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Vacuum formed Mylar Mirror ?

DIY mirror making
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#51 DAVIDG

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Posted 16 January 2020 - 10:45 AM

 The technology to flex a large surface into an accurate optical surface does exist. It is used on the Keck telescope but it requires that the glass segments be made of Zerodur ( ultra low expansion glass). micro actuators to flex the glass at multiple positions,  a complex structural model of the whole system that predicts the flexing of the complete structure, a laser alignment system,  with active feed back and a computer system to control all of it. 

   There are many very smart people in this world and if there were a simpler ways of doing it, someone would done it by now. So when an solution to a problem looks to be simple and no one has done it yet,  what that means is that the problem is much more complex then one thinks and one needs to do more research to truly understand the problem.

 

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#52 GTom

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Posted 16 January 2020 - 11:26 AM

Actually what we need is a material lighter weight and easier to work on than glass. Advanced plastics might evolve to the sufficient characteristics with time.

Beryllium is not exactly cheap.

Edited by GTom, 16 January 2020 - 11:43 AM.


#53 davidc135

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Posted 16 January 2020 - 03:02 PM

Great thread.

I've been using Matt Considine's Schmidt calculator to get an idea of the vacuum/pressure difference needed to create a curve of 240 ins roc. For a film .010 in thick a metre diameter it's miniscule. I'm getting 10 E-4 to 10E-5 psi. (very approx.) depending on material. Unless I made a mistake! Can it be this tiny? Super sensitive to any little influences.

 

www.considine.net/mac/vacpan.html

 

David


Edited by davidc135, 16 January 2020 - 03:10 PM.


#54 dan_h

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Posted 16 January 2020 - 04:34 PM

Great thread.

I've been using Matt Considine's Schmidt calculator to get an idea of the vacuum/pressure difference needed to create a curve of 240 ins roc. For a film .010 in thick a metre diameter it's miniscule. I'm getting 10 E-4 to 10E-5 psi. (very approx.) depending on material. Unless I made a mistake! Can it be this tiny? Super sensitive to any little influences.

 

www.considine.net/mac/vacpan.html

 

David

The surface exceeds 1200 square inches and it is a thin film.  As DAVIDG stated back in post #36, it would be a very sensitive barometer. 

 

dan


Edited by dan_h, 16 January 2020 - 04:34 PM.


#55 Oberon

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Posted 16 January 2020 - 04:39 PM

Yes. The vacuum pump would be a simple screwed plug being drawn out of a sealed thread. Not unlike the methods used on Lunt’s range of solar telescopes for tuning the etalon.



#56 Oberon

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Posted 16 January 2020 - 04:46 PM

I’m more interested to know if a polished spherical glass mirror would form the desired parabola with a negative pressure applied to the back. Goodbye mirror support, hello variable f ratio’s?



#57 GTom

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Posted 16 January 2020 - 04:51 PM

I’m more interested to know if a polished spherical glass mirror would form the desired parabola with a negative pressure applied to the back. Goodbye mirror support, hello variable f ratio’s?

You'd need a quite stable vacuum pump to keep the focal length stable to micron levels and of course a mains plug next to the scope...



#58 Oberon

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Posted 16 January 2020 - 05:08 PM

You wouldn’t need a pump, a sealed enclosure will do, all you need to do is pull on the back wall of the sealed enclosure with a well controlled mechanism. Amateurs have built mirrors with a screw attached to the back for this purpose, buts as its never taken off I presume they were never a great success. I’m mildly curious if a vacuum interface would be an improvement or a waste of time.


Edited by Oberon, 16 January 2020 - 05:08 PM.


#59 GTom

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Posted 16 January 2020 - 05:38 PM

You wouldn’t need a pump, a sealed enclosure will do, all you need to do is pull on the back wall of the sealed enclosure with a well controlled mechanism. Amateurs have built mirrors with a screw attached to the back for this purpose, buts as its never taken off I presume they were never a great success. I’m mildly curious if a vacuum interface would be an improvement or a waste of time.

How much does the focus point have to move before the image gets blurry? I am afraid it would need an autofocus kind of setup, otherwise you'd be fighting with every 0.1 degrees temperature or even 0.01mbar local air pressure change...



#60 davidc135

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Posted 16 January 2020 - 05:40 PM

Yellobeard on the Cats and Casses forum who has made some large, high grade scts has varied the air pressure behind the primary to fine tune spherical aberration.  David


Edited by davidc135, 16 January 2020 - 05:41 PM.


#61 Benach

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Posted 17 January 2020 - 02:10 AM

Yellobeard is by no means comparable. He is a professional optics maker for several decades and he can do tricks and make optics that are beyond any of your wildest dreams.

How do I know this? I know yellobeard personally.
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#62 davidc135

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Posted 17 January 2020 - 01:32 PM

Yellobeard is by no means comparable. He is a professional optics maker for several decades and he can do tricks and make optics that are beyond any of your wildest dreams.

How do I know this? I know yellobeard personally.

You are jumping the gun if you think I was slighting him. I wasn't making a comparison, just pointing out an aspect of his work that should interest members! A mistake, you think?


Edited by davidc135, 17 January 2020 - 01:40 PM.


#63 Oregon-raybender

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Posted 17 January 2020 - 03:58 PM

OK Buzz kill here. I remember we tried very hard to make Mylar mirrors at Tinsley, back in the early 70's

We had so many variables, product processing, surface roughness, mounting issues, gluing issues, vacuum issues, pressure, both air (High and Low atmosphere changes, yes it did come into play) and vacuum holding and metering. We could not make it work. We came close, all we could do was use it for imaging systems, not for telescope quality. We had one system were we made convex and concave surface using pressure and vacuum pumping. I saw several companies at various OSA and SPIE industry conferences, no luck. But new tech always proves one wrong.confused1.gif

 

Not that I am saying it can't be done, BUT, I do mean BUT there is so many things to think about. If I was going to try a thin membrane material, I would suggest Gorilla Glass 6 that is now used for protection of phone and tablets. It bends easy and maybe the process has a surface roughness (polish) that may work. The question is how will a simple AL coat work. I doubt a layer coating would?  Just a thought.

 

One more add, Tinsley was the company who made the Keck Mirrors.

 

Starry Nightswaytogo.gif

 

https://www.youtube....h?v=kF-p9lCpwew

 

https://www.xda-deve...oldable-phones/

 

https://www.corning....la-glass-6.html

 

https://triaticinc.c...ASAAEgIU0vD_BwE


Edited by Oregon-raybender, 18 January 2020 - 12:51 PM.

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#64 ToSpaceOrBust

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Posted 31 July 2020 - 02:26 PM

Hi again, efanton! The boundary condition of how you support and define the edge of the gizmo is critically important. The aperture/frame has to be very circular, very rigid, very flat, electrically-conductive, and grounded... violate any of those four and those imperfections will punch right thru, 1:1 to the topology of your bladder/membrane mirror surface. Whether I call it a "hoop" or you call it "hole in the box" doesn't change or eliminate the extreme difficulty of edge-support. All of these approaches suffer the boundary condition dilemma; camouflaging it is not the same as eliminating it. We addressed it head-on and solved it... but that was expensive and difficult.

 

The other tough nut is the trampoline material itself. Mylar sheet is not nearly as good as we would like to believe. Just take a piece of the stuff and stick it in an interferometer cavity --- 99.9% of it stinks! The only exception I can think of is the Zygo 4-inch and 6-inch pellicle (not Mylar) attenuators @ $1500. And those are only used in transmission... because the reflected wavefronts are loaded with e.g. astigmatism... because the ring support is not flat enough... hence the polished fused silica frames that we made ourselves, in our research labs.

 

Again, I'm not wanting to discourage... just been there, done that, and need to dive in eyes open.

 

This picture indicates the delicacy of what might just be barely good enough... in 3" size. Now imaging a meter across (which we worked with) and gives some flavour of what you would be dealing with. We're talking true optical egg shells here... literally. Touch it, even once, and you're out... the cost of same-sized Primary Mirror.    Tom

hi Tom,

  How about a mylar/thin film reflective surface for optically flat mirrors?  Just stretched across an aperture of say, less than 60mm and of appropriate edge fineness (w.r.t. to your comment about edge preparation).  I'm wondering about a cheap way to make star diagonal mirrors, which seem super duper expensive to me for what they are, small, flat first-surface mirrors.  I read through this thread and have been looking around the Internet but can't figure out if this might be feasible or not.

 I see Peter Drew's comment about degradation when stretched but I'm not sure if or how much it might apply.

 I'd address the same question to DavidG but don't know how to reply to two so I guess I will duplicate it.

randy



#65 ToSpaceOrBust

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Posted 31 July 2020 - 02:29 PM

 I'm  a research Chemist/Engineer for Dupont  also the Director of Mt Cuba Observatory. I  know the properties of Mylar and many other polymer films very well. As others have  found out many times making a precision reflective optical surface  won't work with these films. The way these  materials are  made, results in the stresses and thickness uniformity to not be high enough for a diffraction limited reflected optical surface.  You also have the problem that a very slight change in temperature or air pressure would make a huge difference in the optical figure if the film is pulled to shape under vacuum. You will have  made a very sensitive barometer. Trust me if it worked I would have a 1 meter scope in my backyard made this way many years ago. 

 

               - Dave 

hi Dave,

  How about a mylar/thin film reflective surface for optically flat mirrors?  Just stretched across an aperture of say, less than 60mm and of appropriate edge fineness.  I'm wondering about a cheap way to make star diagonal mirrors, which seem super expensive to me for what they are; small, flat first-surface mirrors.  I read through this thread and have been looking around the Internet but can't figure out if this might be feasible or not.

 I see Peter Drew's comment about degradation when stretching mylar but I'm not sure if or how much it might apply.

I wanted to do a reply/address this question to both you and Tomdey but don't know how besides duplicating it but maybe I can delete one copy later on.

randy



#66 Steve Dodds

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Posted 31 July 2020 - 02:58 PM

hi Dave,

  How about a mylar/thin film reflective surface for optically flat mirrors?  Just stretched across an aperture of say, less than 60mm and of appropriate edge fineness.  I'm wondering about a cheap way to make star diagonal mirrors, which seem super expensive to me for what they are; small, flat first-surface mirrors.  I read through this thread and have been looking around the Internet but can't figure out if this might be feasible or not.

 I see Peter Drew's comment about degradation when stretching mylar but I'm not sure if or how much it might apply.

I wanted to do a reply/address this question to both you and Tomdey but don't know how besides duplicating it but maybe I can delete one copy later on.

randy

They do make those for beamsplitters called pellicles fairly expensive and super delicate, coated with a partially reflective material. No reason you couldn't coat it with aluminum and use it for a flat mirror.  Pellicles aren't mylar but something transparent and slightly stretchy. But are super flat, don't introduce ghosting or spherical aberration.



#67 ToSpaceOrBust

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Posted 31 July 2020 - 03:57 PM

They do make those for beamsplitters called pellicles fairly expensive and super delicate, coated with a partially reflective material. No reason you couldn't coat it with aluminum and use it for a flat mirror.  Pellicles aren't mylar but something transparent and slightly stretchy. But are super flat, don't introduce ghosting or spherical aberration.

Do you have any idea if the film they use could be purchased by a normal person at a non-astronomical price?  I wonder about the camera industry....I see https://en.wikipedia...Pellicle_mirror and it looks like there must have been some plastic foil/film that was used there so I'm guessing it was optical grade and relatively cheap.  I wonder if it's possible to find a small supply of it somewhere, and if it might work.

 

Also, re: this thread, this article might be interesting...a pellicle mirror in space, shaped by laser pressure:

http://adsabs.harvar...A&A....77L...1L

"Standing Wave and Pellicle: A Possible Approach to Very Large Space Telescopes", Astronomy and Astrophysics Vol. 77, no. 1-2 Aug. 1979, p. L1-L2.



#68 TOMDEY

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Posted 31 July 2020 - 07:01 PM

hi Tom,

  How about a mylar/thin film reflective surface for optically flat mirrors?  Just stretched across an aperture of say, less than 60mm and of appropriate edge fineness (w.r.t. to your comment about edge preparation).  I'm wondering about a cheap way to make star diagonal mirrors, which seem super duper expensive to me for what they are, small, flat first-surface mirrors.  I read through this thread and have been looking around the Internet but can't figure out if this might be feasible or not.

 I see Peter Drew's comment about degradation when stretched but I'm not sure if or how much it might apply.

 I'd address the same question to DavidG but don't know how to reply to two so I guess I will duplicate it.

randy

And there's the rub ... the frame. Here's a bit more detail as to why (just no evading the mechanical engineering theory and practice, which I've been trying to avoid) >>> Transmitted wavefront, no problem; bend a pellicle a bit and the transmitted wavefront is unchanged. But the reflected wavefront catastrophically degrades. Here's why: The simplest frame is a hoop. The pellicle is necessarily in tension (potential energy per unit area); this puts the hoop in compressive stress. The system seeks its lowest energy state. This causes the hoop to "want to" flop into a figure eight and fold in half (mechanism related to why hoses kink). Other non-linear contributors don't allow it to get that far, but the result is that the hoop twists enough to manifest a terribly astigmatic reflected wavefront. The math is called ~Knot Theory~. You don't notice this instability just looking at it... but an interferometer clearly shows it.

 

And how we solved that... needing pellicle beamsplitters from 6-inch to 36-inch diameters... which need superb transmitted and reflected wavefronts! >>>

 

Make the frame of solid Fused Silica or ULE. Grind, polish, and figure to twentieth wave surface PV. Then machine out the central part where the pellicle will go. Vacuum Metalize the surface where the pellicle will attach, so it's electrically conductive. Fab/Attach the pellicle (usually takes several tries to get a good one)... Coat it, electrically ground it (to prevent electrostatic puckering), integrate it and use it. So you see, the frame far far exceeds the cost or a solid flat mirror of the same size (which the frame started out as). 

 

Conclusion: The frame is the toughest part... far exceeding the cost of an equivalent solid mirror.    Tom

 

Aside: Most of my training was in optics, math, philosophy, and psychology/physiology. But mechanical engineering would have been my 4th or 5th choice, so I audited a lot of grad courses (gratis, as an alumnus) and picked up a lot, hanging out with the mechanical engineers and scientists. Although telescopes are optical systems... all of the components and assemblies are mechanical builds, subject to all the vagaries of trying to craft things that hold their geometry to millionths of an inch perfection... forever!  Notice that building a good optical flat is much tougher than making a sphere. That's because the radius must be exactly infinity. With spheres, the radius tolerance is generally far far more forgiving.  Tom

 

Bit more on where the astig in the reflected wavefront of a pellicle comes from. Examine a bag of rubber bands. Notice that they naturally relax into 1, 3, 5... odd-loop potential well energy states, because the net twist is zero for all of those. The compressive stress on the pellicle frame upsets the one-loop well. The strain response is to (try to) transition to state 3, by (necessarily) passing through unstable state 2... It never gets there, but becomes severely astigmatic, as a consequence. Related topics from physics: Hamiltonian, Lagrangian.    Tom

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#69 Oberon

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Posted 31 July 2020 - 08:48 PM

Now that was educational! waytogo.gif



#70 ToSpaceOrBust

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Posted 01 August 2020 - 07:24 AM

And there's the rub ... the frame. Here's a bit more detail as to why (just no evading the mechanical engineering theory and practice, which I've been trying to avoid) >>> Transmitted wavefront, no problem; bend a pellicle a bit and the transmitted wavefront is unchanged. But the reflected wavefront catastrophically degrades. Here's why: The simplest frame is a hoop. The pellicle is necessarily in tension (potential energy per unit area); this puts the hoop in compressive stress. The system seeks its lowest energy state. This causes the hoop to "want to" flop into a figure eight and fold in half (mechanism related to why hoses kink). Other non-linear contributors don't allow it to get that far, but the result is that the hoop twists enough to manifest a terribly astigmatic reflected wavefront. The math is called ~Knot Theory~. You don't notice this instability just looking at it... but an interferometer clearly shows it.

 

And how we solved that... needing pellicle beamsplitters from 6-inch to 36-inch diameters... which need superb transmitted and reflected wavefronts! >>>

 

Make the frame of solid Fused Silica or ULE. Grind, polish, and figure to twentieth wave surface PV. Then machine out the central part where the pellicle will go. Vacuum Metalize the surface where the pellicle will attach, so it's electrically conductive. Fab/Attach the pellicle (usually takes several tries to get a good one)... Coat it, electrically ground it (to prevent electrostatic puckering), integrate it and use it. So you see, the frame far far exceeds the cost or a solid flat mirror of the same size (which the frame started out as). 

 

Conclusion: The frame is the toughest part... far exceeding the cost of an equivalent solid mirror.    Tom

 

Aside: Most of my training was in optics, math, philosophy, and psychology/physiology. But mechanical engineering would have been my 4th or 5th choice, so I audited a lot of grad courses (gratis, as an alumnus) and picked up a lot, hanging out with the mechanical engineers and scientists. Although telescopes are optical systems... all of the components and assemblies are mechanical builds, subject to all the vagaries of trying to craft things that hold their geometry to millionths of an inch perfection... forever!  Notice that building a good optical flat is much tougher than making a sphere. That's because the radius must be exactly infinity. With spheres, the radius tolerance is generally far far more forgiving.  Tom

 

Bit more on where the astig in the reflected wavefront of a pellicle comes from. Examine a bag of rubber bands. Notice that they naturally relax into 1, 3, 5... odd-loop potential well energy states, because the net twist is zero for all of those. The compressive stress on the pellicle frame upsets the one-loop well. The strain response is to (try to) transition to state 3, by (necessarily) passing through unstable state 2... It never gets there, but becomes severely astigmatic, as a consequence. Related topics from physics: Hamiltonian, Lagrangian.    Tom

thanks for the in-depth explanation.  It's really interesting, particularly the hoop wanting to flop into a figure 8, which makes sense in retrospect (thinking of folding bike tires when I went on trips).

I used to think of structural parts of machines as being hard and not really considering them much in an overall design but now I tend to just imagine them as rubber and ask myself how they're going to flex because at the tolerances in consideration they basically are rubber.

  That said, I don't really have a huge constraint on the hoop part for the pellicle and since it's a 100% mirror, I can have a solid backing part and the hoop can just be a raised element on a surface, which should provide a ton of stiffness.  It seems like at some point the tendency to flop must be made insignificant with enough frame material.  The structure could be a made from regular plate glass, say, 20mm thick with a 1mm high ridge on top (and a hole in the back to eliminate pressure making a pressurized chamber) which the pellicle stretches over.  I guess that would be stiff enough?

 

So, take a piece of plate glass, cut out a circular body for the pellicle backing structure, say using this simple technique https://www.instruct...y-Sized-Hole-in. then, imagining the raised 'hoop' we want to create, remove glass from outside that diameter with a silicone carbide grinding cup of appropriate diameter (like https://www.google.c...nding&tbm=isch, which I think I've seen at the flea market for a few bucks), and remove glass from the surface inside the hoop/ridge with another small SiC grinding wheel fixed parallel to the surface while the glass rotates below and the two slowly come together and the SiC wheel moves in and out radially to do the entire hoop-enclosed surface area (maybe a grinding bit on a flex extension on a Dremel, or die grinder if the Dremel is too wimpy).  Maybe do those steps submerged to avoid the glass dust danger.  Then polish the top of the hoop/ridge with normal grinding/polishing techniques.  Aesthetically I wouldn't care if the hoop/ridge was not circular but maybe it needs to be for some reason to stretch the pellicle on without making undulations?  (seems like you could stretch a pellicle flat also over a rectangular aperture with proper edge tensioning...?)  I suppose all points on the upper surface need to be planar but maybe for such a small area of glass as the hoop/ridge upper surface is that could be ground out rapidly?  So, ok at this point I'm at a loss for ideas how to make the hoop/ridge upper surface really flat if that is a requirement.  Kind of back at square one, to make a flat glass surface (for a mirror) except that instead of having to make a 50-60mm flat mirror, it's just a ring surface ~60mm in diameter and 1-2mm wide...so ~120mm^2 to polish vs. ~3000mm^2 (for 50x60mm surface).

  Or maybe the backing+hoop/ridge could be machined out of a single chunk of cast iron (but that would not longer be cost effective unless I had the machine shop, which I don't...so, just mentioning it).  Or maybe just take a long-ish (for the stiffness against figure 8 flop) section of thick-walled iron pipe which is 60mm in diameter and prep one end flat and smooth to stretch the pellicle over.  Maybe that would be much easier overall.

 

Not sure how much thermal expansion matters if one is willing to wait a little longer for thermal equilibrium, but just for reference:

0.55 Fused Quartz

2.77 SiC

4.0 Pyrex

5.8 grey cast iron

9.0 Plate glass

... in x 10^-6/°C:

(https://www.engineer...ients-d_95.html)

 

  I guess making the hoop/ridge top planar and flat could be difficult.

hm...


Edited by ToSpaceOrBust, 01 August 2020 - 07:29 AM.


#71 Jon Isaacs

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Posted 01 August 2020 - 07:54 AM

You wouldn’t need a pump, a sealed enclosure will do, all you need to do is pull on the back wall of the sealed enclosure with a well controlled mechanism. Amateurs have built mirrors with a screw attached to the back for this purpose, buts as its never taken off I presume they were never a great success. I’m mildly curious if a vacuum interface would be an improvement or a waste of time.

I think the thermal effects would mean slight changes in pressure which would effect the focal length.  And 15psi negative doesn't create all that much force... 

 

Have you ever worked with vacuum?  

 

Jon



#72 Oberon

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Posted 01 August 2020 - 08:09 AM

Have you ever worked with vacuum?  

 

Jon

You could say so...

 

CFA91703-27E2-4EB2-8033-611A329D8C6E.jpeg



#73 Oberon

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Posted 01 August 2020 - 08:16 AM

But what I had in mind was more along the lines of the vacuum pump built into this Lunt solar scope. That black tubular thing on top of the center, unscrew it and it draws a vacuum within the scope to adjust the spacing of the etalon and so finely tune the frequency of the Ha filter.
 

gallery_217007_5191_98521.jpg



#74 TOMDEY

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Posted 01 August 2020 - 09:15 AM

Hi, ToSp!

 

Yes to all that... comes under the general heading of ~Metering Structures~ which is a giant topic in Aerospace Builds. And your preliminary thoughtful literature research and musings unearth some of the technical challenges and potential ways to address them. What starts out simple quickly morphs to far more than challenging. This is what killed the Superconducting Supercollider, Laser Fusion... and even continues to plague such entrenched mesmerizing initiatives as ~free~ wind, solar, and geothermal energy.

 

Often... most often... we pop the lid on what we think is a Cornucopia, only to discover it's a Pandora's Box!

 

Alternative giant telescopes is worth pursuing. So far, nothing sticks. The devil is in the details.    Tom

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#75 kb58

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Posted 01 August 2020 - 07:37 PM

I don't think it should be thought of more than a fun afternoon experiment. As was said, consistent film thickness is a big one, but also the partial vacuum sucking it down. Since it's partial, both air temperature and barometric pressure will make maintaining a set focal length all but impossible, as it'll be constantly shifting.




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