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satori
2007-Jan-31, 02:51 PM
many years ago, when i was still given to dreaming up great things, i figured, whether it mightn't be possible to blow a huge glass bubble in space. You could coate it then with a metall gas from within to obtain a spherical mirror. Suppose this to be feasible.
Could such a device be of any observational value ?

satori
2007-Feb-01, 02:06 PM
i am still hoping for answers

samkent
2007-Feb-01, 02:20 PM
It would give the Red Necks something to aim at on Saturday nights.

satori
2007-Feb-01, 03:05 PM
you are cruel,
you should rather torture your brain to come up with an intelligent reply
my question couldn't be that stupid (or could it ?)....

loglo
2007-Feb-02, 11:10 AM
Uses for a glass bubble in space:-
1. Coat it with aluminium and use it as a secondary mirror for a large Schmidt-Cassegrain. Creating a matching parabolic primary might be difficult though.

2. Use them to block some of the sunlight coming to Earth, as the US government requested NASA to study recently.

3. Fill them with water and chuck them at Mars. :)

satori
2007-Feb-02, 04:29 PM
thanks for responding , logo,
my understanding of optics is alas not far developed.
I just saw the possibility to create HUGE perfect spherical shapes in space by a cheap natural mechanism. My asking went more in the direction of using this for Primary Mirrors !
Couldn't you correct this not ideal geometric situation with a Secondary optic. Now that is a perfectly resonable question for anybody used to tinker around with telescopic stuff...
There should be some body around here able answer on this !

joema
2007-Feb-03, 06:25 PM
You could theoretically create a roughly accurate mirror "blank" that way. However an actual telescope mirror must generally be polished to within about 1/8th wave of light. Even in space the mirror would require such finishing.

But there's no way in space to perform the final finishing required to make it a usable mirror. Also much more than the mirror is required. You'd have to construct the entire telescope around it. It's hard enough to do crude assembly and repair in space, much less precision construction of a telescope.

Maksutov
2007-Feb-09, 01:32 AM
many years ago, when i was still given to dreaming up great things, i figured, whether it mightn't be possible to blow a huge glass bubble in space. You could coate it then with a metall gas from within to obtain a spherical mirror. Suppose this to be feasible.
Could such a device be of any observational value ?What joema said about having to final polish the mirror is accurate. The final hardened sphere would only approximate that shape, due to residual hysteresis remaining from when it was released, and from uneven cooling due to half of it being in shadow at any given time, as well as other factors such as homogeneity of the material, etc.

Then there's the problem of cutting the sphere into the correct size disc, which would introduce stresses that would distort the shape.

Then, a sphere section isn't what one wants in a telescope mirror anyway. Spherical mirrors suffer from a condition called spherical aberration, which means light reflected from the edges of the mirror wind up focusing at a different position than light from near the center of the mirror, for a given object. Here's a graphic example (http://www.glenbrook.k12.il.us/gbssci/phys/class/refln/u13l3g.html).

In order to use a spherical mirror one must employ a corrective plate, which in most designs is harder to manufacture than the primary mirror itself. This is how catadioptric (Schmidt-Cassegrain and Maksutov-Cassegrain) (http://www.celestron.com/tb-2ref.htm) telescopes work.

As joema pointed out, these are hard enough to produce on Earth. It would be at least an OOM more difficult in space.

trinitree88
2007-Feb-09, 03:07 AM
The Echo satellite, which followed soon after Explorer, was a sizable balloon coated with metal to reflect radar signals......

Maksutov
2007-Feb-09, 05:36 AM
The Echo satellite, which followed soon after Explorer, was a sizable balloon coated with metal to reflect radar signals......Plus it was easy to photograph with a time exposure. But it wasn't of any use for producing astronomical observations.

August, 1960, IIRC.

satori
2007-Feb-09, 06:13 PM
Maksutov, thanks for a well thought out reply!
It was actualy only after the posting , that I ponderd myself about this thing, which I had simply taken out from my memory.
What me attracted to such a (mis)conception must have first and foremost been the wish to come up with a realy HUGE primary mirror. My notion would have been that IF you had really out there a Perfect spherical mirror (come about say through a miracle !) What would the Astonomical Comunity make of it????!!!!
Now I see there are problems with glas ! So what about this :
You take a lump of a plastic kind of stuff. It is transparent (for light and microwaves). There is a certain amout of a certain kind of gas there in a cavity in that lump. You place this stuff in an orbit around sun (just the earth orbit just far enough away to reduce tidal complications).
NOW you start to heat the gas inside the lump with some microwave beam. The gas starts heating up and so does this thermo plastic stuff. A BUBBLE starts to build! You let it grow to size say 1000m across (remind we want to do BIG)!
Now you abort this. Next comes the mirrormaking. You aply some metal gas on a part of the outer surface. Light schould now to be able to get from the opposite side thru the bubble onto the reflecting surface. The bubble could be wobbely I see that. So you keep up a certain pressure level inside of the bubble. Now this was the "miracle" part of the game. Angels have done all this!!! Now for the Astronoms to choose : use it or loose it
??!!
Appendum : I see the Bubble should be further out (sun,heat...).
second : I provide as a gift this additional idea :
the secondary optic (people who really, really understand optics have provided this part) is placed on a "ship" inside of the Bubble and there manouverd by air-jets.
You only do chance observations (kind of) manouvering only this "air ship".....
details spared (must leave terminal. roger. end of transmission)

Maksutov
2007-Feb-12, 02:28 PM
[edit]Appendum : I see the Bubble should be further out (sun,heat...).
second : I provide as a gift this additional idea :
the secondary optic (people who really, really understand optics have provided this part) is placed on a "ship" inside of the Bubble and there manouverd by air-jets.
You only do chance observations (kind of) manouvering only this "air ship".....
details spared (must leave terminal. roger. end of transmission)OK, let's see.

You still have the issues of material homogeneity. Plus few liquids transform into solids without some kind of issues. Thus the resulting "sphere" would not be of optical quality.

But, let's assume it was.

Now you have light entering the bubble and departing it (albeit, internally). Unless it is some material we're not yet aware of, it has an index of refraction. In the first instance it would distort the image upon entry into the "clear" material". Then, upon exit, it would display another distortion.

Since our mirror is placed on the outside of the sphere, the light would then experience another distortion when it approached the reflective surface, which was behind yet another interface with the bubble material.

Any image resulting from these interfaces would be quite distorted and require corrective optics. If you look very carefully, you can see such artifacts in common (back-faced) mirrors, such as those found in bathrooms.

Therefore, so to speak, we're back at square one.

satori
2007-Feb-13, 11:50 AM
Therefore, so to speak, we're back at square one.
..yes I was lastly under the same impression...,but Thanks for taking the yarn up anyway !
To salvage at least part of my glorious idea, I thought first of all to diminish this unholy thing by scale-factor ten and next to maybe use it as a lens. The membrane imperfections woulld surely weigh in less in this scenario, or am I mistaken ?
Also, if limiting oneself to special observations only (taking of spectra etc.) , the lack of optical quality and resolution could probably be accepted if juxtaposed to the high ""luminosity gain"" provided by a really big collector.

My Original starting point (which led me to the Bubble in the first place) was to come up with some method to let physics do the cumbersome and costly job of forming a big and usable primary. The only alternative methods known to me are the forming of a quicksilver paraboloid by rotating a pan filled with this stuff (done and tested) and the forming of galaxy clusters (not exactly easy to achieve)...
bytheby : the latter example strenghens my point, that Optical Perfection may be of second order importance, if counterweighed by Huge light gathering powers !!! (Not so ?)
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we are the stuff that dreams are build on...

Argos
2007-Feb-13, 12:15 PM
Interesting... We had a similar discussion a while back (http://www.bautforum.com/showthread.php?t=1929&highlight=inflatable+mirror). You might like to take a look at Argos´Inflatable Mirror (http://www.wikinews.com.br/argos_toys). :)

satori
2007-Feb-13, 12:31 PM
Whilst I go over to study your ill conceived concepts, you should go to the thread "What Kind of a Super Villain are you ?" !
This idea is most certainly all mine !!!

Argos
2007-Feb-13, 12:36 PM
i am still hoping for answers

Well, you said you wanted answers. I just gave my contribution. :)

joema
2007-Feb-13, 12:54 PM
...the lack of optical quality and resolution could probably be accepted if juxtaposed to the high ""luminosity gain"" provided by a really big collector...
Already, large terrestrial telescopes have vast light-gathering power. Some of these are very low cost, relative to a space telescope.

Magellan: http://www.ociw.edu/magellan/aluminum.jpg
http://www.ociw.edu/magellan/history.html

11.1 meter Hobby-Eberly telescope: http://www.as.utexas.edu/mcdonald/het/het.html

VLT: http://en.wikipedia.org/wiki/VLTI

Planned future terrestrial telescopes are even bigger:

30 meter telescope: http://www.tmt.org/
Giant Magellan Telescope: http://www.gmto.org/
40 meter European Extremely Large Telescope: http://www.eso.org/outreach/press-rel/pr-2006/pr-46-06.html

It's a lot cheaper to build things on earth than in space. Any type of space-fabricated mirror or lens would only be a small part of the overall system.

It's technically possible to build a 100 meter terrestrial telescope, and this was studied as the Overwhelmingly Large Telescope: http://en.wikipedia.org/wiki/Overwhelmingly_Large_Telescope

The OWL wasn't pursued because of the projected cost, at 1.5 billion euros. However this is still less than HST and the associated servicing missions.

Thus any future space-based telescope must compete with future terrestrial telescopes. Light-gathering power has always been a strong point of terrestrial scopes. As adaptive optics and imaging interferometry are further improved, they increasingly compete in resolution.

That leaves spectral sensitivity in the atmospherically-blocked regions as the remaining main advantage of space telescopes. This is a powerful advantage, but doesn't require zero-G-fabricated mirrors or lenses.

satori
2007-Feb-13, 12:57 PM
argos,
I diagonaly read thru the thread you provided (thanks) and will go there back again...
your membrane approach comes (a little) close to the currently aplyed method of prodding up the relatively thin mirrors of the giant telescopes of the last generation with actively controlled support elements (all well known, I know)... did you notice "some similarity" here ?

Argos
2007-Feb-13, 01:06 PM
Frankly, no. :)

satori
2007-Feb-13, 01:18 PM
joema,
the examples you brought are just peanuts , compared to what I am capable to imagine (similes disenabled)...

No, actualy am I under the impression myself (laylaylay-perspective), that groundbound telescopes are the better (cheaper !!) option currently ( I read though, that "atmospheric flimmer" correction was still a hard one for really big telescopes).
Anyway, thanks for the link list !

bytheby : the question still standing unanswered would be:
If you had an awfuly bad but also awfuly large mirror, what would/could you be using that for ??
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think big !

satori
2007-Feb-13, 01:32 PM
frankly, no
Argos, the large "thin" mirrors of today's use, could surely be looked upon as the "upper thickness level" of a membrane (goodwill presumed!). The complexety of the controll techniques employed point to a high level of practical dificulties in the actual realisation of this approach...
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the devil is allways in the details

Argos
2007-Feb-13, 01:47 PM
My concept does not invove adaptive optics. It´s simply a passive mirror. I`m not sure about its feasibility. The gas pressure must be constant and the film deformation must come within very narrow limits.