800mm Telescope Project

[Gina]

Errrrr... that's quite a big scope!  🤣

[Michele Scotti]

On 14/01/2020 at 14:34, Gina said:

Errrrr... that's quite a big scope!  🤣

I have to admit that we were quite stunned by the stance of it when we pull it together.

Also, it was less than 10Kg so it felt weirdly light.

 

When looking at  that as just a sub-assy, the whole scope will look fairly ginormous.

 However my mind goes to the fact that the success of this project is not its size but rather its capability as an remotely controlled, robotizer imager - that's the big challange to me.

[Gina]

Remote imaging with it would be quite something.  Wonderful if you can manage it!!  Good luck.

Edited January 15, 2020 by Gina

[Michele Scotti]

On with the work on the mirror box.

This is just an assembly dry-run to sort out all those details that we've overlooked while producing the single components.

The main piece of work here is to fit some long M10 and M8 studs for wood, preparing the mating surfaces to maximise the contact area and to sort out the right washers.

All of this is to be able to tighten the fixing so that the structure shows its potential and it's not wobbly just because a fixing is not up to the task. With this minimal mirror box concept the structure is as strong as its weakest element or joint.

 

I attach a pic here and I've just uploaded a video on YT with a bit more details if you're interested:

 

 

 

Edited January 19, 2020 by Michele Scotti

[Michele Scotti]

An element of the assembly that I hold dear is the connection of the components. A bad connection of just one of the element will preclude the chance to achieve the modal performance of the mirror box and the telescope itself.

 

The following is my interpretation of the joints we are extensively using on the pac-men and is works around to 2 main constraints:
  • the Pac-men material is a sandwich of foam and plywood
  • all the connecting element are thin walled Aluminum

Wrt sandwich construction the solution was to we carved out the foam and replaced with solid plywood in the areas where fixings were expected. Here is the picture where you can see few of them - some of these inserts have other function such as the manufacturing and assembly process. They improve the performance of the sandwich too.

 

Second challenge lays with thin walled profiles - from 1.5 to 2.0 mm. The load from the fixing tightening needs to be distributed to avoid sagging which spoils the ability to reach high bolt loads. Such deformation is obvious for thin Aluminium but it might be subtle yet relevant for steel as well.

So how to deal with that? 2 things: a generous steel washer under the bolt head/nut and a purposely built spacer to transfer the load through the hollow profile. Here is a picture of the rear main bar.

 

The option to bolt directly on the inner wall in contact with the pacman is far from ideal as it's cutting off some of the material from contributing to the stiffness.

 

Now to the actual joint – as it works on friction 2 main aspects are taken care of:

  • Contact area treatment
  • Bolt load

As per picture here below we’ve sanded down the surface to flatten it. Or better to secure that the contact pressure will be more evenly distributed.

 

   

 

Bolt load is tricky with a wood sandwich especially when the screw is inserted along the layers plan. As we would have to undo the joint several times even during the assembly tuning, we rules out wooden screws. What we opted for instead are these metric studs for wood. They can accommodate a nice M8 or M10 nut.

 

They come with an handy torx at the top.

 

A pilot hole 0.5mm smaller than the screw shank is drilled and some epoxy added in the hole to have a rock solid anchoring.
All of these allows high torque hence high “bolt” load which is transferred evenly through the profile and the pacman.

As always, thnaks for your attention - if you have any comment or if anything is not clear -which is likely!- I'll be please to answer.

Clear skies, Michele

[Rusted]

Don't ignore the option of "furniture nuts." I use them all the time in my constructions. A couple of dozen on my big GEM alone.
These are of galvanized steel to avoid rust. A "gold" finish is optional but I wanted to match the aluminium.
The large, flat heads provide a tidy solution without needing load spreading washers and "Nyloc" nuts.
The hex socket doesn't mar easily no matter how often they are driven and removed.
You also have the option of putting a hex driver in a power tool for speedily applied torque.

 

 

[Michele Scotti]

7 hours ago, Rusted said:

Don't ignore the option of "furniture nuts." I use them all the time in my constructions. A couple of dozen on my big GEM alone.
These are of galvanized steel to avoid rust. A "gold" finish is optional but I wanted to match the aluminium.
The large, flat heads provide a tidy solution without needing load spreading washers and "Nyloc" nuts.
The hex socket doesn't mar easily no matter how often they are driven and removed.
You also have the option of putting a hex driver in a power tool for speedily applied torque.

 

I like this option when low profile - especially the fact that is low profile apart form the large head.

And I like your GEM too! To you have a thread where you decribe its construction? Can you track consistently for long exposure?

[Rusted]

1 hour ago, Michele Scotti said:

 

I like this option when low profile - especially the fact that is low profile apart form the large head.

And I like your GEM too! To you have a thread where you describe its construction? Can you track consistently for long exposure?

Hi,

No mention of my build on FLO. I usually blog and photograph all my projects to death.
The details are all there including my endless discussion of the design as it evolves in my mind.
I rarely bother with drawings and just make it up as I go along. I have a lathe and power tools.

 

The GEM uses multiple, tensioned, threaded rods [studs] in all three planes, to reinforce each other.
Only the longitudinal and cross studs are shown here. A similar set of studs run towards the camera.

They not only resist each other in compression of the flanged bearing boxes,
but all are carefully placed to contact each other to further resist bending.
These studs are all hidden within the bearing housing's 10mm thick, stressed aluminium plates.
The whole thing weighs "a ton" but was intended for permanent mounting on a solid pier.
I need a chain hoist to lift it as one unit.

It has AWR Goto drives. With multiple drive rates and full planetarium support via ASCOM. 
I am primarily a solar imager, these days, with a fast ZWO camera, so I don't need long exposures.
My long focus refractors 180/12 & 150/10 aren't very suitable for imaging the night sky anyway.
Other than the moon and planets of course.
 

[Michele Scotti]

22 hours ago, Rusted said:

Hi,

No mention of my build on FLO. I usually blog and photograph all my projects to death.
The details are all there including my endless discussion of the design as it evolves in my mind.
I rarely bother with drawings and just make it up as I go along. I have a lathe and power tools.

The GEM uses multiple, tensioned, threaded rods [studs] in all three planes, to reinforce each other.
Only the longitudinal and cross studs are shown here. A similar set of studs run towards the camera.

They not only resist each other in compression of the flanged bearing boxes,
but all are carefully placed to contact each other to further resist bending.
These studs are all hidden within the bearing housing's 10mm thick, stressed aluminium plates.
The whole thing weighs "a ton" but was intended for permanent mounting on a solid pier.
I need a chain hoist to lift it as one unit.

It has AWR Goto drives. With multiple drive rates and full planetarium support via ASCOM. 
I am primarily a solar imager, these days, with a fast ZWO camera, so I don't need long exposures.
My long focus refractors 180/12 & 150/10 aren't very suitable for imaging the night sky anyway.
Other than the moon and planets of course.
 

I have to say that it's a very intriguing approach to GEM - I'll dig a bit in your blog.
I like the use of the bearing directly as part of the structure as well as boxing with Aluminium panels and rods.

[Rusted]

22 minutes ago, Michele Scotti said:

I have to say that it's a very intriguing approach to GEM - I'll dig a bit in your blog.
I like the use of the bearing directly as part of the structure as well as boxing with Aluminium panels and rods.

Thanks. My GEM was built back in 2017 with later improvements.
I'm afraid my build diary blogs are horribly "inaccessible." 

I realised early on that I had no access to suitably rigid box sections.
So I made them rigid by constraining the boxes with the massive, tensioned, internal studs.

Anyway, I am going seriously off topic for your fascinating build thread.
Perhaps I should explain my design in a fresh DIY thread.

[Michele Scotti]

21 hours ago, Rusted said:

Anyway, I am going seriously off topic for your fascinating build thread.
Perhaps I should explain my design in a fresh DIY thread.

Yup, I think you should start a thread as your approach is fresh and it can easily inspire somebody else.

No worries for the topic - still relevant and  thnaks for the compliment.

Comments slowed down a bit - I wonder if people are waiting to see whether this is 'real' or not.....me too to be honest!

[Rusted]

Thanks. The sheer scale and technical level of your project is all a bit mind boggling.

I came close to making a [relatively tiny] 16" F/5 until I realised it wouldn't fit in any storage space I owned.
I couldn't even get my arms around the solid PVC tube to lift it!
That was decades ago before truss tube Dobs were visible on the horizon.

A 12" f/5 Dobsonian, again with a plate glass mirror, went much more quickly.
The ease of movement of PVC and Formica on PTFE was an absolute delight.

 

Just testing the bigger mirror at C of C was a struggle to find enough linear space.
Re-figuring the mirror to F/4 proved to be far more prolonged than I'd hoped.
Plate glass is very slow to cool after polishing in a cold shed.
Particularly when bringing the mirror into a much warmer room to test it. Hopeless in fact!

In perfect hindsight I just didn't have the facilities to complete such a project.
I 'm sure you'll have more luck. As long as you have enough room for storage!  

This was back in the 1980s when I first heard of Dobsonians.

 

[Michele Scotti]

Some update on the works on the bottom on the mount.

The 'tripod' is now (almost) completed enough to be tried out on the (almost) completed azimuth table.

The tripod has all bearings in place - at the end of each leg-  to run on a stainless steel track which is placed on the bottom side of the azimuth table.

It was just nice to see the whole thing spinning around....

               

What's left to do on this sub-assy? 

On the tripod it's the implementation of the drivetrain and sorting out a robust way to place the mount on 3 feet.
The azimuth table just needs the track to be properly prepared to function as a generous bearing - I'll come to that in one of the next post.

However, all of that was good enough for a first 'spin'. I uploaded the video on yt:

 

(btw does anybody know how to resize the video preview window? If it's even possible...)

Clear skies, Michele

[Rusted]

Excellent video! It was just like being there!

Have you tried loading it with the expected weight of the completed structure?
Roller bearings can completely change character when subject to loads.
As I learnt to my cost with a 24" cast iron, face plate. Too heavy and too much friction!
It was supported on 4" rollers with journal bearings on the rim.
While tilted up at my latitude on a short shaft with a thrust bearing.
It was supposed to be my [98" Isaac Newton stye] equatorial fork for my 16" f/5. 

[Michele Scotti]

2 hours ago, Rusted said:

Have you tried loading it with the expected weight of the completed structure?
Roller bearings can completely change character when subject to loads.

No loading at all just yet as the SS track still needs to be machined. On top of that I'd like to properly sort out the position of the feet.

I'm going got minimize the distance between the contact point of the bearing and the feet themselves. Ideally they should be just below so that the bending stress and its deformation is minimized.  

[Rusted]

That sounds sensible.

[Michele Scotti]

On to the challenging bits of the build....and the reason why Iwe didn't want to load the strucutre just yet.

Background/assumption: looking at its Periodic Error an ideal mount minimizes the amplitude and it looks smooth – in mathematical terms this means that the higher order content of the Fourier Analysis.
I would add that a desirable element is that the movement is repeatable i.e. it’s ‘smooth’ and without ‘random’ jitters all around the rotation the axis.

To achieve a good tracking capability I’d start from the most precise execution of the components – however soon you realize that even so, errors in the order of few microns will show up in the image given the long focal length.
A bit of math done some time ago showed me that 1 micron error on the azimuth track would generate a 0.22arcsec (arcsin(1/900.000) tilt of the telescope – which is comparable to the sensor pixel resolution.

And of course there is no chance -for me at least!- to produce such an accurate surface. Let alone that all other elements of the system have a tolerance – for instance the roller bearings are specified at around 11microns.

As a consequence, for a telescope to be an imager, autoguiding is a necessity. However a smooth repeatable PE is easier to be guided out.

 

So what’s the plan? A ground track of course! How…?

In the absence of a gigantic lapping machine my choice is going into creating a grinding machine on the table itself.

Here is the sketch which is hopefully self-explaining. Enough to say that it makes use of an angle grinder -with a controlled play- and a controller to limit its speed.

 

The tool will be a “diamond” cup like the one in the pic.

Sure it will take long time to carry out the machining operation but that’s ok if it yields the result. Also the shaft is rotating into the same bearing that will stay on the azimuth table and that is used to center the "tripod".

On a side note: the use of a central bearing is part of an attempt to reduce the random jitters as the rollers will always be running on a constrained radius with no chance to build slack -on a micro-scale. It’s possibly more difficult for me to explain than for you to guess it.

 

Well, that's the plan.....

[Rusted]

With respect, I have some recent  experience in trying to "machine" the inside of a 3m diameter plywood ring using a central pivot and a portable router.
The result was extremely violent chatter! I then duplicated the radius arms into a horizontal triangle and this helped to smooth the chatter but not by much.
I used 20x100mm timber arms to form the horizontal triangle.

What was missing from my radius bar system was 3D limitation on the radial cutter. The router could hop up and down despite the sheer weight of the triangle.

I believe you need a pyramidal radial limit system to avoid tool rotation in all planes.
If you use a simple, vertical triangle, as shown, you cannot restrain the cutter [angle grinder] motor from bodily rotation.
Your cutter is of large radius and has absolutely no reason to obey your reasoning.

The radius bars shown cannot be stiff enough unless you make them seriously large in cross section.
I think you will need to triangulate both your supports in the plane of the drawing to avoid torque effects causing severe digging in!
Admittedly your radius is much smaller. But your demands are for very high precision and the tool radius is uncontrollably large.

Perhaps you can produce a very solid, double triangle, plywood arrangement like a double wishbone, car suspension?
Two solid triangles. One horizontal. The other sloping down to the angle grinder support bar.
The bases of both triangles firmly supported by your bearings, as shown, with serious vertical restraint between the bearings.

I still wouldn't practice on anything precious to you!  

EDIT: Image added for my initial cutting triangle using alloy, tubular radius arms. Pivoted at the center 1.5m away.
This was replaced by a broader triangle of 20x100 planks for better damping. Neither system had any chance of real accuracy.
The crossed timbers formed my central pivot. Again providing too little stiffness.

Edited January 30, 2020 by Rusted

[Michele Scotti]

19 hours ago, Rusted said:

With respect, I have some recent  experience in trying to "machine" the inside of a 3m diameter plywood ring using a central pivot and a portable router.
The result was extremely violent chatter! I then duplicated the radius arms into a horizontal triangle and this helped to smooth the chatter but not by much.
I used 20x100mm timber arms to form the horizontal triangle.

What was missing from my radius bar system was 3D limitation on the radial cutter. The router could hop up and down despite the sheer weight of the triangle

I see what you are saying and I reckon I have contermeasures in the design of the jig - btw as a flat 2D it didn't provide any hint of the width on the jig itself.

Here are a couple of pics of the early stages of jig build - let me know if it makes sense to you - still I would need to run many checkes before giving it a go on the actual azimuth table

 

Bottom side with track:

 

Upper side with the flange to carry the second bearirng:

  

[Rusted]

That tube is not going to offer much support without some help even if you triangulate. Think of it as merely a center guide pin.
What about adding some counterweights on a horizontal extension to balance the angle grinder and jig materials?
You don't need any pressure for "cutting" and fighting the weight will require too much adjustment to take up unknown levels of backlash.
The triangles can be scrap ply. That board isn't going to offer much resistance to twisting on the axis of the angle grinder.
You could add a roller on the tail end. Your track is the only accurate surface you can trust as a register so you might as well use it.
You could add further rollers at the widest part of the jig. Inline skateboard wheels? The jig will have to be widened to suit.

 

Edited January 31, 2020 by Rusted

[Michele Scotti]

On 31/01/2020 at 18:21, Rusted said:

That tube is not going to offer much support without some help even if you triangulate. Think of it as merely a center guide pin.
What about adding some counterweights on a horizontal extension to balance the angle grinder and jig materials?
You don't need any pressure for "cutting" and fighting the weight will require too much adjustment to take up unknown levels of backlash.
The triangles can be scrap ply. That board isn't going to offer much resistance to twisting on the axis of the angle grinder.
You could add a roller on the tail end. Your track is the only accurate surface you can trust as a register so you might as well use it.
You could add further rollers at the widest part of the jig. Inline skateboard wheels? The jig will have to be widened to suit.

The track currently is of unknown geometrical quality - hence I can't use it as a reference. Any roller used to help supporting will inevitably act as a "copy-carver" to some extent.

The challange here is to be able to use he tube as a reliable axis to start with. That is the reason why I extended the distance between the 2 bearings in order to better cope with the bending moments. Clearly success in not a given here...

However my way to evaluate how far we are from a solid jig is to check how close to the ideal situation i.e. how insensistive to loading is the distance between the track and the tool.  .

I agree that the grinding forces are not going to be massive but I'd like to start with the most solid set-up.

 

In the meantime this is the set-up we came up so far. Added quite few reinforcements and a rudimental 'micrometric' adjustment of the head height....still working on that though

[Rusted]

I agree and questioned my motives [afterwards] for using the ring as its own reference.
So what is your reference and what exactly you are trying to achieve?
Is it your pipe? Is it perpendicular to a fraction of a second of arc?
Is it infinitely rigid? Is it equally rigid in all angles of rotation?

Is the base absolutely rigid when fully loaded? If not, the ring will change form.
It will sag between the feet or be "shaved" over them. Giving you a gentle, roller coaster track.

[Michele Scotti]

2 hours ago, Rusted said:

I agree and questioned my motives [afterwards] for using the ring as its own reference.
So what is your reference and what exactly you are trying to achieve?
Is it your pipe? Is it perpendicular to a fraction of a second of arc?
Is it infinitely rigid? Is it equally rigid in all angles of rotation?

Is the base absolutely rigid when fully loaded? If not, the ring will change form.
It will sag between the feet or be "shaved" over them. Giving you a gentle, roller coaster track.

A lot of good points...
Ref is the shaft and its bearing - the one that will stay on the Az table. I'd like to target a few tenths of microns of total vertical run-out with very smooth transitions - somehow the latter is more importand than the first.

The pipe is a go-kart rear axle with 2mm wall thickness - its rigidity will be scrutinized due course. The idea is that it has to allow the generation of an ideal plane which is the new track surface.

f it is slightly tilted - to a reasonable extent- compared to the table itself is not a big deal. What counts is that the track and the 2 Altitude ground bars are 'parallel' - this can be quite easilty checked and rectified with shimming under the Alitude bearing brackets.

The Az table is fairly rigid - at least by design anf for the purpose- as it's the most critical element for the overall mount performance.

The picture here is for the HUGE investment in a micrometer to test the accuracy of the surface and preliminary the stiffness of the jig.

 

[Rusted]

You can use your micrometer head to confirm the uprightness of your central tube relative to the ring in its present form.
It should read close enough to an average figure over one complete revolution of your jig before you even consider a run.
Any bias in one particular direction will thin, or thicken, your track in that direction, depending on the slope of the tube.
I still think the angle grinder is a fierce tool for this task. It has no finesse. Nor any pretensions to accuracy.
If it digs in, then it will pull the tube over in that direction as a form of positive, mechanical feedback. Bad!

I just wish I could think of a tool which better suits the task. Your jig just isn't up to using milling tools or even end face, router bits.
There is bound to be far too much flexibility for the accuracy you desire. Or any accuracy at all!
I wouldn't be surprised if it resonates like hell just with the motor free running.
There just isn't enough mass or stiffness. What about an ablation laser? Shouldn't be any vibration.

Don't you have a tech college nearby which could treat your project as a class exercise? 
The machine tool tutors might like oddball projects. Makes a nice change from the usual humdrum.
They might have some useful ideas before you break something important.
A vertical, turntable lathe would do nicely if anyone had one. Try a crane manufacturer?
 

[MarcusH]

An impressive project and my deepest respect for the courage to start off (and hopefully finish off) such a project while keeping us all informed along the way. You have gotten many well thought out questions and advices from members and you have equally given many thought out replies and you have taken the advise in when needed. So I'm going to feel a little like the odd-ball here when a ask my silly questions, but please bear with me.😄 

Firstly, now that you have invested in a micrometer, have you measured how much vertical run-out you are dealing with ? Is it millimeters, fractions of millimeters or something less ? Why I ask is to find out just how much machining needs to be done to that track. I'm thinking that you have built a great jig, but not for the angle grinder, rather than for the micrometer. You could measure and mark the high spots on the track and then attack them with something with a bit more finesse. Perhaps even a hand tool sufficiently large and stiff with the proper abrasive, just "sanding" or "polishing" off the high spots and not causing any low spots. Now your only worry would be to keep the micrometer at an fixed plane so not to loose the reference point. Quite an easier task for your jig than an angle grinder spinning a 13000 rpm.....

And secondly, why a tripod to support your azimuth table and not for example a hexapod ? Would that just be form over function with no added benefits ?

 

There, that's my silly questions for today.😊