Gregorian telescope 10" f/24 - MDL

[Chriske]

My pal Marc(MDL) is busy making himself a Gregorian. Busy drawing that thing.
It'll be 250mm(10") f/24. He almost finished the optics, still only the secondary to polish.
We discussed how to proceed and decided to use as many printed parts as possible.
Ok we know, a FL that long requires a very sturdy construction. To begin with all is 'attached' to 3 aluminium poles. These poles are not 'in line' but interconnected in a shallow angled, so doing the construction should be very stiff.

This is how V1 looks like :

Maybe, just maybe there will be a very thin connecting rod(3mm or so) to support/connect that secondary ''far point" with that central pole. But before we decide to do that we need to find out what the result will be in the diffraction pattern. That circular shaped spider btw is chosen because it 'nulls out' diffraction in the image compared to a regular spider with 3 or 4 arms. Well, in fact the diffraction of that circular shaped spider is not completely gone of course. That circular shaped spider produces a residual pattern in the image, but that pattern is no longer visible because it is evenly redistributed all over the field of view, so in fact 'invisible'.

This is how the spot of that Gregorian looks like in Winspot. Rather good looking spot, In fact far better result compared to a  Dahll-Kirkham or a Newt. The black circle is the Airydisk. Field of view is 0.5°. It looks like the edge rays are not good at all, but in fact the end result is very good. The size of these patterns (at the edge) can be twice the size of the Airydisk without any problem.

[Gina]

Very interesting.  It looks like the secondary end connection to the bars is within the view of the main mirror but I presume this is an optical illusion.  The primary printed support looks pretty big - will this fit on a 300mm x 300mm print bed or does it need bigger?

[Chriske]

Oops missed this one, sorry Gina...🤨

All parts do fit on a 200x300mm bed. Except for the primary mirror'ring'. That ring will be printed in two parts and bolted together.
holes for these bolts not present in the drawing yet.

 

200x300 bed

[Chriske]

Meanwhile all parts of this one are printed ( all done with a 0.7mm nozzle)
Busy assembling the scope, together with my dear pal Marc. (he's very ill)

The part at the left are actually four separate parts already bolted together.
There was no way I could print this in one piece. But using a handful of M6 bolts this primary cage is very stiff and very strong. There's no way you can call these parts 'slimline', but that was the initial idea.
Lower centre part is the actual mirror-holder and also the primary baffle will be mounted onto this part. Both primary mirror and light-baffle can be adjusted separately.
Upper centre is the part that connects to the mount.
And right is the secondary 'cage' and baffle.
And the last picture is showing the slit that holds the thin shield around the primary mirror(if necessary)

 

 

[andrew s]

@Chriske did you do a 3D  stress analysis of the structure? Alignment is critical in this type of telescope. 

Regards Andrew 

[Chriske]

I know the alignment is very critical  in these scopes. But again I did not print these parts slim-line. That's why.
Although I could do a stress analysis in Inventor I did not preform it because there's no need to. In the past I made a few of these scopes, even single pole-scopes. Never I had any problem with them.
If you could only hold these printed parts in your hand you would not doubt for a moment these parts will ever deform.
As a matter of fact the COG is very close to the primary cage. Next there are three poles in a shallow triangular configuration going upward to the secondary cage that is not to heavy. I'm not worried at all.
Thanks for mentioning this anyway Andrew..😉

[Gina]

Nice job - as always

[Gina]

Hmm...  Looking at the extreme strength you get with your printing Chris I'm thinking I was misled by someone thinking a 3D printed pulley would not be strong enough for a pulley in the chain drive for my roll-off-roof, so I've been trying to turn one on a lathe that is too small for the job!

[Chris]

Remarkable project, looking forward to seeing the end result. 

[Marvin Jenkins]

Stumbled into a whole new world. Eyes as big as saucers. What an amazing project, I will be following this every day. Well done to the both of you and hope   Marc’s health is quickly resolved. I would love to hear about first light with this amazing creation.

Please keep us all informed

Marvin

[Chriske]

Thanks guys for the kind words.

Borrowed an adjustable reamer from a friend to clean up all the holes for these poles.

[tonyowens_uk]

Hi Chris and my compliments on your unorthodox monorail Gregorian. This style of 'monorail' tubeless scope reminds me of the late Horace Dall's planetary Dall-Kirkham.

I had a crack at designing a 14" F4 planetary prime focus Newt a couple of years ago. At the prime focus I could interchange various coma correctors, cameras and filters. I used ITEM alloy extrusions with screwed and epoxy-bonded joints and waterjet-cut pieces of 4mm and 10mm alloy plate to construct a goniometric primary mirror mount and various reinforcements to get the gravitational sag under control.

For a Newt the max tolerable decentration of the 'true' axis of the primary mirror from the eyepiece or camera to maintain no more than a 0.20 drop in Strehl is given approx by DeC = 0.005 . FR^3 [mm]. For my optic I therefore needed to assure no more than around 0.32 mm decentration of the primary from the camera, for all possible poses of the OTA and for the heaviest imaging train payload at the prime focus. This number is somewhat simplistic and debatable and little more than a guideline for acceptable structural sags, but as a target it was fine. In designing a structure I actually worked to around 25% of this target and aimed for a lowest natural frequency of around 20Hz. The thinking here was that the EQ8 mount I was planning to mount this on is marginally capable of supporting a 20Kg payload/counterweight combination with a natural frequency of around this. The natural frequency not just of the mount but of the entire scope/camera system needs to be as high as possible to allow a good percentage of quality frames when lucky imaging Moon and Jupiter at 120 Hz especially with some wind.

Here is what I ended up with:

I used a CFRP flat plate secondary single-stalk design which is magnetically fixed to the OTA using a kinematic mount for interchangeability. The mirror is contained in a lightweight shroud.

 

The structural deflections due to gravity (linear and tilt displacements) of the OTA at zenith pointing looked good.
This was Horizon Pointing showing less than 100um of linear sag of the prime focus group:

This was Zenith Pointing. Note how tilts of Primary and primae focus group matched:

This was another Horizon Pointing scenario with the OTA rolled through 90 degrees w.r.t. the gravitation direction. It proved to be the worst case but still the linear sags fell not too far outside my target:

 

 

This pic shows the lowest natural frequency of the OTA assuming a rigid circular saddle support of diameter 160mm is used. I call this 'Nodding Duck'. The frequency is 34 Hz which is perfectly acceptable:

 

This is the second lowest vibration mode, at 38 Hz. I call this 'Secondary Stalk Waggle':

 

Finally, this is the third lowest vibration mode which I named 'Primary Waggle'. It appears at 42 Hz so is not a concern:

 

 

My experience was that structural FEA was of tremendous help in arriving at something that almost met my stiffness requriements under all possible poses and imaging payload configurations. It never occurred to me to 3D print structural parts in something like PET-G using lightweight cellular infill at that time because to be honest I did not take FDM printing seriously. I since revised that silly opinion and am now gradually learning the practice of large 3D printing with a large printer. I am very encouraged to see your project and hope your practical findings about collimation stability show my own analysis to be too conservative!

best

 

Tony Owens

Edited August 16, 2019 by tonyowens_uk

[Chriske]

Hey Tony,

Thanks for the info.
To be honest when we first started working and investigating the possibilities of 3D printers we were not planning to do the things I do right now.
About 6 years ago when we made our first we were eager to find out what we could do with these 'toys'(as my my wife used to call it). Nowadays my wife don't call it no more 'toys for boys' as she very often ask me if I could repair or even make items to be used in combination with her own hobby(s).
Anyway, I'm always pushing things, see how far I can go(story of my life...). As a mechanical engineer I do have a good idea how things should be built strong and most of all very stiff.
But in this case, working with printers, (making plastic parts) it is a completely other story. I have to 'reinvent' again all the techniques I am familiar with. I do design all projects myself. One of he first things I had to learn was how to make printed parts as strong as aluparts. First rule : NO slim-line parts ever..! Second was the major advantage compared to classical (mechanically) made parts : all parts that has been drawn can be printed. Even the most complex parts are printable...! Not feasible, using the most sophisticated CNC machines. I do have discussions about this with other CNC(mill) users, until I show them a few parts I printed, asking if they’re capable of making that in aluminium. Not..!
But..! I always combine printed parts with metal or alu parts. There's no way I print an entire scope just using PLA(or whatever). I carefully combine metalwork and printed parts. In fact the major structure as in this project, is 3 alupoles mounted in a shallow triangle. These poles are the basis of the scope. When I print a few parts for holding these poles tightly in place, I can do whatever I want to finish it of with printed parts to make a scope out of it.

The primary cage of this Greg. has 6 separate parts bolted together with M6 bolts. Additionally : inside the mirror-cell are a few long 8mm rods(not visible) to make things very stiff. I can assure you Tony there's no way that thing will sag or bend.

Yesterday I reprinted the secondary cage because : not stiff enough at all. So this is the one I'll be using now.
On the left the old and on the right the new secondary cage. The new one has the same wall-thickness and is very stiff. (I still need to remove a few burrs and some stringing, I know...)
To be clear on the matter. This new circular 'spider' is very high. I can use this high version because this scope has a very narrow TF(focal length 6(!)meter). If it were a short(er) focus 'light-bucket' I would not go for this solution of course. I'd probably go for a completely other setup to build that secondary cage.

[tonyowens_uk]

I think your design and design approach are fantastic Chris!
Like you I had to change my whole thinking about pathways to structural stiffness as a result of commissioning 'cheap plastic' prototypes in SLS or FDM from service bureaus. The apparent stiffness and damping with the parts perplexed me until I discovered the routine use of low density infill to cut material use. Hey presto, cellular lightweight plastic 'castings', with resonant frequencies comparable to homogenous metal parts!. My emerging practice now is to use like you a mixture of materials and processes in which FDM and SLS fit. Where ultra-stable 'mechanical grounding' of parts in the face of temperature humudty and load variations is required (as in some optics and some grinding operations) I specify waterjetted granite or moulded polymer concrete. For thermal stability but lower stability requirements I use steel or Permali Wood. For general structures and parts without any special strength or stability aluminium in all its forms was what I used.

There has been a developing problem over the last 25 years with the gradual disappearance of jobbing machine shops in Anglo countries, presumably linked to the financialisation of their economies and a wholesale shift to Asian sourcing for consumer goods. This has made both my professional and 'hobby' work more difficult. I have responded by investing in basic machining and printing capability. I'm seeing a gradual substitution of SLS and FDM printed parts, with critical surfaces machined if necessary. I also source gravity and pressure die casting parts from India and mouldings from many sources including Poland and Spain, where previously I would have turned to UK and German firms.

The main reason I posted details of my 14" planetary Newt concept (I havent had time to start building one!) was to offer you a quantitative datapoint about gravitational sags and frequencies in a similar truss-type telescope structure for high resolution use. My concept used machined and extruded aluminium for most of the structure. I am not judging your structural concept about which I know little. FWIW given the cellular parts and super-light weight at the secondary end, I would be surprised if you were troubled by structural issues.

There are some unique advantages that apply to 3D printed parts for telescopes too. One of those, is high levels of insulation. For things that are within or close to the optical beamline, where metal parts like tubes and spiders if made of metal tend to sub-cool during clear nights, there is far less of an issue if cellular dimensionally-stable resins are used. Another is relative freedom from the need for coatings and finishes. Finally there is the ease of incorpating things like brackets, reinforcements, pneumatics and wiring. As you say, our profession is having to reinvent again all the established design techniques, in a world where the West now struggles to manufacture things profitably and where additive technology is finally becoming reliable and good enough for first-class products. 

I'd be very interested in your progress on the Gregorian once the secondary is polished!
Good luck with your endeavours!

Tony Owens

 

[Chriske]

Thanks for the kind words,

And one more thing Tony, I forgot to mention here above(mentioned it somewhere else before if I'm not mistake).
I never, ever, use dark colours printing telescope-parts if that scope (or other project in the garden) are exposed to the sun.
Did some thorough tests with printed parts, in different colours, in the past. White filament (even PLA(!)) is the absolute winner whenever these printed parts are exposed to the sun for months in a row. Needles to say black is the big loser here. It deforms and sags enormously in the sun.

The inner surfaces of all my scopes and parts are painted black of course.

[Chriske]

Almost done this scope, optics not finished yet(working on it). Primary needs to be parabolized and secondary needs to be polished altogether.

Sorry about the 'mount', it is rather unstable I know, but it will be replaced later on...🤭

[Chriske]

The mount is called Marc-I btw...😁

[Rusted]

21 hours ago, Chriske said:

Almost done this scope, optics not finished yet(working on it). Primary needs to be parabolized and secondary needs to be polished altogether.

Sorry about the 'mount', it is rather unstable I know, but it will be replaced later on...🤭

My wife suggested you were about to play "Stairway To Heaven" on that thing.

I tried 4" wide builder's straight edge profiles for a beam OTA for my 10" f/8 mirror.
It was fine "up and down" for an altaz but had awful torsion problems on an equatorial. 
The offset mass of the mirror twisted the whole OTA so that collimation became impossible.
I hope your straight tubes can resist such torsion.

 

[Chriske]

The COG is VERY low in this case. The sec cage is very light + the sec. mirror has almost no weight. It's only 10mm thick.
In case of torsion, as you mentioned, I do have plan 'B'.

Thanks for the warning.

Chris

[Chriske]

The man in the picture is my pal Marc btw...😉

[Rusted]

They always want to be lead guitar. Never just rhythm.

The mass offset of the secondary arrangements are absolutely tiny compared with the massive primary.
The short distance between the cradle and the mirror cell fixing allowed the OTA to twist on mine.

 

[Chriske]

Lead guitar or not, what would we do without them...😊

[Chriske]

Not stiff enough, so plan 'B' is activated.

These part will be added.

[andrew s]

How did you decide the lack of stiffness was in the secondary rather than the primary support given there relative moments?

Regards Andrew 

[Chriske]

I always use the same procedure to find out if a scope does sag (or not) under it's own weight.
Installing a laserpointer at the very end of the scope, make a marking where the dot is(at the other side of the scope) and start rotating the scope in every possible direction, while observing what happens with the green/red dot. If the dot does move away from it's initial point while rotating, then you do have a problem.
Embedded in Inventor is a 'Strain detector', but I hardly use it.

Making a scope using tubes or trusses (correctly installed) there's no problem most of the time. But in a cases like this one there are many possible things that could go wrong.
It is not the first time I make a scope this way. I anticipated possible failure, and so it did. Because I wanted this scope to make a bit more elegant I used smaller tubes(22mm) and positioned them in a shallow angle. To correct this I added a extra tube, but this time at a 90° angle. It's less elegant but this version will not sag at all, I'm sure.