1000mm long and is intended for the trunnion outer diameter to sort out the Altitude position.

The next step is to test the two RGH22 read-heads with 0.1micron resolution.

If you're curious I'm using a DSO138 oscilloscope kit - first time for me working on such an instrument but it was quite straightforward after a couple of tutorials. It's like 20$/£/Euros but I mean...it works for me.

I opened a thread on encoders some time ago - I'm planning to add some findigns there for whom is interested to adapt this type of encoders to their scope.

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  • 3 months later...

Some time ago I introduced the H-frame being a thin Aluminum bonded structure at the bottom of the mirror box. Here is a pic of the item:

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Bonding was a purposeful choice to stay in line with the spirit of the project i.e. a build that is approachable by as many people as possible that may do not have access to or be skillful with welding. It goes without saying that if you can weld, by all means you should go with that.

Now bonding, especially with Aluminum makes people itchy. And for a good reason. The oxide layer can impair a good adhesion with catastrophic results - conversely if taken care of it can yield outstanding performances. Key here is that eppoxy is applied just after the surface is roughed up with sandpaper so that Oxygen doesn't virtually have time to cause harm. Not that is strictly needed, as Aluminium oxidation is not instantaneous although certainly faster than the more obvious ferrous reaction.

What I came up with is a specific, unscientific, uncontrolled stress test.

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The result is ongoing: as of now 6cm^2 (ca. 1in^2) could take 25kg (60lbs) for 2 months.

 

Edited by Michele Scotti
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  • 1 month later...

I was in two minds if this is going on the mirror thread or here on the project. Anyway....

Sub-frame and whiffle-tree almost completed.The frame came up pretty light as it’s all Aluminium yet sturdy – there’s a couple of details left to sort out but so far so good.

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Here is the sub-frame along with the H-frame.

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Whiffle-tree awaiting the nylon-covered bearings

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Here is the video of the build:

 

 

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17 hours ago, Rusted said:

A naked circular saw and a bent twist drill remind us all how we only survived DIY telescope making by the skin of our teeth! ;)

That blade is the scariest this in the garage! I was surprised when I watched the video - I usually wear heavy gloves and goggles. Safety first! 😬

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NO! Gloves and machines in rotation are even more dangerous!
Gloves can easily grab hold of polished shafts and wrap the victim tightly around that shaft!
That is why tractors must have covers over their external drive shafts.
Any blade which can cut through harder materials than flesh will give you a life changing injury!

Goggles are usually considered a good idea.
It keeps the spraying blood out of your eyes!  :crybaby2:

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  • 7 months later...

Wow I really haven't posted in a while! Guess a mix of work, covid other stuff and a dash of laziness maybe.

Anyway here's a quick update on the status of the project: https://www.youtube....h?v=pmk9X52cC0A

We made progress on few areas -I have pics and footage to post at some point- however the biggest endeavor was to find a solution to grind the trunnions rolling surface to a good level of accuracy - more to come on that.
 
Cheers, Michele

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  • 1 month later...

Hi everybody, we've just completed the assembly of the mirror cell and here is a VERY lengthy video of the process: - but at the end we try the big mirror on!

 

It's a 27-point cell with contact made out of Delrin 14mm flat pads. I'm tempted to slightly dome them off but I'll pass it for now. Eventually the cell will support a meniscus hence a smaller contact surface - which I don't even know if it's really desireable.

A note on the strange/fancy/unusual shape of the triangles: they are optimized trading off cost, flexure and weight. The 3 main ones are mild steel, blued for some rust resistance. They are 5mm thick with cut-outs to match the deformation to the secondary Aluminium triangles.

The 3 big triangles would be pretty expensive if therey were made of 8mm Aluminium. I've used FEA to size the cut-outs to match also the weight of a 8mm Aluminium, 580gr or slightly more than a pound.
They are all laser-cut.

The hinging or pivoting mechanism is based on spherial bearings - these are PTFE lined to provide smoother operation. The bearing sits against a modified washer that is secured with screws. Gravity makes it work or at least rest in place.

I think I have a video about steel blueing but not for the triangles hinges.

There are still one or two clearances to fix but overall we're happy with what we've got.

Also the mounting 'pillars' for the whiffle trees are set to accomodate a thicker mirror - they need some adjustment to properly sit on this thin mirror.

Clear sky,
Michele

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