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800mm f/3.3 Telescope Project

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#51 TOMDEY

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Posted 01 September 2019 - 07:41 PM



Thanks for sharing!

Is that a sort of manual version of active optics? Wouldn't be dependent on the altitude angle?

The wavefront correction is almost entirely independent of altitude angle aka the direction of the gravity g vector relative to the OTA coordinates. That's because the shaping forces are introduced by springs ~force actuators~  between OTA components, that follow the OTA coordinate system. For anyone/everyone out there... if you are going to try to stress a mirror into better topology... use force, not displacement actuation! That is blissfully easy to do and works... because it reduces gravity and geometric "punch-through" effects by (typically) at least 1,000x and (careully done) 1,000,000x. Our aerospace optics take full advantage of that. Build and certify in one-g environment... deploy and use in zero-g.

 

PS: There of course remain other g-sensitive alignment/wavefront contributors that can still manifest... e.g. differential drooping of the SM held by the spider, upper cage, and trusses.

 

[I worked on a lot of active/adaptive optical systems before retirement.]    Tom


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#52 TOMDEY

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Posted 01 September 2019 - 08:22 PM

Very nice design.  Are those rungs permitted some rotation to help redistribute load or are they locked in place with those clamps between the rungs?

I was hoping someone would ask that! Their positions along the rope are fixed... which is fine. Each contact sees a small contact load. After initial installation, point horizontal and just walk fingers around the edge and wiggle each one to relieve stiction stresses. The nature of a "rope ladder with rigid rungs" is that each rung will self-adjust (twist about the ladder axis) to maintain equal loading on either side, against the mirror (front and back plate) edges. Then, by induction/summation, over all 15 rungs, this also assures that both plates are equally and adaptively supported. Note that the ladder can be loosely-toleranced because of that and because any residuals are all local cosine effects aka change hardly at all until the build errors are huge. An even more deluxe ladder sling would further-comprise a central rope with far greater stretch constant k than the outer two ropes. So the load is mostly carried by the center rope, with the outer ropes just indexing the positions of the contact points... One can get away with that, because no one is actually "walking up the ladder." We just didn't have to get that fancy... KISS... >>>and it works fine!<<< Ehhh... there's actually a lot of interesting M.E. theory involved in something as fundamental as a rope ladder (and related structures)... This picture >>> shows thoughtful design in utilization/stabilization of rope/rung structures... In this case, the load is the guy trying to ~safely~ walk over Niagara Falls! (There are six crucial design decisions manifest in that picture!) Tom

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#53 ButterFly

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Posted 01 September 2019 - 10:14 PM

For convex backed mirrors, this looks far superior to Glatter cables just due to not having to accurately assess the center of gravity of the mirror in the thickness direction.  That might be reason enough to avoid a central guy.  The tensions on either side of the knots should take care of the rest.

 

The possibilities to tailor the resultants at the rungs to the mirror are endless: adjust the spacing between the rungs, adjust the offset of the cable from the axis of each rung ... .  Less squeezing from the edges but each rung adequately supporting the weight above it!

 

I presume from your earlier posts that this has been in service for about a year.  Any edge defects on the mirror due to slipping or, deformation of the rungs (especially the middle ones that get the brunt of the weight)?



#54 TOMDEY

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Posted 01 September 2019 - 11:10 PM

For convex backed mirrors, this looks far superior to Glatter cables just due to not having to accurately assess the center of gravity of the mirror in the thickness direction.  That might be reason enough to avoid a central guy.  The tensions on either side of the knots should take care of the rest.

 

The possibilities to tailor the resultants at the rungs to the mirror are endless: adjust the spacing between the rungs, adjust the offset of the cable from the axis of each rung ... .  Less squeezing from the edges but each rung adequately supporting the weight above it!

 

I presume from your earlier posts that this has been in service for about a year.  Any edge defects on the mirror due to slipping or, deformation of the rungs (especially the middle ones that get the brunt of the weight)?

Yep... one can engineer things to death --- but when you get to the point where it works... time to move on to the next project!

 

We did indeed consider rung spacing etc etc ad nauseam... but just wasn't needed; and 90% of engineers' intuition fails as one overdesigns. A friend of mine continued designing his perfect APO triplet for many years, always a little better and a little better according to Code Five... and never got beyond that point... never built it!

 

PS: My assignments at work were substantially troubleshooting problems... most often when the "regulars" on this or that program had thrown in the towel, or burning thru time and $$$ but stalled out. I'd go in, nice and ignorant... (hopefully) figure it out and --- bow out the moment things were back on track! Then, on to the next forest fire. Routine work bored me to tears; thankfully, managenent let me seek out and address problematics.

 

Our mirror/ladder seem to like each other just fine. I have indeed scrutinized that occasionally --- even yesterday evening (picture >>>), as a matter of fact! Looked at each contact point with a flashlight. All is well! The slumped mirror edges are not absolutely/perfectly cylindrical or blem-free, the usual little imperfections seen in any big precision mirror --- don't need to be perfect, because the ladder adapts/conforms. It's really a very gentle hammock for the mirror. Little force at any one contact point, and the Delrin provides just a blush of desired softness.    Tom

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#55 Oberon

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Posted 01 September 2019 - 11:46 PM

In a vertical position -as assumed- each spider handles the load transfer of 1/4 of the mirror assy (imagine to place 4 scales at the ends of the vanes in a stand alone condition).

If you are thinking of factoring in the tension on the vanes that doesn't change that much.

 

What if instead of 4 vanes we use one? Would you still make it thicker at the mirror side?   

In a classic spider the tension on the vanes is the primary load. The mass of the mirror is (or should be) relatively minor, almost incidental. 

A single vane spider is not in tension, so its a completely different beast. It isn’t a quarter of a classic spider.


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#56 skround

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Posted 02 September 2019 - 09:03 AM

In a classic spider the tension on the vanes is the primary load. The mass of the mirror is (or should be) relatively minor, almost incidental. 

A single vane spider is not in tension, so its a completely different beast. It isn’t a quarter of a classic spider.

The mirror load -however smaller than the vanes tension- is reason itself why you have a supporting system i.e. the spider. If you engineer a structure for pure tension than there no point in having a triangular shape. It can be constant section, no need for any triangle shape.

 

At this point we might just agree to disagree. Having say that, I found this discussion very useful as it made me ponder on the UTA itself and opportunities to further simplify it - possibly for future projects.

 

Please stick around so that when manufacturing the spider and the UTA we can re-vamp the discussion if new elements are clearer - I always appreciate any kind of constructive critiquing



#57 skround

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Posted 02 September 2019 - 10:18 AM



The wavefront correction is almost entirely independent of altitude angle aka the direction of the gravity g vector relative to the OTA coordinates. That's because the shaping forces are introduced by springs ~force actuators~  between OTA components, that follow the OTA coordinate system. For anyone/everyone out there... if you are going to try to stress a mirror into better topology... use force, not displacement actuation! That is blissfully easy to do and works... because it reduces gravity and geometric "punch-through" effects by (typically) at least 1,000x and (careully done) 1,000,000x. Our aerospace optics take full advantage of that. Build and certify in one-g environment... deploy and use in zero-g.

 

PS: There of course remain other g-sensitive alignment/wavefront contributors that can still manifest... e.g. differential drooping of the SM held by the spider, upper cage, and trusses.

 

[I worked on a lot of active/adaptive optical systems before retirement.]    Tom

I reckon I need to learn more about astigmatism! Thanks for the initial clarification. 

So apart from a 27-point cell and a sling/wire lateral support that we are planning, you'd suggest a manual wavefront jig? How many points and where do you place them? If there's already a thread on CN I'm happy to go thru that.  



#58 gregj888

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Posted 02 September 2019 - 11:02 AM

I'm not going to help much on the design with the pro's commenting but will throw in a couple of items.

 

Many of your constraints have been discussed/solved/explored in the Alt-Az group.  Information on Russ' 1m portable scope  (there was a 2m played with).  Direct drive, Mel's modification to allow an Alt Az to cover zenith...  should be worth a look:

 https://groups.yahoo...Initiative/info

 

You really need to define "science" and establish your science program.  It will be important to look out 5 years too as amateur astronomical science is likely to change dramatically with the onset of LSST and other big survey instruments.  High cadence work- double star speckle, fast light curves, planetary probably will not, but others will.  Google "Time Domain Astronomy" and do some reading.  IMHO an area of low hanging fruit for us will be fast response to the Alert streams.  That likely means a robotic scope when in that mode.  

 

Last, the control systems are pretty well understood.   For long exposures you either need auto guiding or high resolution encoders (> 1m tic, >3m is better) and depends on the focal length.  If unguided, you also need a good mount model, which is more difficult on a portable scope but not impossible (I haven't don it, but know several the use modeling routinely.  

 

Great project, back to lurking... :-)



#59 TOMDEY

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Posted 02 September 2019 - 01:31 PM

I reckon I need to learn more about astigmatism! Thanks for the initial clarification. 

So apart from a 27-point cell and a sling/wire lateral support that we are planning, you'd suggest a manual wavefront jig? How many points and where do you place them? If there's already a thread on CN I'm happy to go thru that.  

If your mirror is a very good one, excellent wavefront in a relaxed state... should be no need for introducing tunable forces.

 

My cheap mirror suffered inherent astigmatism. Many (most?!) mirrors out there do. That's because Zernike Primary Astigmatism is the lowest bending mode of a thin, homogeneous, isotropic solid wafer. (I think I got that terminology correct?!) In layman's terms... just look at a thin mirror cross-eyed and it will manifest astig. Turns out that power and coma don't matter, because they will be focused or aligned out... and the other higher freq and higher order Zernikes take way more force to induce and are (therefore) less likely to be inherent in a mirror.

 

The choice menu comprises:

 

>live with it; try to ignore the poor imagery... enjoy observing!

>replace the mirror with a good one

>refigure the mirror at home or have it done

>add an optical (lens) global astig corrector

>stress the astig out of the mirror

 

Most people opt for #1 = always annoyed, but at least observing

Some people opt for #2 = expensive, should have bought a good mirror to begin with

Some people opt for #3 = redoubtable challenge at home or expensive to have done professionally

Few people do #4, even though it is very easy and works ... probably just never realized that can be done

Few people do #5, even though it is pretty easy and works ... probably just never realized that can be done

 

When I had the problem... I just thought thru it and came up with options #4 and #5; chose the latter; executed it --- it worked!

 

At that point I wrote a brief, informal white-paper on the subject and presented it here and there at local astronomy clubs, universities, and aerospace groups.

 

Some others have since successfully used variations on the themes. Here are a couple of excerpts from that (nearly all the text omitted) >>>

 

Oooh! Just remembered... I wrote a 4pp article on that, which appears in the Fall 2017 Issue #96 of Amateur Astronomy Magazine.

 

http://amateurastronomy.com/ 

 

Highly recommended, complete series available on CD/DVD!    Tom

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Edited by TOMDEY, 02 September 2019 - 01:34 PM.

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#60 skround

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Posted 03 September 2019 - 10:54 AM

Many of your constraints have been discussed/solved/explored in the Alt-Az group.  Information on Russ' 1m portable scope  (there was a 2m played with).  Direct drive, Mel's modification to allow an Alt Az to cover zenith...  should be worth a look:

 https://groups.yahoo...Initiative/info

That group is very interesting - I new about he initiative but not the Yahoo group.

It looks like they stalled though - where did they arrive to?



#61 skround

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Posted 03 September 2019 - 11:26 AM

Soooo...the 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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#62 Astronorm

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Posted 03 September 2019 - 04:42 PM

Soooo...the 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.

Do you already have a 800mm F/3.3 mirror for this telescope?

Normand Fullum



#63 skround

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Posted 04 September 2019 - 11:01 AM

Do you already have a 800mm F/3.3 mirror for this telescope?

Normand Fullum

We don't have that physically with us although we are pre-checked the availability. In the design phase we are accounting for the weight of a 50mm thick mirror



#64 gregj888

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Posted 04 September 2019 - 11:51 AM

That group is very interesting - I new about he initiative but not the Yahoo group.

It looks like they stalled though - where did they arrive to?

Still there but has backed off with a lot of the harder problems more or less solved.  Had the Portland workshop in Aug. much of it was on spray silvering.  There's a trailer mounted scope coming together, will probably pick back up soon.

 

I'm one of the moderators, after a spam outbreak.  If you have questions or want to discuss, most of the folks still watch the site.



#65 tommm

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Posted 04 September 2019 - 12:29 PM

With a f.l. = 2640mm, distance from pm optical axis to tube O.D. = 480mm, and an arbitrary 0.5 deg fov I get a secondary minor axis of 6.5" just to get the primary image plane at the O.D. of the tube. To project it 3" beyond this I get 7.4" minor axis.  That agrees with the result given by Rutten and Venroog, Suiter gives 7.6". 

 

Newt for web is assuming visual use.  How far does the image plane need to be outside of the tube to be in the plane of the imaging sensor?  That will likely require a significantly larger secondary and will determine your primary-secondary distance. I am using a 23% CO for visual use on an f/3.6. No way I could use it for AP without decreasing the primary-secondary distance.

 

If you use a secondary holder with a lip over the secondary mirror like the Astrosystems holders don't forget to account for that in specifying secondary minor axis (smaller).

 

Btw, please do think about spider vanes some more.  Yes, gravitation does apply a torque to the vanes through their moment arm which tends to rotate or bend them.  You seek to counter this with compressive forces in rigid beams. Oberon is saying that normally you counter that with tensile force in the vanes by tightening screws to put them in tension so they pull on the secondary holder rather than supply a reactive force pushing against it as beams would.  It is easiest to see this in a wire spider which cannot have significant compressive force, and only has tensile force. A vane spider under tension works similarly, but also has some internal stresses which further stiffen it.  It is due to the tensioning that you can use thin vanes, otherwise much thicker ones would be required for the rigidity required to hold optical alignment.


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#66 skround

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Posted 05 September 2019 - 06:04 AM

Btw, please do think about spider vanes some more.  Yes, gravitation does apply a torque to the vanes through their moment arm which tends to rotate or bend them.  You seek to counter this with compressive forces in rigid beams. Oberon is saying that normally you counter that with tensile force in the vanes by tightening screws to put them in tension so they pull on the secondary holder rather than supply a reactive force pushing against it as beams would.  It is easiest to see this in a wire spider which cannot have significant compressive force, and only has tensile force. A vane spider under tension works similarly, but also has some internal stresses which further stiffen it.  It is due to the tensioning that you can use thin vanes, otherwise much thicker ones would be required for the rigidity required to hold optical alignment.

That's how the spider we've design works - I trust that when I'll post some more details of the design our positions will be closer that they appear now.



#67 skround

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Posted 05 September 2019 - 06:39 AM

Still there but has backed off with a lot of the harder problems more or less solved.  Had the Portland workshop in Aug. much of it was on spray silvering.  There's a trailer mounted scope coming together, will probably pick back up soon.

 

I'm one of the moderators, after a spam outbreak.  If you have questions or want to discuss, most of the folks still watch the site.

Just submitted request to join the group 



#68 skround

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Posted 05 September 2019 - 06:55 AM

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.

Attached Thumbnails

  • pacman.JPG


#69 skround

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Posted 07 September 2019 - 01:23 PM

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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#70 skround

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Posted 09 September 2019 - 07:39 PM

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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Edited by skround, 10 September 2019 - 04:15 AM.

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#71 skround

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Posted 12 September 2019 - 05:00 PM

This is the focuser board to house a generous focuser/derotator, connect the UTA hexagons and provide attachment for one of the 4 spider vanes.
Being part of the upper assembly this 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....

vac.JPG  plank.JPG

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#72 skround

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Posted 15 September 2019 - 07:43 AM

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 rules out a commercially available 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 Picture3.jpg Picture2.png

 

Finished parts - with a little reinforcement added because, why not?

IMG_0172 (2).jpg


Edited by skround, 15 September 2019 - 12:58 PM.

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#73 Bob4BVM

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Posted 15 September 2019 - 10:27 PM

Those are some nice looking truss brackets, but they bring to mind something I have wondered about for a long time- why do we see so few (if any ?) truss scopes that are built as a true truss, ie the poles meeting at a single point instead of being separated by some ( however small ) space & attached with 2 bolts instead of one ?  That arrangement effectively creates a hinge joint with accompanying flex effect, which gets worse as the pole end separation increases.  A single bolt attachment for a pair of poles is easy enough to accomplish, as shown in this drawing I made back when I was planning a redesign of my old truss dob.  I have since abandoned that project for another use of those optics, but if I ever build another truss scope this is how I will attach the pole ends to form a true-triangle truss structure.

 

 I am enjoying watching your scope go thru this ' ideas stage ', and wish you the best with the project.
CS
Bob

 

dob truss pole single-bolt-lap joint.jpg


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#74 skround

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Posted 16 September 2019 - 11:50 AM

Hi Bob,
 first of all let me state that for no reason I think the design I came up with is the best or that suites all needs. Rather I try and spend some time on each and every component to maximize the system performance that in my case is stiffness/weight ratio.

 

So what is the benefit of having a true-triangle truss?
- one or more degree in the truss structure: that's always welcome - how much does it increase the overall system performance (heigenmode and deflection)?
- one bolt less: another good point - although longer, still lighter
- forces aligned to one fixing point - if you use fixed joint that is actually the same as a virtual single point further up

 

I see 3 constrains:
1) small contact area - I want to use the joint as a fixed support i.e. a joint able to carry bending moment which means less deflections
2) massive flange thickness - that's unnecessary weight at the top, where is more critical
3) conflict between beam diameter -that gives a great stiffness- and flange length which becomes heavier. In our case the flange is an aluminium part that I'd like to minimize for material cost

 

Having said that I like your idea and got me thinking. A way I would interpret the true-tria truss design to overcome some of the constrains could be: the flanges placed at opposite sides of the bracket and the flanges themselves being offset enough to make it work. It is definitely something worth having a look at in CAD. The bracket would be smaller too - that's very good. I should add a sketch but I hope it's clear enough.

 

Finally to your comment about "hinge":

 

That arrangement effectively creates a hinge joint with accompanying flex effect, which gets worse as the pole end separation increases

I would say that is correct when using Heim joint whereas that's not the case for a "fix support" i.e. the flange and the bracket acts as a single part when loaded

 

That arrangement effectively creates a hinge joint with accompanying flex effect, which gets worse as the pole end separation increases. 

 

Thanks for your post - I really appreciate this kind of contribution and if there's anything that I was not clear don't hesitate to let me know
Michele



#75 tommm

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Posted 16 September 2019 - 12:12 PM

I think two separate bolts are typically used because people like to handle the truss poles in pairs that fold together.  You can't fold the poles together with the design as shown in Bob's post, but you can if you place the pivot point below the ends of the truss poles. This is what I came up with to try to get close to a true truss but still have the poles in pairs:

 

Truss brackets, upper.JPG

 

The notch cut in the right side part is to accommodate the truss pole which is bolted onto the left side of the bracket with one bolt through the hole in the edge of the angle. Another pole is similarly bolted to the right side of the bracket.  The pole end just clears the notch.  It does require somewhat longer brackets.  Left side bracket is how it appears with the two poles folded together, right side is as it appears bolted to the UTA truss bracket.


Edited by tommm, 16 September 2019 - 12:15 PM.

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