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Vacuum formed Mylar Mirror ?

DIY mirror making
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#1 efanton

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Posted 10 January 2020 - 07:20 PM

I have always dreamed of a really  large aperture mirror, who hasn't, but I had this mad thought earlier this week.

 

1 metre mirrors are out of the realms of amateur astronomy,  but maybe they aren't

Here's the mad idea.

 

Basically you make a box that is air tight except for a circular hole the size of the aperture you wish to have on one surface,  scale the box appropriate to the size of aperture you wish to have.  

Inside you mount a vacuum pump and an associated controller to ensure that the vacuum generated remains constant and drill a hole to accept a one way valve to allow the pump to remove air from the box.

Then across the hole you have created fix a mylar sheet in such a way that it forms an airtight barrier.

 

Now you simply start the pump and the mylar sheet will deform always keeping a perfect parabola. 

Now obviously  the properties of the mylar sheet (how much it can be stretched without tearing), will determine the maximum or minimum focal length that can be formed or used.

But the basic principal is there.  As long as the vacuum does not fail. and remains constant,  for a given pressure a specific focal length will be formed.

 

 

I have tried this out in a very rough fashion.  Basically a plywood box I put together, a vacuum cleaner, and a piece of mylar,  and in principal the idea works.

I would imagine the key to the whole thing would be keeping the pressure in the vacuum chamber constant and accurate.

 

So the question is,  has anyone tried something similar?

I have search the web but have not found anything similar being used as a telescope mirror,  but have found similar setups being used for solar cookers and reflectors.

 

Not being an engineer or having any great experience of using vacuum pumps or being able to control the pressure, is this a non starter.

I amm tempted to try develop this further just for the hell of it,  but scientific grade vacuum pumps arent cheap but still many many times cheaper than a large aperture mirror.

I have no idea whatsoever if it is even possible to keep a vacuum at the pressure that might be required accurately.

 

Looking forward to feed back,  even if it is negative.  If you can see a fatal flaw let me know. 


Edited by efanton, 10 January 2020 - 07:23 PM.


#2 MikiSJ

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Posted 10 January 2020 - 07:24 PM

 Try this:  https://tinyurl.com/wbzm4vo

 

I have seen where clear Mylar has been used as dew shield for newts but I have never seen a successful parabolic mirror made from Mylar.


Edited by MikiSJ, 10 January 2020 - 07:26 PM.


#3 gregj888

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Posted 10 January 2020 - 07:31 PM

It's been tried at least 100s of times, perhaps 1000s.  Can cook a hot dog, but as a telescope, keep looking.

 

The available film doesn't have constant enough thickness and other properties to allow success.

 

You could slump and grind a very thin meniscus but that may need active technologies to make work.  There is an example in the book "unusual Telescopes" with magnetic flotation...

 

Mustard, the brown kind, no catsup...  at the Ball Park, it's important ;-)


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#4 efanton

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Posted 10 January 2020 - 07:55 PM

Yes 

 

 Try this:  https://tinyurl.com/wbzm4vo

 

I have seen where clear Mylar has been used as dew shield for newts but I have never seen a successful parabolic mirror made from Mylar.

Yes, that's the basic principal.

 

I was not thinking to put a backing on it as even a slight imperfection, or as happened in the video a warp,  would make it useless for use in a telescope.

 

The pressure alone forms a perfect parabola, so why mess with perfect. 

Obviously there would be much testing and experimentation in finding the correct pressure for a given focal length,  but getting an accurate measurement of the focal length once a mirror is formed should be relatively simply. It would be a case of simply increasing the vacuum in very small increments, being able to measure the vacuum pressure accurately, and then measuring the resultant focal length.

Once you had got the focal length right, then for a given thickness of the mylar sheeting, the process of recreating the mirror should be fairly straight forward.  That would mean that even if you had to replace the mylar sheet on a fairly infrequent period (every year or two) as long as you were using an identical thickness of mylar, with an identically sized aperture and the exact same pressure issus such as the mylar slowly degrading or deforming or  surface oxidation and corrosion would not be a big issue.  We have to re-collimate telescopes on a regular basis,  this would be no more difficult or time consuming


Edited by efanton, 10 January 2020 - 07:57 PM.


#5 dan_h

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Posted 10 January 2020 - 08:18 PM

I think you have to consider the edge support for the mylar. How can you make it perfectly round and perfectly within spec so that the defined parabola doesn't suffer badly from astigmatism. Essentially, the support will have to be as accurate as the edge of  a mirror would be. A plywood box isn't going to it. 

 

Variations in mylar thickness will result in zones in the surface when the vacuum is applied. 

 

dan


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#6 Jon Isaacs

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Posted 10 January 2020 - 08:19 PM

The pressure alone forms a perfect parabola, so why mess with perfect?

 

 

As Greg said, it's only a perfect parabola if the material is perfectly uniform. That not only means a material of uniform homogeneity but also a material of uniform thickness. Polymers are nott homogenous materials nor are the their properies uniform. The Young' s modulus is variable within a sample and the thickness is as well.

 

And the boundary conditions at the edge where it's attached to the support must be perfectly uniform.

 

One would have to do a FEM analysis to determine the tolerances but clearly they're extremely tight. 

 

There is also a gravitational component that must be included.. The mass of the Mylar.

 

This is an engineering problem that needs to take into account the very smallest detail. Physics might tell you that a perfect membrane with perfect edge supports under uniform loading will produce a perfect parabola.. An engineer will look at the real world, perfect doesn't exist in the real world.

 

Jon


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#7 JoeVanGeaux

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Posted 10 January 2020 - 08:20 PM

I am not even close to being an expert, here, but I had the same "vision"  since I got my first Mylar "survival blanket" in the mid 1990s.

Oddly enough, before I even began to cobble together a crude test, I first researched why it would not work (at least, why it may not work very well).  I came to an immediate halt in that pursuit when I looked into the "surface roughness" of good quality mirrors and saw numbers (at that time) of around, or below, 2nm.  Mylar, as per modern specs, are still showing an average surface roughness at around 38nm.  So, although I would still maintain it will work as a novelty, I suspect the image quality will really suffer and not be good enough to replace any "real" telescope.

If proved wrong, I will start hoarding Mylar film. lol

-Joe



#8 Migwan

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Posted 10 January 2020 - 08:32 PM

Maybe use a barrel in the best condition you can find.   Might have to control temperature and expectations.  jd 



#9 dogbiscuit

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Posted 10 January 2020 - 08:36 PM

It would be a drum.

Sound would vibrate the mylar.

Wind would cause constant changes of pressure.


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#10 efanton

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Posted 10 January 2020 - 08:40 PM

It's been tried at least 100s of times, perhaps 1000s.  Can cook a hot dog, but as a telescope, keep looking.

 

The available film doesn't have constant enough thickness and other properties to allow success.

 

You could slump and grind a very thin meniscus but that may need active technologies to make work.  There is an example in the book "unusual Telescopes" with magnetic flotation...

 

Mustard, the brown kind, no catsup...  at the Ball Park, it's important ;-)

If you are going to buy the mylar from ebay or alibaba then I might agree. 

But Mylar is produced  in many many grades for may different purposes.  Some grades are designed specifically for military, medical and scientific purposes.

 

Obviously the way to go about this is to use the cheap stuff during testing and development and buy scientific grade Mylar once you have a design that works (or should).

 

For instance Dupont produce Mylar® E a polyester film that is a clear smooth base film engineered specifically to provide superior optical and physical properties required for critical dying, metallizing, laminating, labeling, and coating applications while processing excellent winding and handling characteristics.

If you go to https://usa.dupontteijinfilms.com/ they have data sheets available with specifications.


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#11 gregj888

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Posted 10 January 2020 - 08:55 PM

There are a lot of things that work in theory with some hand waving and the proverbial spherical chicken in a vacuum concept.  This is one of those.  Could it be done, probably.  just musing, maybe with a stainless sheet or gold foil or exotic polymer other than the standard "space blanket" but there are a lot of things to work out.  Edge supports will probably need to be ground in, and it may only work at zenith (like the spun mercury mirrors, but that might be OK.  Spun polymer mirrors is a similar thread that shows up- hold that though...

 

I'm not saying don't do it, I'm about the last one to take that stand.  I am saying the google is your friend.  Don't assume you are the first with a simple idea...  you may be, but assume otherwise and go look for it.  A few years ago I came up with an idea for a rather simple and I though unique polarimeter. After 30 or so minutes with google I found not only had it been done before, the data reduction had been embedded into a released image processing program.  A lot of good information here to get you going, see if you can find a solution to them... an do the math.

 

So if you want to try it, go for it.  Oh, on the spun polymer mirrors, I think NASA has some that are functioning in the IR/NIR.  There's a thread here on it in this forum if you look... the person well PHD that took it up is a Polymer Chemist who worked the problem and found solutions.  I've tested one of her mirrors, and 3-4 years ago they were pretty dang good..  She did not use an old turn table... :-)

 

Might thy a bicycle wheel rim... :-)



#12 efanton

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Posted 10 January 2020 - 08:55 PM

It would be a drum.

Sound would vibrate the mylar.

Wind would cause constant changes of pressure.

Good point.  Sound might be an issue, thats something that will have to be considered and tested.

 

Wind would not be an issue, nor changes of atmospheric pressure as the mylar would be forming the top surface(which would be totally enclosed) of a vacuum chamber.

In fact because I was deliberately planning not not to mount the mylar on a formed surface but instead use it as one surface of a vacuum chamber   the ambient temperature changes should have no significant effect.  Mylar is extremely stable as regard temperature change,  what will affect the film more will would be any surface it is in contact with (which would be none), and the reflective coating used on the mylar.



#13 gregj888

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Posted 10 January 2020 - 09:04 PM

If you are going to buy the mylar from ebay or alibaba then I might agree. 

But Mylar is produced  in many many grades for may different purposes.  Some grades are designed specifically for military, medical and scientific purposes.

 

Obviously the way to go about this is to use the cheap stuff during testing and development and buy scientific grade Mylar once you have a design that works (or should).

 

For instance Dupont produce Mylar® E a polyester film that is a clear smooth base film engineered specifically to provide superior optical and physical properties required for critical dying, metallizing, laminating, labeling, and coating applications while processing excellent winding and handling characteristics.

If you go to https://usa.dupontteijinfilms.com/ they have data sheets available with specifications.

efanton, you are on the right track...  understand this comes up every 6-12 months along with multiple mirrors (we know how to do that) and Adaptive optics (doesn't do what most want), replica mirror in epoxy (know how to do that too) and of course spun epoxy mirrors .    

 

If me, I would use something thicker and permanently deform it with vacuum so it can hold up to sound an a little breeze or an insect landing on it.

 

All the best, keep us informed and you would at the Portland workshop assuming we have one this summer (which I think is the plan).
 



#14 MitchAlsup

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Posted 10 January 2020 - 09:06 PM

I am pretty sure that the film thickness of mylar is no where close to being better than 1/4 wave of light.

If it is not, then it does not matter how perfect the parabola is.


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#15 TOMDEY

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Posted 10 January 2020 - 09:09 PM

Hi, efanton; interesting topic!

 

For a homogeneous, isotropic thin linear elastomer sheet-material... the circularly-bound inflated shape is not paraboloidal. Although the quadratic term dominates, the quartic and higher even terms are significant... aka spherically-aberrated, and badly so. You are confusing it with the meniscus surface of a rotating fluid in a uniform g-field parallel to the axis... which is the vector sum of the potential and meniscus least-energy states... very nearly paraboloidal for large surfaces.

 

As Jon enumerates, there are myriad gremlins further exacerbating the challenge.

 

I did a white-paper on the generalized topic back in 1965. The analysis is easiest if one just solves for the least-energy state. The context was inflatable satellite balloons... clear on the incoming side and metalized on the back side. For your particular posit... the circular plane ring is the problem. We made them for pellicles in sized never before achieved. And the only way to accomplish that was to grind and polish fused silica hoops to optical quality, then metalize so that the frame would bleed static electricity from puckering the pellicle. That worked... and was astoundingly difficult and expensive.

 

What I'm saying is --- just try to build a rigid hoop that is a meter across and planar to a tenth-wave P/V. It can be done; we did it --- and is tougher than just making a traditional 1-meter telescope mirror... to begin with.

 

But... but! Not to get discouraged! The far-out concepts are the best ones to work on. Even if to just unearth, address and maybe... just maybe... make some progress!

 

[I was 17 in 1965; they let me work on some challenging stuff!]    Tom

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#16 efanton

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Posted 10 January 2020 - 09:11 PM

There are a lot of things that work in theory with some hand waving and the proverbial spherical chicken in a vacuum concept.  This is one of those.  Could it be done, probably.  just musing, maybe with a stainless sheet or gold foil or exotic polymer other than the standard "space blanket" but there are a lot of things to work out.  Edge supports will probably need to be ground in, and it may only work at zenith (like the spun mercury mirrors, but that might be OK.  Spun polymer mirrors is a similar thread that shows up- hold that though...

 

I'm not saying don't do it, I'm about the last one to take that stand.  I am saying the google is your friend.  Don't assume you are the first with a simple idea...  you may be, but assume otherwise and go look for it.  A few years ago I came up with an idea for a rather simple and I though unique polarimeter. After 30 or so minutes with google I found not only had it been done before, the data reduction had been embedded into a released image processing program.  A lot of good information here to get you going, see if you can find a solution to them... an do the math.

 

So if you want to try it, go for it.  Oh, on the spun polymer mirrors, I think NASA has some that are functioning in the IR/NIR.  There's a thread here on it in this forum if you look... the person well PHD that took it up is a Polymer Chemist who worked the problem and found solutions.  I've tested one of her mirrors, and 3-4 years ago they were pretty dang good..  She did not use an old turn table... :-)

 

Might thy a bicycle wheel rim... :-)

Which is precisely why I posted here.  

 

The difference with regards to what I had in mind is the fact that the aluminised mylar will not be attached to solid surface. My thinking was there is no way I could produce an accurate enough mould or surface to adhere it too,  If I could why bother using the mylar in the first place. 

It will simply have a vaccum chamber on one side of the sheet and the optical tube on the other and only the vacuum and stretch properties of the mylar will determine the shape.

 

Why would I  attempt to replicate what has already failed except in the hands of those that have skills, machinery and tools accurate enough to produce almost perfect surfaces. By avoiding the use of a mounting surface then in principal I have eliminated no end of problems from the get go.

 

 

Will try to dig up the links to the threads your are referring to and thanks for point out they are there, somewhere. Also those web links.  Maybe I wasnt using the correct search terms.



#17 Jon Isaacs

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Posted 10 January 2020 - 09:13 PM

If you are going to buy the mylar from ebay or alibaba then I might agree. 

But Mylar is produced  in many many grades for may different purposes.  Some grades are designed specifically for military, medical and scientific purposes.

 

Obviously the way to go about this is to use the cheap stuff during testing and development and buy scientific grade Mylar once you have a design that works (or should).

 

For instance Dupont produce Mylar® E a polyester film that is a clear smooth base film engineered specifically to provide superior optical and physical properties required for critical dying, metallizing, laminating, labeling, and coating applications while processing excellent winding and handling characteristics.

If you go to https://usa.dupontteijinfilms.com/ they have data sheets available with specifications.

 

The requirements for a telescope mirror are far different than any normal optical application. Deflections have to be controlled to about 50 nm over a 1 meter diameter.. 

 

Jon



#18 efanton

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Posted 10 January 2020 - 09:21 PM

I am pretty sure that the film thickness of mylar is no where close to being better than 1/4 wave of light.

If it is not, then it does not matter how perfect the parabola is.

But it will not be the film thickness of the mylar that would be the issue.  It would be the surface coating on the mylar itself, which I assume would be adhered to the mylar in a very similar way that aluminum is adhered to glass mirror blanks.  The aluminsed coating will to some degree reduce the roughness of the mylar backing.

Also because the materially will be in a permanently stretched state any minor aberrations should also be smoothed out.  To what degree I do not know, so your point is still valid to a reasonable degree.

 

 

I have sent an email to DuPont in the hopes that they can give me the exact specifications of any aluminised Mylar they produce.  Hopefully they will be able to give me exact measurements with regards to surface quality and uniformity with regards the material being stretched.



#19 dogbiscuit

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Posted 10 January 2020 - 09:32 PM

Good point.  Sound might be an issue, thats something that will have to be considered and tested.

 

Wind would not be an issue, nor changes of atmospheric pressure as the mylar would be forming the top surface(which would be totally enclosed) of a vacuum chamber.

In fact because I was deliberately planning not not to mount the mylar on a formed surface but instead use it as one surface of a vacuum chamber   the ambient temperature changes should have no significant effect.  Mylar is extremely stable as regard temperature change,  what will affect the film more will would be any surface it is in contact with (which would be none), and the reflective coating used on the mylar.

The vacuum doesn't pull the mylar to curve.

Atmospheric pressure pushes the mylar into the vacuum.

Air movement, wind alters air pressure.

 

Perhaps if other troubles were overcome and a truly optical quality surface was achieved, on a very calm night pressure changes would not be so fast they couldn't be compensated.


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#20 efanton

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Posted 10 January 2020 - 10:11 PM

The vacuum doesn't pull the mylar to curve.

Atmospheric pressure pushes the mylar into the vacuum.

Air movement, wind alters air pressure.

 

Perhaps if other troubles were overcome and a truly optical quality surface was achieved, on a very calm night pressure changes would not be so fast they couldn't be compensated.

 

Actually that's a good point, although I dont think wind will be an issue, but pressure fluctuations might.

 

Maybe fitting baffles along the inside of the tube might alleviate this.

Being that we are talking about the primary mirror which will be at the bottom of a fairly substantial tube, I would imagine that pressure differences would be minimal.  But minimal might be enough to cause problems.

I guess that's something that I will have to find out if this project ever gets to the build stage. 

 

Until I get a response from DuPont answering a fairly long list of technical questions regards specifications of their products and hopefully some really detailed specification sheets, I cant really proceed.

Hopefully they come back with the right answers, and the mylar sheet is a viable solution.

 

But even if they have a the Mylar sheet with the correct specifications, there is so much that can kill this project, such as monitoring and stabilising a vacuum among other things.

 

But thats the reason for this thread.  Already some people have raised a few points I had not considered, and thats exactly what I was looking for.

So thank you all for the responses so far.


Edited by efanton, 10 January 2020 - 10:13 PM.


#21 Jon Isaacs

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Posted 10 January 2020 - 10:23 PM

But it will not be the film thickness of the mylar that would be the issue.  It would be the surface coating on the mylar itself, which I assume would be adhered to the mylar in a very similar way that aluminum is adhered to glass mirror blanks.  The aluminsed coating will to some degree reduce the roughness of the mylar backing.

Also because the materially will be in a permanently stretched state any minor aberrations should also be smoothed out.  To what degree I do not know, so your point is still valid to a reasonable degree.

 

 

I have sent an email to DuPont in the hopes that they can give me the exact specifications of any aluminised Mylar they produce.  Hopefully they will be able to give me exact measurements with regards to surface quality and uniformity with regards the material being stretched.

 

How do you plan to attach the Mylar to the OTA?

 

From my point of view, I see various over simplifications in your analysis. I won't attempt to discuss them.  I will only offer a suggestions:

 

Start with a small scale mirror. In the beginning, probably no larger than 1 inch with the goal to produce a 6 inch mirror. 

 

I will add, the pressure deforming the the Mylar is the difference between the vacuum which is measured in absolute terms, that is 0 Torr is a perfect vacuum, and the atmospheric pressure. When the atmospheric pressure changes, the shape of the mirror will change. You will need to be able control the vacuum very precisely and your pumping system will need to have zero fluctuations. 

 

I looked around for thin membrane equations to estimate the shape of the surface. I didn't find any simple ones. I'd be looking into Tom's statement that a circular membrane under uniform pressure is not a parabola. 

 

Jon



#22 BGRE

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Posted 10 January 2020 - 10:24 PM

If the mylar sheet isn't attached to a solid surface at the edges how do you maintain a vacuum??

 

The boundary conditions for the Mylar film are important they have an effect on the shape of the film as well as the pressure differential between the 2 faces of the mylar sheet..


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#23 efanton

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Posted 10 January 2020 - 10:26 PM

The requirements for a telescope mirror are far different than any normal optical application. Deflections have to be controlled to about 50 nm over a 1 meter diameter.. 

 

Jon

I am very much aware of that.

 

Immediately, before even posting here, I realised that getting Mylar sheets for ebay or alibaba wasn't going to work. Which is why I had already tried to contact DuPont.

 

To be honest,  I'm not sure if it is possible to get close  to the same quality as a standard glass blank mirror.  But ever so close might just be good enough.  We will have to see what comes back,  you never know I might be pleasantly surprised.  



#24 efanton

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Posted 10 January 2020 - 10:52 PM

How do you plan to attach the Mylar to the OTA?

 

From my point of view, I see various over simplifications in your analysis. I won't attempt to discuss them.  I will only offer a suggestions:

 

Start with a small scale mirror. In the beginning, probably no larger than 1 inch with the goal to produce a 6 inch mirror. 

 

I will add, the pressure deforming the the Mylar is the difference between the vacuum which is measured in absolute terms, that is 0 Torr is a perfect vacuum, and the atmospheric pressure. When the atmospheric pressure changes, the shape of the mirror will change. You will need to be able control the vacuum very precisely and your pumping system will need to have zero fluctuations. 

 

I looked around for thin membrane equations to estimate the shape of the surface. I didn't find any simple ones. I'd be looking into Tom's statement that a circular membrane under uniform pressure is not a parabola. 

 

Jon

My intention was to start with a 4 inch or 6 inch prototype.

The thing to bear in mind here (as I see it)  is the bigger the mylar sheet used the more even any stretch or surface imperfections are going to be.  It is going to be easier to get an appropriate focal length on a slightly bigger sheet, and for those reasons I will not be starting at 1 inch or 2 inches.

 

The mylar will not be attached to the OTA.  As described it will be part of a separate vacuum chamber,  that is then housed in the OTA.  The vacuum chamber will essentially be a self contained unit that consist of the chamber, the mylar membrane, the vacuum pump, a controller for the pump and the necessary equipment to monitor the pressure inside the vacuum chamber.

That in itself will be quite an undertaking, but if I can get that to work, then the rest is similar to building a home made Newton.  Obviously What I am looking for here is input from others, especially regarding things I might not have considered.

 

Essentially the vacuum chamber will be a box, with a circular hole on one side that the mylar membrane will be fixed over to form an air tight seal.  I have been playing with various ideas on how to achieve that.  Ideally I would like to be able to remove and replace the Mylar membrane but being able to do that and still keep a perfect seal is an issue.  The alternative is to permanently fix the mylar membrane to the vacuum box,  but that makes it difficult if you would like to try mylar materials of different thickness, or replace the mylar at a later date due to corrosion or blemishes.  I think for the prototype it will simply be glued on and I will work on making it more user friendly if that works. Biggest issue will be machining a perfectly circular aperture,  but I will deal with one problem at a time as they bite me in the butt.


Edited by efanton, 10 January 2020 - 10:54 PM.


#25 wells_c

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Posted 10 January 2020 - 10:53 PM

I’ve been down this very same wormhole for a fairly different application with fairly demanding requirements, yet not as demanding as those of a telescope mirror.

I used to make holograms, which oddly enough has led me to take up telescope making :)

One piece of kit that holographers like to use is a parabolic mirror, which is used to collimate (or even converge) the reference beam, and coverage of a particular size of hologram is limited by the size of the mirror, so there was a very similar desire for big mirrors, equally limited by their expense.

The subject of a membrane, or pellicle, vacuum mirror cane up fairly often, and if I remember a few people tried it with mixed results. If I recall, one issue was the interface between the rim of the hoop/barrel/cylinder and the Mylar; a sharp corner would produce edge distortions. There’s a patent out there for such a mirror and if I recall they spent some effort describing both the clamping mechanism, and the shape of the rim where the membrane lost contact. I think it was curved gently...

I’ll see if I can find the patent reference, but (and I’m the last person to discourage) I feel like getting a quarter wave or better parabola that is stable enough for astronomy will be very difficult at any size, particularly larger than 12” or so diameter.

That said, check out some of the holography forums out there, they may be a good source of info and examples of amateur attempts at construction.

I had to give up holography because life. I sold most of my gear but kept the 8” parabolic mirror, in part because it wasn’t going to get me a pile of cash but also because I thought one day I might look into telescope making.

10 years later it now sits happily at the bottom of a piece of sonotube with a wooden contraption surrounding it, and when it’s clear and I’m not too busy, it gets to check out the night sky :)
  • Astrojensen and happylimpet like this


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