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800mm f/3.3 Telescope Project

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#226 skround

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Posted 29 April 2020 - 11:45 AM

Hopefully the COVID-19 has not impeeded you cool project and you are able to complete your structure. Really interesting project.

 

My 2 cents, there is a very painful world of difficulty between someone making a few 500mm mirrors at F4-5 and figuring a perfect 800mm at F3.

The level of difficulty is really not linear from one size to another, especially at faster F-Ratios.

 

What places have you quoted for the mirror out of curiosity?

 

Cheers.

Hi Hugo thanks for checking out - as a matter of fact the assembly is taking place in Cremona which was the epicenter in Italy. All the guys supporting the project are doing well although the activity is halted as the lockdown is going to be partially lifted starting next week.

 

To make the most out of this situation we are having call confs to decide the strategy over the making of the mirror - a meniscus is one of the option. Mel's - and other people's - work is inspirational and somehow the complexity is recognized. We have slumped mirrors and have some experience in mirror making but as you are pointing out, a bigger faster mirror is a double step-up. 

But hey, don't we like challanges?

 

I requested for quotation soem time ago...I think I recall Hubble Optics, Fullum and a couple of other chaps in the US.



#227 skround

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Posted 18 June 2020 - 01:46 PM

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…

 

deform iso 1.jpg

 

deform front 1.jpg

 

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:

mirror y mm deform.jpg

 

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.

 

UTA def.JPG

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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#228 skround

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Posted 01 July 2020 - 09:04 AM

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

 

ig1.jpg

 

Anyway, focusing on the Lower Tube Assembly I've finished installing all the stud-screws. They are all pre-drilled and epoxied in place. These are crucial elements and, just like any joint, the connections work by 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

 

ig2.JPG

 

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

 

ig6.JPG   ig4.jpg

 

Here is how the Aluminum 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

 

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

 


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#229 skround

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Posted 04 July 2020 - 10:39 AM

LTA assembly dry-run.

 

IMG_7543.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_7584.jpg

 

IMG_7593.JPG

 

IMG_7600.JPG

 

Next up is grinding the Altitude bearing surfaces.

 

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.



#230 skround

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Posted 04 July 2020 - 10:42 AM

LTA assembly dry-run.

 

IMG_7543.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_7584.jpg

 

IMG_7593.JPG

 

IMG_7600.JPG

 

Next up is grinding the Altitude bearing surfaces.

 

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.


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#231 tommm

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Posted 04 July 2020 - 12:46 PM

A lot of work! What conditions do you assume at joints in your FEA model - where parts are held with fasteners such as the truss poles/brackets, struts between the two alt bearings, etc?  Seems the devil is in the details there.



#232 TxStars

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Posted 04 July 2020 - 03:55 PM

Are the joints in the H frame welded or glued?



#233 Bob4BVM

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Posted 05 July 2020 - 12:54 AM

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

 

attachicon.gifig1.jpg

 

Anyway, focusing on the Lower Tube Assembly I've finished installing all the stud-screws. They are all pre-drilled and epoxied in place. These are crucial elements and, just like any joint, the connections work by 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

 

attachicon.gifig2.JPG

 

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

 

attachicon.gifig6.JPG  attachicon.gifig4.jpg

 

Here is how the Aluminum 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.

 

attachicon.gifig31.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.

 

attachicon.gifig5.JPG

Nice work on that H-frame !   Can't see exactly but those look like some very clean weld joints, did you use TIG of are they Al-brazed ?



#234 skround

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Posted 11 July 2020 - 10:20 AM

Nice work on that H-frame !   Can't see exactly but those look like some very clean weld joints, did you use TIG of are they Al-brazed ?

 

Are the joints in the H frame welded or glued?

Bob, TxStars,

 

In a nutshell: joints are bonded and riveted with gussets.

 

Long one – if you really want it as somewhere on this thread we touch based this and had my load of cussing wink.gif

 

The two main motives for a non-welding solution are:

 

1) The structure is thin – 1.5mm/0.06” and I don’t have capability to weld that thickness – I actually don’t trust me around any welding. Brazing could have been an alternative. However the heat involved in both processes can cause warping of the structure - which is designed to not deal with twist but rather only forces on its main plane

2) The spirit of the project: we firmly believe that this project should be executable even by people that do not have special equipment. Bonding is not risk-free but it’s well established in many industries and with the right precautions can be carried out by any DIYer



#235 skround

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Posted 23 July 2020 - 01:52 PM

A lot of work! What conditions do you assume at joints in your FEA model - where parts are held with fasteners such as the truss poles/brackets, struts between the two alt bearings, etc?  Seems the devil is in the details there.

Hi Tom, such a good point you’re bringing up – could talk for hours.

What’s peculiar in a telescope is that the majority of the structure works in a load range that is far away from yielding. It’s more the deformation that we are interested in. Transportation can be a case load to be taken into account, sometimes – the extreme case being with spacecraft I guess.

 

This can be at times misleading though – like for joints that if not properly executed can undermine the structure integrity and be nasty weak-points difficult to eradicate.

 

A couple of examples of joints in this project:

The truss flange to bracket: one being Aluminium the other one being CF or Aluminium with an M8 bolt.

 

IMG_7923.JPG

 

Such a joint is not critical as the combination of flat surfaces, coeff of friction, bolt load that can be easily imparted – in other words that joint can carry a high torque before failing.

 

Much trickier are the joints around the pac-men perimeter.

 

IMG_7947.JPG

 

Points of attention are the bolt load and the effective coeff of friction. I tried to solve the first by using hefty stud/screws that were inserted with some bonding agent to have the good anchorage – I’m not really a wood-worker but the first tests are showing a good level of torque.

 

On top of that, spacers were adopted to properly transfer the load - in the above pic only one is shown inside the tube on the right.

 

To maximize the friction -the effective area to be more specific- between the mating parts I flattened the wooden part with a sanding block.

 

000.JPG

 

Under these precautions, the assumption that joints are FEA welds is not crazy. Happy to hear any concern from your side and see if I still have to address it.

 

You didn't ask for such a sesquipedalian answer did you.



#236 Bob4BVM

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Posted 24 July 2020 - 04:23 PM

Bob, TxStars,

 

In a nutshell: joints are bonded and riveted with gussets.

 

Long one – if you really want it as somewhere on this thread we touch based this and had my load of cussing wink.gif

 

The two main motives for a non-welding solution are:

 

1) The structure is thin – 1.5mm/0.06” and I don’t have capability to weld that thickness – I actually don’t trust me around any welding. Brazing could have been an alternative. However the heat involved in both processes can cause warping of the structure - which is designed to not deal with twist but rather only forces on its main plane

2) The spirit of the project: we firmly believe that this project should be executable even by people that do not have special equipment. Bonding is not risk-free but it’s well established in many industries and with the right precautions can be carried out by any DIYer

..."I don’t have capability to weld that thickness"...

 

Don't limit yourself to your own capability for those structural joints !

 

What you will have is a nicely assembled (glued) structure that is perfectly set up for a competent welder !

When you are ready with the full assembly, take the whole thing to a good welding shop ( one with experienced TIG operators), and have them run beads on every joint. Cleaned and done properly, the welds will look beautiful and you will NEVER have to worry about anything coming apart whether in use or in transportation, it will all be rock solid. That will also add to the salability & value of the completed scope should you ever decide to sell it.

Plus no more worries about how close you may come to FEA limits under stress conditions.

 

Since much of the shop cost for such a weld job is jigging and setup, you have already done that.  As a welder myself, if i were to receive such a project all ready to weld, i'd be thrilled.  Your pre-assembly will bring the welding cost down markedly.

CS

Bob

TIG on Alum.jpg


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#237 PrestonE

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Posted 25 July 2020 - 01:13 PM

..."I don’t have capability to weld that thickness"...

 

Don't limit yourself to your own capability for those structural joints !

 

What you will have is a nicely assembled (glued) structure that is perfectly set up for a competent welder !

When you are ready with the full assembly, take the whole thing to a good welding shop ( one with experienced TIG operators), and have them run beads on every joint. Cleaned and done properly, the welds will look beautiful and you will NEVER have to worry about anything coming apart whether in use or in transportation, it will all be rock solid. That will also add to the salability & value of the completed scope should you ever decide to sell it.

Plus no more worries about how close you may come to FEA limits under stress conditions.

 

Since much of the shop cost for such a weld job is jigging and setup, you have already done that.  As a welder myself, if i were to receive such a project all ready to weld, i'd be thrilled.  Your pre-assembly will bring the welding cost down markedly.

CS

Bob

attachicon.gifTIG on Alum.jpg

Bob, as I also do Tig Welding the above is troubling!!!

 

The epoxy holding the joints together is going to contaminate the weld zone almost instantly...as

it gets hot and vaporizes.

 

Ask me how I know???

 

And thus, nothing will hold due to the entire weld being contaminated.

 

Additionally, aluminum is cleaned immediately by sanding and then with solvent Just Prior

to welding...thus, even if the epoxy was away from the weld zone...the aluminum surface would

have oxidized and thus not weld well at all.

 

Best Regards,

 

Preston


Edited by PrestonE, 25 July 2020 - 01:15 PM.

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#238 MitchAlsup

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Posted 25 July 2020 - 05:12 PM

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.

 

You are "in the right ball park" since 0.29mm ~= 0.012"

 

You should be shooting at something closer to 0.005" (0.12mm) for focus stability over altitude with F/3.3 light cone.



#239 Bob4BVM

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Posted 25 July 2020 - 05:24 PM

Bob, as I also do Tig Welding the above is troubling!!!

 

The epoxy holding the joints together is going to contaminate the weld zone almost instantly...as

it gets hot and vaporizes.

 

Ask me how I know???

 

And thus, nothing will hold due to the entire weld being contaminated.

 

Additionally, aluminum is cleaned immediately by sanding and then with solvent Just Prior

to welding...thus, even if the epoxy was away from the weld zone...the aluminum surface would

have oxidized and thus not weld well at all.

 

Best Regards,

 

Preston

Ah, you are right Preston ! Of course the epoxy is a contaminant... and aluminum is especially touchy to anything foreign when welded

 

So is there an option that would allow TIGing ?  I wonder if it is possible to clean off the epoxy and join where possible with single bolt & nut. Could not do that with butt joints, maybe those could be clamped in place after glue is removed...

IIWM i would go to whatever trouble is necessary to get a welded structure.

So not as simple as my first knee-jerk idea, but the added time & cost is justified IMO.

I know it's been covered here already, but i cannot get out of my head all of my attempts to glue aluminum, none succeeded in the long term. And all were much less stressed than this frame structure.


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#240 BGRE

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Posted 25 July 2020 - 06:01 PM

Adhesive bonded aluminium panels have been successfully used in aircraft construction (e.g. Fokker F27 Friendship).
The secret lies in the pretreatment of the aluminium lap joints.
Essentially they are anodised cold and not heat sealed before bonding.

#241 Bob4BVM

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Posted 26 July 2020 - 01:37 AM

Adhesive bonded aluminium panels have been successfully used in aircraft construction (e.g. Fokker F27 Friendship).
The secret lies in the pretreatment of the aluminium lap joints.
Essentially they are anodised cold and not heat sealed before bonding.

Correct. For panels and lap joints. Joints with plenty of surface area.

What i see here is lots of end of beam butt joints with little contact surface area , as shown clearly in post#228. Thinwall tubing ends glued at 90 & 45 degrees...


Edited by Bob4BVM, 26 July 2020 - 01:47 AM.

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#242 Benach

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Posted 26 July 2020 - 06:00 AM

Skround: where did you get the material properties of your plywood from? Matweb? How did you model the boundary conditions of your truss in Ansys Workbench? How did you convert from a global sag of the telescope to a local deviation of your secondary wrt the focusser?

Have used ANSYS and Hyperworks professionally for a good few years.

#243 skround

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Posted 26 July 2020 - 07:17 AM

Correct. For panels and lap joints. Joints with plenty of surface area.

What i see here is lots of end of beam butt joints with little contact surface area , as shown clearly in post#228. Thinwall tubing ends glued at 90 & 45 degrees...

For reference the overall surface is equivalent to a credit card



#244 skround

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Posted 27 July 2020 - 12:58 PM

Skround: where did you get the material properties of your plywood from? Matweb? How did you model the boundary conditions of your truss in Ansys Workbench? How did you convert from a global sag of the telescope to a local deviation of your secondary wrt the focusser?

Have used ANSYS and Hyperworks professionally for a good few years.

For plywood I’ve used the material card coming from the supplier – that’s a marine grade plywood in the hope it’s going to hold better against seasons.

 

How did you model the boundary conditions of your truss in Ansys Workbench?”  I have to revert back to the chaps that actually ran the analysis – I was only supervising. Can you elaborate more on this question in the meantime?

 

The focuser axis deviation vs the optical path due to the overall telescope deformation was done old style with basic trigonometry, once we had guaged the deformation on relevant points.

It doesn’t want to be 100% accurate but gave me confidence that the deflection is within an acceptable (and arbitrary?) limit. I was concerned by the twist of the secondary mirror although it came out pretty insignificant.

 

I’m very curious to validate the result when the all thing is pull together – in a similar fashion as Oberon is doing in another thread.



#245 skround

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Posted 15 August 2020 - 03:25 PM

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

 

2.jpg

 

5.JPG

 

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

 

4.JPG

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.

 

3.JPG

 

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. .
 



#246 skround

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Posted 19 September 2020 - 09:43 AM

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.

 

ezgif.com-resize.gif

 

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.

 

IMG_8969.PNG

 

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!.

 

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

 


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#247 tommm

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Posted 19 September 2020 - 12:30 PM

 

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

 

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

Indeed! In the 1970's we would have thought we died and went to heaven experiencing all the capabilities today, from the internet and Google search to Apps, to a laptop more powerful than the old mainframe Amdahl, to the plethora of data available from the government.
 


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