Huge Balloons in Space as Telescopes?

[MrUniverse]

http://en.wikipedia.org/wiki/Echo_1
Echo 1a was a 100 foot balloon launched into space and inflated. Echo 2 was even larger. They were Mylar, coated with shiny material. It was launched to reflect radio signals back to Earth.

Some (including myself) have suggested similar designs for space based solar power satellites. One hemisphere could be transparent, and the other reflective to focus light toward some sort of reflector.

:?:
Here's my question: Could the hemispherically reflective balloon be used to focus light onto optical sensors? Could we inflate a telescope in space that'd be hundreds of feet in diameter?

I remember the 'trouble with Hubble'. Would the accuracy of the geometry of the telescope matter too terribly much with a telescope this size?

I don't know much about telescopes, so I apologize if these are stupid questions. If it make for a good telescope, then I'd like to know why. I've been thinking about this for a long time. Obviously there's something wrong with the idea; I imagine it's that the curvature of the balloon would be too unpredictable.

 

[MeteorWayne]

A spherical balloon makes a lousy telescope. There's no focal point. The mirror in a reflector telescope is a parabola, which focuses all the light at one point.

 

[MrUniverse]

A spherical balloon makes a lousy telescope. There's no focal point. The mirror in a reflector telescope is a parabola, which focuses all the light at one point.

If there were a hemisphere of the balloon, opposite the field of view that was covered with reflective material, and the side towards to the view were transparent; then wouldn't the rays of light from the object of interest be at the center of the balloon? I must be missing something here.?
I want to learn. :geek:

 

[MeteorWayne]

Yes you are. As I said, a sphere does not make a reflector telescope. A parabola is required. Look it up. I've already told you once, now it's up to you.

 

[MrUniverse]

Thanks for the input, I will. And some day, once I've invented the world's largest space-based telescope, I will thank you again for pushing me to find things out on my own.

 

[adrenalynn]

From a free perspective - this is an excellent beginning textbook for optical physics:
http://www.lightandmatter.com/area1book5.html

It does require some basic physics and math background, but it's very well written. I've sent some of my students to it as a supplement.

 

[MrUniverse]

Thanks, Adrenalynn. I'll have to read that.

Looks like my telescope idea would suffer from spherical aberration (https://secure.wikimedia.org/wikipedia/en/wiki/Spherical_aberration), unless a Schmidt corrector is employed. I've found out that there are two types of Schmidt correctors - refractive (a big lens in front of the scope), and reflective. I'm not exactly sure what a reflective one would look like, but apparently the LAMOST scope in China employs one.

According to Wikipedia (https://secure.wikimedia.org/wikipedia/en/wiki/LAMOST), the corrector on LAMOST is made of "24 segments, fitting in a 5.72m x 4.4m rectangle." that's huge.

 

[MeteorWayne]

Thanks, Adrenalynn. I'll have to read that.

Looks like my telescope idea would suffer from spherical aberration

I think that's what I said. A sphere makes a useless telescope reflector.

 

[Andorfiend]

Thanks, Adrenalynn. I'll have to read that.

Looks like my telescope idea would suffer from spherical aberration

I think that's what I said. A sphere makes a useless telescope reflector.

Which is not quite the same thing as saying the fundamental idea is useless. A sphere is hardly the only possible shape of an inflated object. I doubt it would be much of a feat to design a ballon that would inflate into a useful shape.

However to make that into a useful space based telescope you'd have to prove the design offered significant advantages over existing telescope designs. The theoretical advantage is that you'ld be creating telescope with a low launch weight compared to a rigid mirror substrate of the same size. The question is whether or not the problems with flying a ballon erase that advantage.

The three principle problems with the idea that occurr to me off the top of my head are the transparant layer over the face of the mirror, the gas within the mirror and the rigidity of the structure. Thermal issues would also be interesting.

The fact that you have to have some solid material covering your lens is going to be a problem since whatever you use is going to be sure to limit the range of wavelengths your telescope can view. Glass for example is transparent to visible light but opaque to infrared. Any material you choose will have similar problems.

Likewise your ballon will have to contain enough gas to inflate it during deployment, and then maintain enough pressure to hold your mirror rigid during dust impacts and any manuvering acceleration. So that obviously will block it's own absorbtion spectra and have optical effects which will vary over time as the ballon slowly loses pressure (an inevitability.)

And a telescope mirror needs to be extreamly precise. your mirror backing substrate would need to be both light and flexible and yet capable of taking launch stresses and inflation without stretching at all. And it will need to survive the stresses of being in space without wear and tear for long enough to make good on the investment in the project.

I'm pretty sure the thermal effects of cyclical heating and cooling of the ballon and it's gasses as well as having a tempurature differential across the ballon would all make the whole thing useless, so it'd have to fly behind a solar shield large enough to prevent heating. That's hardly a unique problem, but it would at least double the weight to the mirror itself.

All told it would take a pretty convincing feasability study to convince me it's a good idea as an earth launched system. Sounds like a dandy way for someone living in a space station or the asteroid belt to whip up a backyard telescope however.

 

[adrenalynn]

Not so dandy in either place, given that a balloon doesn't do well in a pure vacuum.

 

[Andorfiend]

Not so dandy in either place, given that a balloon doesn't do well in a pure vacuum.

True, but if you were on site to actual make the thing in space a ballon would probably make a handy temporary mold onto which you could deposit a thin layer of metal to serve as your reflector. I'm not sure it would be superior to ice though... Tell ya what, you give me a place to stay in orbit and I'll experiment.

 

[Mee_n_Mac]

Just an FYI re: balloons in space.

http://www.space.com/businesstechnology ... 01112.html


The primary mirror on the HST weighed in at 1700 lbs and the OTA trusswork, about 250 lbs. The whole HST came in about 24,500 lbs so a "balloon-o-scope" would only save 10% of the mass to be launched (for something akin to the HST). Certainly nice but not revolutionary or a game changer. One wonders what could be done today using some sort of carbon fiber "blank", instead of the ~13" honeycombed glass that the HST started with. Would such a thing possess the needed stability to perform as a primary mirror ??

To get back to the OP's question :

Would the accuracy of the geometry of the telescope matter too terribly much with a telescope this size?

I just can't imagine a balloon getting to needed accuracy in it's curvature to be a proper telescope. From :

http://www.scienceclarified.com/scitech ... z1557nuNBy

Laser tools were then used to polish the surfaces so that they would not deviate from the desired curve by more than 1/800,000th of an inch.