800mm Telescope Project

[Michele Scotti]

1 hour ago, MarcusH said:

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.....

 

4 hours ago, Rusted said:

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?
 

I try to bundle up answers - in first place there's no such thing as a silly question. Only asnwers can be silly.

Also, I totally share with both of you the fact that the likelyhood that all of this is going to be a big mess is very high - I'm not shooting for first time quality. I'll learn by mistakes along the way.

There's a specific reason though why I'm going down this routet. And it's about the spirit of this project - this is a pilot project and an (ambitious) blueprint for a large, attainable imager that should be potentially built around the world without access to expensive/porfessional/dedicated machinery. A go-kart shaft and a couple of bearings are fairly common and inexpensive. Surely we might fail - then I'll reconsider what to do.

 

Anyway - I took 2 measurements every 15deg - orange and grey lines. I applied a sinusoidal correction to simulate a tilted shaft being straighten-up - in blue.

The result after such correction is the amber line.....which is more than I expected! We are looking at a ca. 1.3mm P-V or run-out. Quite some material to remove....

Worst scenario if (likely) the steel left is too thin I'll bond another set of arches like the first time - it probably costs less than 100Euros

[Rusted]

12 hours ago, MarcusH said:

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.😊

Marcus, I agree and it's not silly.

If it were my project I'd just be using long sanding blocks. With various paper grades getting finer over time as needed.
Or even a diametrical, cross board pivoted on the pipe/bearing with an abrasive paper covered block running around the track.
As you say, use the jig only for measuring.
Double check that every single radial measurement is fully repeatable before even trusting the jig at all.
Using maths to analyze "a length of string" is a common fallacy.

EDIT: Can't understand the strike through text! Can't get rid of it either!

Edited February 4, 2020 by Rusted

[Michele Scotti]

3 hours ago, Rusted said:

Marcus, I agree and it's not silly.

If it were my project I'd just be using long sanding blocks. With various paper grades getting finer over time as needed.
Or even a diametrical, cross board pivoted on the pipe/bearing with an abrasive paper covered block running around the track.
As you say, use the jig only for measuring.
Double check that every single radial measurement is fully repeatable before even trusting the jig at all.
Using maths to analyze "a length of string" is a common fallacy.

EDIT: Can't understand the strike through text! Can't get rid of it either!

I might actually resort to sand blocks, eventually! At least they'll provide smooth transitions although not going to do much for the overall run-out. 

"Using maths to analyze "a length of string" is a common fallacy."          what do you mean with this specifically?

[Michele Scotti]

15 hours ago, MarcusH said:

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 ?

To start by stating the obvious: the tripod has 3 legs and at the end of each leg there's a roller. So this set of 3 points defines a plane - adding any additional supporting point will introduce an unwanted constrain. Chairs have 4 legs and they are 'stable' because they deform under load allowing a distribution of weight on 4 points.

So 3 is actually the only choice for such structure.

Class-meter teelscopes uses a different approach - they usually have 2 very accurate flat rings that are part of a hydrostatic bearing where pressurized oil keeps the 2 rings apart.  

Hope this answers your question?

[Davey-T]

5 hours ago, Rusted said:

EDIT: Can't understand the strike through text! Can't get rid of it either!

You must have accidently hit that S  at the top of here, you should be able to highlight it all then click the S  to get rid of it.

Dave

[Rusted]

1 hour ago, Michele Scotti said:

I might actually resort to sand blocks, eventually! At least they'll provide smooth transitions although not going to do much for the overall run-out. 

"Using maths to analyze "a length of string" is a common fallacy."          what do you mean with this specifically?

Simply, that one cannot make false assumptions to obtain true accuracy.
For example: You cannot add a flexible extension to a micrometer to measure a longer distance.
Though you can use a rigid extension and make relative measurements using a micrometer.
You have absolutely no idea of the true length of the rigid extension to the same accuracy as the micrometer.

Your jig offers almost zero hope of true accuracy but might offer repeatable relative measurements.
Emphasis on repeatable. You are making relative measurements.
Which require close repeatability to have any value to you in this context. 

The question remains: Is your pivot tube perfectly upright and absolutely inflexible?
If not, then you aren't measuring your track at all.
You are only measuring errors in the pivot tube relative to the track.

If you falsely assume that the pivot tube is accurately set and rigidly so,
then you might remove half a meter of track depth for no good reason.

I hope this makes it clear.
 

[Rusted]

1 minute ago, Davey-T said:

You must have accidentally hit that S  at the top of here, you should be able to highlight it all then click the S  to get rid of it.

Dave

Thanks Dave

I tried all the usual ploys to undo it no effect. Nothing I did made any difference to the strike through.

[MarcusH]

4 hours ago, Michele Scotti said:

To start by stating the obvious: the tripod has 3 legs and at the end of each leg there's a roller. So this set of 3 points defines a plane - adding any additional supporting point will introduce an unwanted constrain. Chairs have 4 legs and they are 'stable' because they deform under load allowing a distribution of weight on 4 points.

So 3 is actually the only choice for such structure.

Class-meter teelscopes uses a different approach - they usually have 2 very accurate flat rings that are part of a hydrostatic bearing where pressurized oil keeps the 2 rings apart.  

Hope this answers your question?

Well, sort of. I can easily understand that a tripod is far superior when it comes to position something on an uneven surface. It's very easy to make the tripod to conform to that surface and that's why it's the go-to solution for mounting your camera, telescope mount etc. But what happens when you start rotating the tripod around it's Z-axis. As the "legs" are spread out 120 degrees there's nothing straight opposite (180 degrees) to each leg to counteract a force acting on it. E.g. when you grab one leg of a tripod it's pretty easy to lift it and tilt the whole tripod, the two other legs just create a nice pivot point. So the three rollers on your tripod wouldn't have any problem to "follow" the high and low spots on your track, tipping and tilting your telescope at the same time, making it paramount to get the track as even and smooth as possible.

What I was thinking (possibly erroneously) that going from a tripod construction to a hexapod (6 -legged or double tripods) you would create a resistance to this tilt with every roller having an opposite counterpart. There would not be any nice pivot point there anymore. Now any single roller hitting a high spot would have to lift the whole construction resting on top of it (supposing the hexapod would be stiff enough), more likely something would "give" resulting in flexure at that point either in the track or the tripod "leg". The same would apply for a low spot, the two adjacent rollers would take the load causing the roller hitting the low spot to coast over it. In a sense the hexapod would not eliminate the unevenness of the track, but would "filter" the high and low spots relaxing the requirements for the surface evenness a bit. 

This subject is clearly way over my head......😅

[Rusted]

The weakest factor of any tripod is the radius out to the tilt line [hinge] between any two feet.
Which is why tripods are so unstable relative to any increased number of points of contact.
A four legged [quadruped] is far more stable and the reason there are no tripodal animals.
A tripodal mountain goat is basically a non-starter in Darwinian terms.

The Martians wouldn't stand a chance in a real War of The Worlds.
It would only have taken a car and a brave chap with a rope to pull one over.

For the same reason a tricycle is hideously unstable compared to a quadricycle of the same track and wheelbase.
And a three legged stool, only has value on a rudimentary, brick or paving slab floor.

Every additional leg provides greater stability because it better equates to the full radius of a circle.
Which is why executive, swivel chairs [and my own computer chair] have five feet or castors. 

 

Edited February 4, 2020 by Rusted
New image

[Michele Scotti]

22 hours ago, Rusted said:

The weakest factor of any tripod is the radius out to the tilt line [hinge] between any two feet.
Which is why tripods are so unstable relative to any increased number of points of contact.
A four legged [quadruped] is far more stable and the reason there are no tripodal animals.
A tripodal mountain goat is basically a non-starter in Darwinian terms.

The Martians wouldn't stand a chance in a real War of The Worlds.
It would only have taken a car and a brave chap with a rope to pull one over.

For the same reason a tricycle is hideously unstable compared to a quadricycle of the same track and wheelbase.
And a three legged stool, only has value on a rudimentary, brick or paving slab floor.

Every additional leg provides greater stability because it better equates to the full radius of a circle.
Which is why executive, swivel chairs [and my own computer chair] have five feet or castors. 

I trust you realized that all the examples you brought are 'dynamic' at a certain extent - even your swivel chair.

It totally makes sense to have more 'legs' to achieve a more stable set-up simply  by increasing the area on which the CoG can fall on.

One of the perks of a AltAz mount is that all loads are pretty well centered. The CoG of the 'OTA' rotates around on a 124mm circle - well within the 'triangle' set by the 3 rollers. I do not foresee any stability issue.

To me one of the design aspect that I tried to optimized is related to the load transfer from the Alt bars thru the Az table then Az track to the rollers on the tripod and from there to the feet on the ground - which incidentally I haven't designed yet so they are not shown in any CAD so far.

As a matter of fact the FEA we ran was on the worst case where one of the Alt bearing is placed half-way over 2 bearings

Edited February 5, 2020 by Michele Scotti

[Michele Scotti]

The quest for a stiff and reliable platform

I've used some counterweights at the end of the board to evaluate the stiffness of the system. Not a good starting point... 0.9mm with something like 6Kg placed some 30cm outboard, just to exacerbate the flexure.

I've then tighten all screws and added M6 bolts with inserts to cement the position of the wooden blocks. Better but still 0.6/0.7mm.

Where's the flexing coming from?? Maybe from the element assumed to be the most rigid? Let's check the pipe...

A cheap laser was clamped on the top of the pipe -see pic- pointing down. Some paper tape on the bottom to act as a 'screen' for the phone to record just under it.

And there you go – the pipe flexes big time!

Movement -subtle-is caused by pushing on the board and releaseing - laser dot moves 1 or 2mm. Smoking gun!

So the pipe that fits perfectly in the bearings and we made all the wooden blocks around is not up to the task. Shall we throw it away? Nah!

The pipe needs stiffening on the vertical plane where it meets the angle grinder – so next step is to insert a 5mm flat bar that is as wide as the ID of the pipe. Possibly a few welding points at the ends. For a structural application this would be gruesome borderline criminal – in this case it really needs to work under small stress.

[Rusted]

If the pipe is disposable you could fill it with something rigid like concrete. 

But is the pipe anchored immovably at the bottom? This is where flexure is most likely.

[Rusted]

On 05/02/2020 at 13:26, Michele Scotti said:

One of the perks of a AltAz mount is that all loads are pretty well centered. The CoG of the 'OTA' rotates around on a 124mm circle - well within the 'triangle' set by the 3 rollers. I do not foresee any stability issue.

To me one of the design aspect that I tried to optimized is related to the load transfer from the Alt bars thru the Az table then Az track to the rollers on the tripod and from there to the feet on the ground - which incidentally I haven't designed yet so they are not shown in any CAD so far.

As a matter of fact the FEA we ran was on the worst case where one of the Alt bearing is placed half-way over 2 bearings.

The moment you shove your altazimuth around manually you may apply an overturning force well beyond the CofG's theoretical comfort zone.
If the static : dynamic friction ratio goes sideways you may well have a distinct "timber!" scenario.
Particularly when pointing into the velocity "black hole" up around the zenith.
The azimuth cannot rotate so all the manual effort goes directly into tilting instead.
All this, just as you have achieved the highest C of G of the whole instrument.

Edited February 7, 2020 by Rusted

[MarcusH]

57 minutes ago, Rusted said:

But is the pipe anchored immovably at the bottom? This is where flexure is most likely.

Perhaps the test could be reversed to find out. I.e. clamping the laser to the bottom of the tube and apply pressure to the jig. If the bottom is immovable the laser dot shouldn't move. If it moves any added rigidity to the pipe itself would be a moot point.

[Michele Scotti]

2 hours ago, MarcusH said:

Perhaps the test could be reversed to find out. I.e. clamping the laser to the bottom of the tube and apply pressure to the jig. If the bottom is immovable the laser dot shouldn't move. If it moves any added rigidity to the pipe itself would be a moot point.

I've check most of jig elements -and specifically the upper and lower bearings- is in a pretty funny and maybe novel way.
I pointed the smatphone at the area where movements would manifest and shoot a video while applying load. 
As funny as it sounds you can detect way finer movment than naked eye.
I was trying to put some maths behind it...maybe later.- you can see sub 0.1mm for sure. I have few videos where the check is OK but can't find the one that was clearly showing a minimal movement in a different area of the jig that was later fixed.

Bottomline - to my surprise- bearings are ok.

Anyway if they moved the laser pointer wouldn't move as the pipe would not bend but rather tilt.

[MarcusH]

18 hours ago, Michele Scotti said:

I was trying to put some maths behind it...maybe later.- you can see sub 0.1mm for sure.

No rush, but please do, or a video. Seeing sub 100u movements sounds intriguing...😮

[Michele Scotti]

On 08/02/2020 at 10:10, MarcusH said:

No rush, but please do, or a video. Seeing sub 100u movements sounds intriguing...😮

Rather than a video let's see some maths behind this - it applies to any modern smartphone with Mpixels.

At the minimum focus distance - usually 50mm or something- this is a clear picture of a caliper.

A crop-up shows that 5mm takes 77pixels. This equals to 65um/pxl

By the way in a video you wold appreciate sub pixel movement for sure. 

Out of curiosity this is what happens using the zoom (digital in this case)

Reoslution here is 14um/pxl

 

Not maybe the most accurate tool but it detects tiny movements very efficiently - it might be handy when checking mounts for slack

[Michele Scotti]

New CAD model update, now version #13 with few improvements - mainly:

- model aligned with actual built parts
- new lower part sub-assembly
 

Next step is another run of FEA for modal analysis and displacement.

After that we are going to modify the structure from the original f/3.75 to the currently assumed f/3.3. Consequently the focal will be reduced from 3000mm to 2640mm.

 

 

[R26 oldtimer]

Hi Michele.

You've mentioned that you intend to make this an imaging scope rather than visual. If that's the case, then wouldn't be easier to skip the secondary mirror and place the camera directly to the upper cage assembly?

This would give you far less weight, up there, so the cage could be designed to be substantially lighter, and a round body camera will give you a smaller secondary obstruction. My guess is that it would also be easier to colimate and the only downside would be that probably you could not use a filter wheel.

[Rusted]

The OTA would also need to be considerably longer.
Not a trivial matter with an 800mm aperture.

[R26 oldtimer]

Yes, I guess the poles will have to be 50-60 cm longer, but I don't think this would be any problem?

[Rusted]

42 minutes ago, R26 oldtimer said:

Yes, I guess the poles will have to be 50-60 cm longer, but I don't think this would be any problem?

Probably not, but an on-axis camera needs quite a fat cable to run it. Major diffraction spikes! 
A tensioned wire spider to carry the signal and power via insulated strands might just be possible? 

[Michele Scotti]

17 hours ago, R26 oldtimer said:

Hi Michele.

You've mentioned that you intend to make this an imaging scope rather than visual. If that's the case, then wouldn't be easier to skip the secondary mirror and place the camera directly to the upper cage assembly?

This would give you far less weight, up there, so the cage could be designed to be substantially lighter, and a round body camera will give you a smaller secondary obstruction. My guess is that it would also be easier to colimate and the only downside would be that probably you could not use a filter wheel.

Kalimera! We considered this in the early stage of the project and that's one of the reason why the spider has a big cilindrical section to let the cone of light go thru it. 

However we decided to drop that - an additional add-on section and the cabling can be sorted out - no big deal.

The showstopper is the focusing and de-rotator mechanism - they are part of the project in the standard configuration but in that position they'll add significant complexity.

As a final note -  we would really want to experience the magic of 'guessing' the colour of some DSOs - feature that only this kind of diamater can untap.

[Geoff Barnes]

Hi @Michele Scotti 

It's been a long time since you've posted, any new developments with your amazing project?

Regards, Geoff.

[Michele Scotti]

Hi Geoff thanks for checking out.- as a matter of fact the astro-club and the assembly is taking place in Cremona which was the epicenter in Italy.

I'm actually based in the UK whereas all the guys supporting the project are doing well although the activity is halted as the lockdown is going to be partially lifted from today.

To make the most out of this situation we are having call confs to decide the strategy over the making of the mirror but we can't really progress on the assembly.

Cheers and thanks for following,,

Michele