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Slumped mirrors - any drawback?

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#1 GTom

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Posted 14 January 2020 - 03:14 PM

Saw some Newtonian primary mirrors made from slumped blanks with an extreme thickness:diameter ratio, I wonder what is the downside? Obviously mirror support has to be very carefully designed, maybe even a central hole added. Other than that? The thin glass cools very fast, helps tremendously with transport - but can it hold a nice high-class (Strehl 0.95+) shape with no astigmatism when tilted? Logic says provided decent support it should, because the weight to be supported is also less...

 

Obviously it is the big scopes game, there is not much to gain with an 8" OTA.


Edited by GTom, 14 January 2020 - 03:15 PM.


#2 hakann

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Posted 14 January 2020 - 07:56 PM

You might has read this ?
https://www.bbastrod... Telescope.html

 

On the drawbacks.
Well, it’s very hard to do one, especially if go very thin.
If not slumped one can get a core that are machined meniscus, but it’s very hard to place it down when polishing.
Then the mirror cell etc.

 

This 25” has a slumped meniscus plate glass mirror and its little over 1/2” thick and use a plywood triangle 9 pt cell and edge support is at 90 degree.
As meniscus and a deep sigatta as f/2.6, so the idea it’s a eggshell and also that its basically thicker vs edge thickness/sigatta, comparing to a traditional plano. ( whatever that is the case I can’t say )

This telescope design is very light and compact in design and its foot on the ground observing.

 

At low power as I seen it at its really cool as wider field.
I can’t say on Strehl and maybe this instrument came to be by using Ethos 21/17 mm EP and max pupil.
High power is little over 500X ( E3.7 mm ) but of course a Powermate can be used.

Its also has a rather small secondary at 5.5” and that is because the focuser and the CC intrude some.
This scope has no fans.

 

If it can compete with a optical glass and more ratio and maybe a machined core at ex f/4.5 might be a non meaning discussion.

It shore it’s a hard work do one of these and this is ATM at its best.

 

 

More app is to come I read !


Edited by hakann, 15 January 2020 - 10:47 AM.


#3 hakann

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Posted 16 January 2020 - 12:18 PM

A nice fun idea when thinking of a meniscus would be a 29" core and get out a +28" in machined Zerodur at f/3 and 1" thick, so around +25 kg or +11 lbs.
Whit E21 mm it would be around 115X and a pupil at 6 mm and near 0.9 in FoV.
A step-up is needed, but no ladder maybe.
If the paracorr tube end is at mirrors edge it need a 6" diagonal ( mag-drop not over 0.2 ) and 21% obstruction.
But who can do one of those is not easy and has it IF tested as I see a near must a f/3.


Edited by hakann, 16 January 2020 - 12:32 PM.


#4 GTom

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Posted 16 January 2020 - 12:43 PM

Regarding plate glass: sounds like an extremely low budget project, still viable. 1/2" or 3/4" plate glass sheets can be bought by the square metre. Being very thin, bad thermal conductivity won't destroy equilibration. Expansion/focus shif still have to be dealth with.

Slumping might go easier with plate glass too.

#5 Starman1

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Posted 17 January 2020 - 03:15 PM

Saw some Newtonian primary mirrors made from slumped blanks with an extreme thickness:diameter ratio, I wonder what is the downside? Obviously mirror support has to be very carefully designed, maybe even a central hole added. Other than that? The thin glass cools very fast, helps tremendously with transport - but can it hold a nice high-class (Strehl 0.95+) shape with no astigmatism when tilted? Logic says provided decent support it should, because the weight to be supported is also less...

 

Obviously it is the big scopes game, there is not much to gain with an 8" OTA.

--not stiff enough due to the thickness.

--difficult to design a good back support

--no one has figured out an optimal edge support.  If the mirror is fast, the COG might be behind the mirror edge.

--it's not needed.  A series of fans on a thin flat-backed mirror will cool the mirror just fine.

--It will make balancing with a normal UTA impossible without adding weights to the LTA.

--difficult to make


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#6 GTom

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Posted 17 January 2020 - 04:54 PM

--not stiff enough due to the thickness.

--difficult to design a good back support

--no one has figured out an optimal edge support.  If the mirror is fast, the COG might be behind the mirror edge.

--it's not needed.  A series of fans on a thin flat-backed mirror will cool the mirror just fine.

--It will make balancing with a normal UTA impossible without adding weights to the LTA.

--difficult to make

1. Valid. Less weight to carry though.

2-3. Valid. Thought about a hole in the middle adding a support there as in SCT's, mitigating the COG problem 

4. There are other applications, where a 3/4-1" mirror might come handy: mobile telescopes, where the heaviest part is usually the primary. Counterweights can travel in a less mission-critical package. 

Recently thought about blank prices too: a 39mm thick 24" borosilicate blank will cost you around €1500. If you say this cools OK, then a plate glass mirror with half the thickness will be just as good, for 1/5th price. Difficult to work on though...

5. Absolutely right, unfortunately no PRO mirror makers take orders on slumped thin mirrors afaik....


Edited by GTom, 17 January 2020 - 05:07 PM.


#7 hakann

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Posted 17 January 2020 - 05:15 PM

GTom,
Youre right no ’PRO’ makers do this and when I say pro I’mean Zygo, QED etc ( visit several of them personally )

——

-My guess no ATM ’pro’ will do it either.
It’s to go the miles..

Mel is hard to core ATM, and I lift my hat for him and his works.

Before any critics I would suggest look into his telescopes.
Mel shore say it’s all a compromise and he do it for the love, fun and do it by his own hands.

In the end, we fight seeing and going fast and big, might get outstanding views at a 6 mm pupil, and not heawy, no ladder, not much money into it.

In my case I think going bigger the best deal for a ATM like to get the optics is one from Dream.
Decent price, very stiff, not heawy vs diameter and IF tested and ribs/surface at 5 mm is a killer hard to beat.

#8 GTom

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Posted 18 January 2020 - 08:29 AM

What commercial makers actually do are conical mirrors


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#9 Oberon

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Posted 20 January 2020 - 03:50 AM

What commercial makers actually do are conical mirrors

Now that is tempting!



#10 GTom

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Posted 20 January 2020 - 05:53 AM

Now that is tempting!

It's about time for the industry to move on and show some development. I believe ATM will follow or even take the lead producing more sub 100lbs, 28" and sub 80lbs 24" portable scopes, aperture has to grow!



#11 Jon Isaacs

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Posted 20 January 2020 - 06:07 AM

It's about time for the industry to move on and show some development. I believe ATM will follow or even take the lead producing more sub 100lbs, 28" and sub 80lbs 24" portable scopes, aperture has to grow!

 

Shake rattle and roll... 

 

Ever used a big scope in the wind?

 

Jon



#12 GTom

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Posted 20 January 2020 - 08:11 AM

Shake rattle and roll... 

 

Ever used a big scope in the wind?

 

Jon

Haven't tried these, but I guess they behave much better in the wind, than my high GEM mounted full 12" f5 OTA:

 

g2.jpg

 

With shroud it's another story, but the point to have a mobile scope is that you move it to a dark site...


Edited by GTom, 20 January 2020 - 08:18 AM.

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#13 Jon Isaacs

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Posted 20 January 2020 - 08:37 AM

Haven't tried these, but I guess they behave much better in the wind, than my high GEM mounted full 12" f5 OTA:

 

 

With shroud it's another story, but the point to have a mobile scope is that you move it to a dark site...

 

That's part of the story, transporting it.

 

But once you get it to that dark site, the plan is to use it. How stiff and rigid those scopes are, I don't know but they don't look like they're rock solid to my engineering eye.

 

Jon


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#14 hakann

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Posted 20 January 2020 - 08:37 AM

When I was at LaPalma/Canaries under Christmas it was one night we decided to observe from the new visitors centre at +2.000 meter on Roque but it was not that windy but to much to be there for observing.
Sky was dark near 21.9 but we moved to a location to east in le, but then we missed sky at some parts.
( open truss scope )
I been in stabile 25" telescope in some wind so it's also about stability.
This scope I rented can't stand any winds, even open truss.

My own classis Dob at 18" will SDM do a 25" wide mirror box to and I has a very stabile CF trusses.
Portable is great vs travel and I seen very light weight 25" from Germany, but that is a compromise I don't personally like.

I also like a shroud around the trussers more than get cross stars/light issue.

 

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#15 GTom

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Posted 20 January 2020 - 09:04 AM

That's part of the story, transporting it.

 

But once you get it to that dark site, the plan is to use it. How stiff and rigid those scopes are, I don't know but they don't look like they're rock solid to my engineering eye.

 

Jon

Right, splitting up the trusses in two sections, joint at an angle <180degrees, you'll get a better structure. Still far less wind cross section than a tube. Lots of construction freedom here, one has to get a general idea then do a decent FEM before ordering any raw materials.



#16 Jon Isaacs

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Posted 20 January 2020 - 09:20 AM

Right, splitting up the trusses in two sections, joint at an angle <180degrees, you'll get a better structure. Still far less wind cross section than a tube. Lots of construction freedom here, one has to get a general idea then do a decent FEM before ordering any raw materials.

 

Good luck implementing that into FEM.. 

 

I once was a very small part of a large project to design and build a 4 lane vehicle bridge made from composite materials.  It never got built. The conclusion was that weight and mass are important factors in stability.

 

Jon


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#17 GTom

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Posted 20 January 2020 - 09:21 AM

Good luck implementing that into FEM.. 

 

I once was a very small part of a large project to design and build a 4 lane vehicle bridge made from composite materials.  It never got built. The conclusion was that weight and mass are important factors in stability.

 

Jon

That might be a little bit pushing the boundaries grin.gif



#18 havasman

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Posted 20 January 2020 - 01:44 PM

Edge support for a >20" slumped mirror made thin seems an extremely daunting task. Edge support for any Newtonian mirror in a Dobsonian structure is difficult to optimize even when structures are robust and mirrors have stable configurations.

 

My Starmaster was built to an old-school model applying mass to gain rigidity. Not only is it very rigid but it is also quite stable in windy conditions. 



#19 GTom

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Posted 20 January 2020 - 02:14 PM

Edge support for a >20" slumped mirror made thin seems an extremely daunting task. Edge support for any Newtonian mirror in a Dobsonian structure is difficult to optimize even when structures are robust and mirrors have stable configurations.

My Starmaster was built to an old-school model applying mass to gain rigidity. Not only is it very rigid but it is also quite stable in windy conditions.


Edge support should be changed to center support I am afraid. Like the sct's and the conical thin mirrors.

#20 Starman1

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Posted 20 January 2020 - 02:26 PM

Edge support should be changed to center support I am afraid. Like the sct's and the conical thin mirrors.

So the mirror would have a multi-point back support and be held in the middle?

Wouldn't that cause serious astigmatism when the scope is pointed low and the top of the mirror falls away from its support points?

That only works on SCTs because they aren't extremely large and because the center of the mirror is a LOT thicker than the edge.

The meniscus shapes being discussed have uniform thickness from center to edge.

 

I'm wondering if a large thin meniscus mirror shouldn't have, say, 54 points of support or more, and be glued to a ring-shaped support at the COG from front to rear.

It's well known gluing causes deformation of the wavefront in small area deformations, but it might be superior to supporting the bottom of the mirror if the entire bottom edge

is in front of the mirror's COG.  And having so many points might reduce overall wavefront error compared to a loose support and/or having only the glued ring. 


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#21 GTom

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Posted 20 January 2020 - 04:23 PM

So the mirror would have a multi-point back support and be held in the middle?

Wouldn't that cause serious astigmatism when the scope is pointed low and the top of the mirror falls away from its support points?

That only works on SCTs because they aren't extremely large and because the center of the mirror is a LOT thicker than the edge.

The meniscus shapes being discussed have uniform thickness from center to edge.

 

I'm wondering if a large thin meniscus mirror shouldn't have, say, 54 points of support or more, and be glued to a ring-shaped support at the COG from front to rear.

It's well known gluing causes deformation of the wavefront in small area deformations, but it might be superior to supporting the bottom of the mirror if the entire bottom edge

is in front of the mirror's COG.  And having so many points might reduce overall wavefront error compared to a loose support and/or having only the glued ring. 

Good point, the center supported conical mirrors are also far much thicker in the center. Can PLOP model a Cassegrain type support?

 

Combining edge and center support might be difficult, like balancing a 5-leg chair.

 

Someone with more modelling experience could chime in, my gut feeling is, that with edge support, a normal flat-bottom mirror, even if very thin, could beat a slumped one.


Edited by GTom, 20 January 2020 - 04:43 PM.


#22 havasman

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Posted 20 January 2020 - 04:29 PM

Support for such a mirror whether rear, edge, center or by magic yet undiscovered presents a daunting task. As does the creation of a complex lightweight large aperture Dob.



#23 GTom

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Posted 20 January 2020 - 04:55 PM

Support for such a mirror whether rear, edge, center or by magic yet undiscovered presents a daunting task. As does the creation of a complex lightweight large aperture Dob.

There is only one thing I hate more than gravity: the atmosphere grin.gif


Edited by GTom, 20 January 2020 - 04:55 PM.



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