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800mm f/3.3 Telescope Project

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#1 skround

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Posted 27 August 2019 - 12:35 PM

Hi everybody,

i'd like to share with you the design, making and progress of a project my astro club endeavoring to. I'll post here relevant info in a run-up  to where we got to so far.

 

Here is the mission: 800mm in diameter. That's it. A lot of inspiration out there but it needs to be able to do science. So it's a relatively fast Newtonian capable of tracking for tens of minutes.
So we realized that we were facing 2 projects in one. The mirror on one side and the mount on the other. Shall we start with the mount? What's the best compromise in terms of ease to build and cheap components and the chance to have a stiff yet light structure. An alt-az, like the biggest telescope! - or rather a glorified dobsonian in this case.

 

Ambitious? Definitely

 

Any comment/suggestion is welcome!

 

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#2 junomike

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Posted 27 August 2019 - 12:54 PM

See this thread.



#3 photomagica

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Posted 27 August 2019 - 01:01 PM

Lowell Observatory just installed a 32" StarStructure, which is now the largest telescope on Mars Hill. I was part of the team that selected this. It has just had first light so it is too early to evaluate it as a science instrument but it would seem to meet your criteria of tracking for tens on minutes. You may want to use this as inspiration for your project.

 

Consider going to an f2.8 mirror or faster - as fast as your field flatner (assuming your need one) and instrumentation will tolerate. This will make your mount more compact and easier to engineer.

 

Likely you should fabricate the mirror prior to finalizing the mount, as the exact mirror diameter, thickness and focal length will be needed to make final mount dimension determinations. Also consider the mass and dimensions of your science instrumentation and guide telescope or tracking camera pick-off if needed.

 

Or you could just buy time on the 32" on Mars Hillsmile.gif

Bill


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#4 ButterFly

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Posted 27 August 2019 - 01:28 PM

I presume "do science" means imaging?  If that is the case, a Cassegrain (or at least Nasmyth) is much easier to implement than a Newtonian design - the camera moves much less.  With tracking for tens of minutes, it is also much easier to track on one axis if possible.  This class of instrument lends itself to an open fork on a wedge or a horseshoe mount for slightly larger mirrors.  Serrurier truss designs help with flexure.  A regular un-wedged alt az will benefit from a field de-rotator and needs to stay away from zenith.  Counterweights should be avoided at all costs.

 

If it is just for visual, a dob is fine.

 

The "do science" needs to be clarified before any plans really begin.  The mirror and the mount must be designed together for the intended class of targets.


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#5 havasman

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Posted 27 August 2019 - 01:30 PM

The drawn scope in the original post seems to have opportunities for flexure in its minimalist design that might best be overcome with exotic materials, violating the cheap materials requirement. Cheap, stiff and light as design concepts might present another of those "pick two as you can't have all three at once" situations.

 

The Dob, in relatively traditional form, has stood well for some time as a flagship for the functional aperture at relatively low cost. Getting one to track for many minutes or even hours at a time is simple via standard off the shelf gear. Why not build or buy a Dob?


Edited by havasman, 27 August 2019 - 01:33 PM.


#6 TOMDEY

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Posted 27 August 2019 - 01:58 PM

Well, that's challenging. I see that your cartoon shows a standard truss dob configuration. I'd recommend, in that size and the tracking/imaging restriction... to seriously consider a permanent installation. Because portable means every single time you use it... well, you know the drill (or not?) My scope is 900mm F/3.75 and tracks Alt/Az... allowing short sub-exposures and EAA imagery. These big Newts are not for the faint of heart.... tiger by the tail territory.    Tom

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#7 skround

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Posted 27 August 2019 - 02:34 PM

Thanks - I'll take time to read it. Looks interesting



#8 ButterFly

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Posted 27 August 2019 - 03:01 PM

Well, that's challenging. I see that your cartoon shows a standard truss dob configuration. I'd recommend, in that size and the tracking/imaging restriction... to seriously consider a permanent installation. Because portable means every single time you use it... well, you know the drill (or not?) My scope is 900mm F/3.75 and tracks Alt/Az... allowing short sub-exposures and EAA imagery. These big Newts are not for the faint of heart.... tiger by the tail territory.    Tom

Ah!  So that's how you track so accurately: there's a little mammal in there this tall ...



#9 skround

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Posted 27 August 2019 - 05:53 PM

Lowell Observatory just installed a 32" StarStructure, which is now the largest telescope on Mars Hill. I was part of the team that selected this. It has just had first light so it is too early to evaluate it as a science instrument but it would seem to meet your criteria of tracking for tens on minutes. You may want to use this as inspiration for your project.

 

Consider going to an f2.8 mirror or faster - as fast as your field flatner (assuming your need one) and instrumentation will tolerate. This will make your mount more compact and easier to engineer.

 

Likely you should fabricate the mirror prior to finalizing the mount, as the exact mirror diameter, thickness and focal length will be needed to make final mount dimension determinations. Also consider the mass and dimensions of your science instrumentation and guide telescope or tracking camera pick-off if needed.

 

Or you could just buy time on the 32" on Mars Hillsmile.gif

Bill

That's an interesting one Bill - looking forward to seeing the images it can take. Does it come with a field de-rotator? In visual they say you can see the colours of nebulae and galaxies....

 

The original project was actually f/3.75 but we shortened it to 3.3. I agree the mount would perform better and that we need a Wynne! 



#10 skround

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Posted 27 August 2019 - 05:57 PM

I presume "do science" means imaging?  If that is the case, a Cassegrain (or at least Nasmyth) is much easier to implement than a Newtonian design - the camera moves much less.  With tracking for tens of minutes, it is also much easier to track on one axis if possible.  This class of instrument lends itself to an open fork on a wedge or a horseshoe mount for slightly larger mirrors.  Serrurier truss designs help with flexure.  A regular un-wedged alt az will benefit from a field de-rotator and needs to stay away from zenith.  Counterweights should be avoided at all costs.

 

If it is just for visual, a dob is fine.

 

The "do science" needs to be clarified before any plans really begin.  The mirror and the mount must be designed together for the intended class of targets.

With "science" I mean the ability to make minutes-long integration with imaging systems.

 

I agree - a de-rotator is part of the project. Kind of a project by itself to be honest...



#11 skround

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Posted 27 August 2019 - 06:03 PM

Well, that's challenging. I see that your cartoon shows a standard truss dob configuration. I'd recommend, in that size and the tracking/imaging restriction... to seriously consider a permanent installation. Because portable means every single time you use it... well, you know the drill (or not?) My scope is 900mm F/3.75 and tracks Alt/Az... allowing short sub-exposures and EAA imagery. These big Newts are not for the faint of heart.... tiger by the tail territory.    Tom

That sir is a heck of a telescope. You must enjoy it a lot! 



#12 skround

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Posted 27 August 2019 - 06:35 PM

First of all thanks everybody for your warm welcome! Lots of hints and suggestion. I actually thought to be roasted!

 

Anyway let me give you a bit more of background on this project.

 

 

Over the years Charly -on the left-  had achieved a mastery in working mirrors such as three 500mm objectives. One of these mirrors is dedicated to his personal telescope.
The dream in the drawer was always to venture on higher diameters: 700mm, 800mm even 1 meter.
Perhaps the very structure of the telescope beyond is the burden. In fact, his 500mm equatorial mount had required various improvements over the years and the feeling is that there's a lot of unexplored potential in that mirror.

 

I hear many of you suggesting to sort the mirror out in first place. And that's a valid point. However the accent on this project is more on the mount that needs to be capable of tracking for several minutes.

Don't get me wrong - a 800mm mirror is a huge challenge and we are not underestimating that.

 

Cheers,

Michele

 

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#13 photomagica

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Posted 27 August 2019 - 07:10 PM

As far as I know the 32" StarStructure at Lowell will not have a field derotator, at least not initially. Over the next year a 24" PlaneWave will be installed that will have a derotator, and I expect this will get more use for heavy-duty science imaging. This 24" will be a short distance from the famous 24" Clark Telescope in a small dome that now houses a 16" Boller and Chivens.

 

If you are planning a primarily imaging telescope, consider making it not a Newtonian but a prime focus instrument, with the camera, field flatner and possibly the derotator where the Newtonian diagonal would be.

Bill


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#14 ButterFly

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Posted 27 August 2019 - 09:45 PM

With "science" I mean the ability to make minutes-long integration with imaging systems.

 

I agree - a de-rotator is part of the project. Kind of a project by itself to be honest...

Keep the chip size small at first to kick the derotator down the line a bit.  You can software test the two star guiding and derotating along the way.  Instability will creep in well before the one minute mark and is the biggest priority.  After that it's just guidance.

 

The observatories of the last century dealing with this class went to EQ designs mostly becuase it was hard to track in two directions and rotate cameras.  Even modest computers are more than up to the challenge now.  Keeping the camera close to the center of gravity is still a good idea to eliminate the potential for huge vibrations there.

 

Have a lot of fun along the way and post lots of pictures.


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#15 skround

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Posted 28 August 2019 - 04:58 AM

Have a lot of fun along the way and post lots of pictures.

That's exactly the plan! ;)



#16 skround

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Posted 28 August 2019 - 06:35 AM

At this point, I reckon I feel the need to clarify the vision.

This project is aimed at science not just visual - all inspirational big apertures out there are mostly visual. The French T1000 is actually achieving some sub-minute integration as far as I know: http://www.astrosurf...ltaz/T1000.html

 

It needs to fill the gap between amateur and pro. Up to 500-600mm there are choices commercially available - then you jump almost directly into the meters-class from professional observatories.

 

Success looks like other people will build a better version of this scope. Hence this is an open-source exercise where this specific project is just the execution of a sort of pilot project. Its realization will be just a reassurance of its capabilities.

 

Affordable - being open source is not enough. The project is intended to utilize modern material in a smart way with a limitation to the use of fancy materials - rather we will develop new ad-hoc processes to extract the most from common materials.

 

Being transportable is a plus but in all fairness  not a must. As long as it can be put in a small van that's good enough before it finds its final home.

 

Again very ambitious but that's what motivates us. If we fail well we won't regret to have tried at least! It'll be painful though!

 

Attached a preliminary strain energy analysis:

 

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#17 skround

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Posted 28 August 2019 - 06:26 PM

Having set targets and goals what follows is drafting the type of mount. I suppose this part will be pretty arbitrary...
Few types were casually taken into consideration with their pros and cons. Equatorial as GEM is ruled out as it would have to be huge and heavy - mainly driven by the overhang, big bearing, big shafts likely in steel to contain costs, last but not least the drive would require large worm gears. Too heavy and expensive.
So we shift our thoughts to Alt-az. Weight load is inherently more balanced leaving room to more humble materials. Only drawback that I can really see? Field rotation. Will come back to the de-rotator at a later post.

Drafting the scope we ended up in something I'd call a glorified Dobsonian. A Dob in its essence with few upgrades to turn it into a mount that can track for imaging. Big appeal is represented by the friction drive which perfectly fit this configuration.
In hindsight, the horseshoe set-up earns a special mention - it's very palatable as is equatorial and it would adopt very similar material/manufacturing processes as the glorified Dob.

How to turn a Dobson-like mount into a proper imager? What makes a mount an accurate tracking system? I reckon this boils down to 2 main elements - Low backlash and sound structure.
How to achieve that? friction drive, stiff yet light structure, autoguiding capability.

 

Quick math check to start with.
A perfect structure with 1micron (0.00004") error on the friction rollers equals a drift on 0.23arcseconds on the focal plane. Now, if you consider a sensor with a generic 10micron pixel, every pixel equals to 0.79arcseconds (at least for this mount)
Reality check: a commercially available ground bar or shaft has a run-out of 13 microns (ISO h6) - just to consider a relevant geometric error.
If you are following me in this generic and very simplified calculation, long focal length optical systems cannot perform accurate tracking based solely on geometrical accuracy. It's just not robust enough.

Hence we need autoguiding...how does a mount look like to achieve an effective autoguiding? Again low backlash via adopting friction drive paired to a stiff and light structure that promptly reacts to tracking adjustments. As you can see it's a whole package of traits that needs to be developed and implemented holistically.
If one aspect is missed or poorly executed the entire system is not capable.

 

Pics of parts I had a head start with:

  • 2 driven azimuth roller assemblies . The third one will be the driving one.
  • The wood preparation for the upper cage.

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#18 Mike Lockwood

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Posted 28 August 2019 - 08:50 PM

As far as I know the 32" StarStructure at Lowell will not have a field derotator, at least not initially. Over the next year a 24" PlaneWave will be installed that will have a derotator, and I expect this will get more use for heavy-duty science imaging. This 24" will be a short distance from the famous 24" Clark Telescope in a small dome that now houses a 16" Boller and Chivens.

The main purpose of the 32" f/3.0 is public outreach, and it is set up for visual observing in the normal way, just a standard focuser, no derotator.


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#19 Oberon

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Posted 28 August 2019 - 09:09 PM

Are you planning to exploit the hexapod arrangement for collimation?



#20 Pierre Lemay

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Posted 28 August 2019 - 09:15 PM

In hindsight, the horseshoe set-up earns a special mention - it's very palatable as is equatorial and it would adopt very similar material/manufacturing processes as the glorified Dob.

Did you ever consider the English yoke? It's the mount the 100 inch at Mt Wilson uses. You don't have access to the immediate area around the pole but, it's VERY easy to make, with cheap materials and fairly easy to make robust and vibration free. The equatorial drive would be fairly easy to do as well. You would have a true equatorial mount, no field rotation. The eyepiece would sometimes end up in awkward observing positions but it is possible to make a rotating UTA or multiple eyepiece positions (rotating diagonal) to compensate.



#21 m. allan noah

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Posted 29 August 2019 - 06:30 AM

I'm not a fan of how two of your truss tubes meet in the middle of a horizontal bar on top of the mirror box. You've just made a leaf spring. Also, I think your spider vanes are tapered the wrong way. They should be wide at the hub and narrow at the outer wall.

 

allan



#22 skround

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Posted 29 August 2019 - 09:01 AM

I'm not a fan of how two of your truss tubes meet in the middle of a horizontal bar on top of the mirror box. You've just made a leaf spring. Also, I think your spider vanes are tapered the wrong way. They should be wide at the hub and narrow at the outer wall.

 

allan

Very well spotted Allan - it is actually a CAD flaw as we are re-constructing it now.

You'll notice 2 flat spots on the 'mirror box' elements - those are to support 2 smaller beams to reinforce the area you are concerned of.

 

 

About the spider vanes I actually tricked me as well. But if you look at that as a shelf the weight is the secondary mirror assy whereas the upper cage is the 'wall'. Hence the higher moment/stress is close to the wall. Conveniently this allow the secondary central hub to be shorter and avoid the overall upper cage to be higher.

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#23 skround

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Posted 29 August 2019 - 03:26 PM

Did you ever consider the English yoke? It's the mount the 100 inch at Mt Wilson uses. You don't have access to the immediate area around the pole but, it's VERY easy to make, with cheap materials and fairly easy to make robust and vibration free. The equatorial drive would be fairly easy to do as well. You would have a true equatorial mount, no field rotation. The eyepiece would sometimes end up in awkward observing positions but it is possible to make a rotating UTA or multiple eyepiece positions (rotating diagonal) to compensate.

To be honest I haven't but I know about it. Having a look at it now I'd say that there are bending moments in the rotating frame so that part would need to be very robust. Consequently the fixed frame would need to be even more robust - and it's already inherently a big structure by itself. It's equatorial though - which would be great! It would make an excellent mount for smaller diameter and short F - say a 400mm f4 for instance.

 

The advantage of Alt-Az is that the main loads are vertical and they create little bending moment. That's why it's the only way to go for big observatories - they have to cope only with the bending of the OTA.

 

That's my humble opinion though.



#24 skround

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Posted 29 August 2019 - 03:30 PM

Are you planning to exploit the hexapod arrangement for collimation?

I'm afraid I don't fully understand your question but I try to answer anyway. The secondary mirror can be 'giggled' around for collimation by pulling the 4 spiders vanes. A bit like when you center a piece into a lathe chuck (??)



#25 Oberon

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Posted 29 August 2019 - 05:40 PM

OK, then allow me to explain. If a hexapod truss is made with adjustable poles, as a few of are are doing, then there is no need to provide any other collimation mechanism in any of the mirror supports. This allows both your primary and secondary supports to be sturdier, stiffer, simpler and lighter, collimation be be quicker, simpler and more intuitive, and even permits star collimation from the eyepiece. It also greatly relaxes construction tolerances with respect to focal length.

 

Your design so far allows for it. It is the obvious thing to do. Check out my Merope thread in my signature for more detail.


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