800mm Telescope Project

[Michele Scotti]

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!

[Raph-in-the-sky]

19 minutes ago, Michele Scotti said:

Ambitious? Definitely

Any comment/suggestion is welcome!

How about sending it in orbit? Too ambitious?

[Mr niall]

I would say without question, sort the mirror first. The sheer challenge of making and fitting one of those in a cell would keep anyone busy for years. Its going to weigh an absolute ton. And probably cost several thousand pounds to construct.

I've seen people make parabolic mirrors from those silver emergency blankets on inflatable jigs; but I've never seen one work. And they'd never be much use for visual. What are you planning on using as a blank? Are you going to grind it yourself?

[Dr_Ju_ju]

An 800mm diameter mirror is going to weigh a LOT (simple calculation based on 800mm x 25mm = 125 Kg) [https://www.glass-ts.com/glass-weight-calculator] so I'd forget timber laminates & instead go for Aluminium.... 

[JamesF]

1 hour ago, Dr_Ju_ju said:

An 800mm diameter mirror is going to weigh a LOT (simple calculation based on 800mm x 25mm = 125 Kg) [https://www.glass-ts.com/glass-weight-calculator] so I'd forget timber laminates & instead go for Aluminium.... 

Ummm.  I did: volume = r * r * pi * thickness = 40 * 40 * 3.14159 * 2.5 (in cubic centimetres) = 12566 cm^3.  Borosilicate glass is about 2.23g/cm^3, so 12566 * 2.23 = 28022g, or 28kg.

Am I being dim?

James

[Michele Scotti]

4 hours ago, Dr_Ju_ju said:

An 800mm diameter mirror is going to weigh a LOT (simple calculation based on 800mm x 25mm = 125 Kg) [https://www.glass-ts.com/glass-weight-calculator] so I'd forget timber laminates & instead go for Aluminium.... 

The overall weight target is 130kg mirror included. Don't mix r with D in your calculations

Edited August 27, 2019 by Michele Scotti

[JamesF]

11 minutes ago, Michele Scotti said:

Don't mix r with D in you calculations

Ah, yes, that would make it broadly in line with my calculation.

James

[PeterW]

Any reason for 6 truss poles? Fast optics need stiffness to hold collimation. Do you have the blank? Some people make “dummy mirrors” out of concrete to help them in making the structure before the mirror is ready. GOOD LUCK!!!!!

 

Peter

[Peter Drew]

We have a similar project, currently "on hold" due to problems caused by being ring fenced by wind turbines that create bad seeing turbulence. The scheme was initiated before the turbines were erected or we would not have started the project in the first place. A first priority, if you don't already have one is to source a mirror blank.  An 800mm diameter thin blank and fast focal ratio would be a challenge for a top professional and would probably need to be a slumped meniscus configuration to provide adequate stiffness. Our mirror blank is 1070mm diameter and 76mm thick fused ceramic, pre-generated to curve and a 76mm bored core, weight is 160k.

An alt-az mount would be relatively easier and cheaper to make than most other mounts  and best made of metal for long term stability. I went ahead at the time and built the all aluminium tube assembly once the mirror blank was secured, as mentioned in an earlier post, a concrete dummy mirror was used for handling experimentation. Despite heavier weight, I think a traditional 8 component truss would be needed for stiffness if longish exposure imaging is planned.  😀 

[R26 oldtimer]

Single or segmented mirror?

[Michele Scotti]

9 hours ago, Peter Drew said:

We have a similar project, currently "on hold" due to problems caused by being ring fenced by wind turbines that create bad seeing turbulence. The scheme was initiated before the turbines were erected or we would not have started the project in the first place. A first priority, if you don't already have one is to source a mirror blank.  An 800mm diameter thin blank and fast focal ratio would be a challenge for a top professional and would probably need to be a slumped meniscus configuration to provide adequate stiffness. Our mirror blank is 1070mm diameter and 76mm thick fused ceramic, pre-generated to curve and a 76mm bored core, weight is 160k.

An alt-az mount would be relatively easier and cheaper to make than most other mounts  and best made of metal for long term stability. I went ahead at the time and built the all aluminium tube assembly once the mirror blank was secured, as mentioned in an earlier post, a concrete dummy mirror was used for handling experimentation. Despite heavier weight, I think a traditional 8 component truss would be needed for stiffness if longish exposure imaging is planned.  😀 

Holy cow - that's a monster telescope right there! What focal ratio are you planning? Keep it up - I'm sure there are some other places you can home your telescope.

[vlaiv]

Yep, check your calculations.

No way such a large mirror can be 25mm thick - most of that thickness is going to be ground away if mirror is to be fast, and it ought to be. F/3 800mm scope is going to have 2400mm FL - or be almost two and a half meters long. Anything slower than that is going to be as tall as a house

Next thing to worry about is impact of mirror thickness on ability to maintain figure. When you have a chunk of glass that thin it is going to bend by its own weight and that will produce astigmatism in virtually any part of the sky apart when looking straight up at zenith.

I know that newtonian seems like easiest thing to do, but you might consider two mirror system instead? Instruments of that size are probably best made in Nasmyth configuration. Here is schematic:

Such telescope does not need central hole in the back and hence no hole in primary, but uses two curved mirrors and one flat. It can be mounted on dob type motorized mount - alt/az, with addition of camera rotation device if you need to counter field rotation (not needed for visual, but probably needed for imaging and science stuff). Flat mirror and "focuser" are mounted on alt rotation axis, so it is pretty much stationary. It is actually fixed in place if you have your scope on large platform where observer is rotated with the scope. In any case it is easily accessible - no need for ladders or anything.

[Rusted]

You could try asking Mike Lockwood how much he charges for such a pre-formed, 800mm F3 mirror.
Think of the savings in time, weight, OTA size and money in getting it right the first time.

Lockwood Custom Optics - In Stock & Standard Sizes

Difficulty of support, handling and figuring rises as the square of aperture.
Thermal issues alone make a thin & shaped mirror essential.

8 strut Dobsonian with all the latest and best US practices in primary and secondary cell design and altazimuth OTA structures/support/bearing systems.
Mr Lockwood is a global resource and discusses all of these factors on his commercial website with constant updates.
Successful telescope designers and builders run in parallel with his optical work.
Note that the mechanical and optical frontiers move constantly forwards.
As some of the brightest minds apply themselves to fast, large aperture telescope, PRACTICAL designs for those who can afford them.
The compact instrument would be mounted on a well proven, commercial, equatorial platform.

At a site where the average seeing and average cloud cover and security are actually worth investing in such a large and expensive project.

Or you could spend years/decades building a white elephant at enormous cost in wasted time and resources like everybody else.

[DaveS]

OK, focal length? What camera are you planning? Pixel scale? Have you got an image rotator planned? They tend to be pricey.

I would agree with Vlaiv that a Nasmyth focus would be best for mounting equipment. It also gives the possibility of, for instance having an imaging camera on one side and a spectrograph on the other.

[Peter Drew]

17 minutes ago, DaveS said:

OK, focal length? What camera are you planning? Pixel scale? Have you got an image rotator planned? They tend to be pricey.

I would agree with Vlaiv that a Nasmyth focus would be best for mounting equipment. It also gives the possibility of, for instance having an imaging camera on one side and a spectrograph on the other.

I have been through all the ramifications of such a project, believe me!. The Nasmyth configuration looks good on paper but the convex secondary imparts an amplification to the focal length as with traditional Cassegrains, increasing the already long focal length of a  large primary mirror, not ideal for imaging. The extra weight  (and expense) of the additional optics and their support  requires an even more robust construction. I've been looking into the possibility of re-imaging via a Mersenne arrangement or a flat circular secondary and transfer system directing the focus through the primary as a relatively non amplified focal length, again expensive.  😀

[Michele Scotti]

22 minutes ago, Peter Drew said:

I have been through all the ramifications of such a project, believe me!. The Nasmyth configuration looks good on paper but the convex secondary imparts an amplification to the focal length as with traditional Cassegrains, increasing the already long focal length of a  large primary mirror, not ideal for imaging. The extra weight  (and expense) of the additional optics and their support  requires an even more robust construction. I've been looking into the possibility of re-imaging via a Mersenne arrangement or a flat circular secondary and transfer system directing the focus through the primary as a relatively non amplified focal length, again expensive.  😀

Good point Peter - although a basic Newtonian set-up is a stretch for such diameter and ration I reckon it's the only way for ATMs. We have some experience with parabolic mirrors - an;t got no clue about all other convex secondary mirrors.

I'd prefer to spend some money on a Wynne and go through a careful design of the mount

[Michele Scotti]

First of all thanks everybody for your warm welcome! Lots of hints and suggestions. 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.

Sometimes passion leads to focus on one aspect and to overlook others. Making a big mirror inevitably unbalances the whole telescope project. As a result mounts suffer. To me, the bigger the mirror the more important the mount.

From the beginning it was clear that the project was twofold. If on the one hand the mirror is in itself an exciting challenge -demanding and scary- it became clear that the telescope structure had to be in the same league.

Cheers,

Michele

[Peter Drew]

Do you already have a suitable mirror blank. I really do advise that this is imperative before other considerations as it could significantly determine the mounting  design.   😀

[Michele Scotti]

57 minutes ago, Peter Drew said:

Do you already have a suitable mirror blank. I really do advise that this is imperative before other considerations as it could significantly determine the mounting  design.   😀

Well we dont have that physically with us although we pre-checked the availability. In the design phase we are accounting for a 50mm thick mirror 

[vlaiv]

19 minutes ago, Michele Scotti said:

Well we dont have that physically with us although we pre-checked the availability. In the design phase we are accounting for a 50mm thick mirror 

Worth checking out is "honeycomb" cast of blank - it can save quite a bit of weight while still maintaining figure. Here is image of it:

It is pretty much same as regular blank with a difference that casting was done "over" honeycomb pattern so one side of blank is flat and used for figuring while other is sort of "hollow" with support structure that gives it enough rigidity to hold shape.

[Rusted]

Thickness increases not only weight but thermal mass. Hours spent waiting for the thick mirror to cool soon wears thin.
Forced air fans can help scrub surface thermal current but wont help the optical figure.
 It will still lag in a typically cooling regime where the blank is thick. As will a massive, metal, primary cell.

Does the blank vendor guarantee precision annealing? What is the LE substrate material of choice?
Can your optician work at or below f/4 in such apertures with guaranteed results to allow a practical sized instrument?

Fast mirrors have serious coma. For which they invented Paracorr SIPS and Starlight Instruments.

Search - paracorr

Support for a large mirror is critical to its figure and much more easily achieved, using modern methods, in an altazimuth.

Mirror cells by JP Astrocraft

A  well proven, tracking, equatorial platform is an off-the-shelf purchase. No need for a massive equatorial and crane hire to lift each separate component.

Equatorial Platforms: Home Page

Here is an example of a fast, modern Dobsonian. Just one of many in an expanding, global market, for mobile, large aperture telescopes.

Observing with Webster 28" f/2.7 | Astromart

Do an image search for Lockwood Dobsonian for more examples of large, fast, PRACTICAL telescopes.

No personal commercial interest in any of the posted links.
 

[Peter Drew]

Search www.astronomycentre.org.uk and click on "instrument special feature" to see our 750mm F4.1 Dobsonian to give you some idea of overall sizes.    😀

[Michele Scotti]

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.com/altaz/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 is a strain energy analysis:

 

[Michele Scotti]

22 minutes ago, Peter Drew said:

Search www.astronomycentre.org.uk and click on "instrument special feature" to see our 750mm F4.1 Dobsonian to give you some idea of overall sizes.    😀

That's a great project Peter!

[vlaiv]

43 minutes ago, Michele Scotti said:

This project is aimed at science not just visual

What sort of science are you looking at with this scope?