Spin-cast a mirror in space?

There's been discussion about liquid mirror space telescopes.

a perennial issue is that in addition to centrifugal force, you have to keep a constant "downward" force. so for instance you spin it along one axis, and then rotate it around another (e.g. a big counterweight on the antenna). but this means the this means the telescope is now scanning and can't focus on a point.

but... if you could somehow solidify the liquid lens - or at least the surface of it (could be e.g. just a thin silver coating) - in space, then you wouldn't need to maintain a constant force. you'd only need to maintain the force long enough for the surface to "dry".

you could use a foldable tarp as the "casting mold", so you could make a really really big mirror.

it seems to me the biggest obstacle here is finding the right materials and process solidify the surface.

spin-casting the mirror in space. could dramaticaly increase the size and decrease both the weight and the cost.

Dear happy...:

On Wednesday, May 11, 2016 at 11:36:38 AM UTC-7, wrote:
There's been discussion about liquid mirror
space telescopes.

Mirrors make sense on Earth, because they are relatively rigid to changes in orientation, and you only have one surface to control the quality of. Lenses make more sense in space, I think, because they are no longer a problem to support. Chromatics aside...

a perennial issue is that in addition to
centrifugal force, you have to keep a constant
"downward" force. so for instance you spin it
along one axis, and then rotate it around another
(e.g. a big counterweight on the antenna).

So you have two-axis rotation, which is inherently unstable.

but this means the this means the telescope is
now scanning and can't focus on a point.

Lots of spin-to-scan technologies exist, and conversion to raster images is not a problem either.

but... if you could somehow solidify the
liquid lens - or at least the surface of it
(could be e.g. just a thin silver coating) -
in space, then you wouldn't need to maintain
a constant force. you'd only need to maintain
the force long enough for the surface to "dry".

You could make truly spherical lenses in space without spinning, or oblate spheroids. And likely much less problem with dissolved gases when forming them.

you could use a foldable tarp as the "casting
mold", so you could make a really really big
mirror.

But the inherent instability of two axis rotation (albeit with little friction), might be a big problem.

it seems to me the biggest obstacle here is
finding the right materials and process
solidify the surface.

No, two-axis rotation becomes a problem too.

spin-casting the mirror in space. could
dramaticaly increase the size and decrease
both the weight and the cost.

Maybe.

David A. Smith

On Wednesday, May 11, 2016 at 2:24:41 PM UTC-5, dlzc wrote:
Dear happy...:

On Wednesday, May 11, 2016 at 11:36:38 AM UTC-7, wrote:
There's been discussion about liquid mirror
space telescopes.

Mirrors make sense on Earth, because they are relatively rigid to changes in orientation, and you only have one surface to control the quality of. Lenses make more sense in space, I think, because they are no longer a problem to support. Chromatics aside...

a perennial issue is that in addition to
centrifugal force, you have to keep a constant
"downward" force. so for instance you spin it
along one axis, and then rotate it around another
(e.g. a big counterweight on the antenna).

So you have two-axis rotation, which is inherently unstable.

but this means the this means the telescope is
now scanning and can't focus on a point.

Lots of spin-to-scan technologies exist, and conversion to raster images is not a problem either.

but... if you could somehow solidify the
liquid lens - or at least the surface of it
(could be e.g. just a thin silver coating) -
in space, then you wouldn't need to maintain
a constant force. you'd only need to maintain
the force long enough for the surface to "dry".

You could make truly spherical lenses in space without spinning, or oblate spheroids. And likely much less problem with dissolved gases when forming them.

you could use a foldable tarp as the "casting
mold", so you could make a really really big
mirror.

But the inherent instability of two axis rotation (albeit with little friction), might be a big problem.

it seems to me the biggest obstacle here is
finding the right materials and process
solidify the surface.

No, two-axis rotation becomes a problem too.

spin-casting the mirror in space. could
dramaticaly increase the size and decrease
both the weight and the cost.

Maybe.

David A. Smith

you could also use a constant linear acceleration for the "down" force, e.g.. through an ion thruster. that way you'd only have one-axis rotation.

more i think about it though you might need something more rigid as a tarp, so as not to cause too much deformation with the "down" force. maybe keeping it tight, maybe having support "spokes". i'm sure some compromise can be found there between rigidity and size/weight.

but anyways, yeah... you can just use constant linear thrust for the down force, that way you'd only need one-axis rotation.

another approach might be to blow a bubble, but that may be tricky with almost zero external pressure. then if you solidify the bubble or something inside the bubble, you can just cut the bubble in half (in the first case) or pop it (in the second). though not sure how easy it'd be to get anything close to parabolic, that way.

I wonder what the fluid would look like in a two-axis rotation system. That
is, you would have the fluid rotating around a vertical axis through the
center. But also have the entire spacecraft subjected to a centrifugal
rotation via a connection to a tether with a counterweight at the other end,
as with proposals to simulate gravity with long space missions.

To simulate this on Earth you would probably have to do it in a zero-g,
i.e., parabolic arc, aircraft. The presence of gravity would likely ruin the
actual effect of the double rotation.

Bob Clark


----------------------------------------------------------------------------------------------------------------------------------
Finally, nanotechnology can now fulfill its potential to revolutionize
21st-century technology, from the space elevator, to private, orbital
launchers, to 'flying cars'.
This crowdfunding campaign is to prove it:

Nanotech: from air to space.
https://www.indiegogo.com/projects/n...ce/x/13319568/
----------------------------------------------------------------------------------------------------------------------------------
wrote in message
...

On Wednesday, May 11, 2016 at 2:24:41 PM UTC-5, dlzc wrote:
Dear happy...:

On Wednesday, May 11, 2016 at 11:36:38 AM UTC-7, wrote:
There's been discussion about liquid mirror
space telescopes.

Mirrors make sense on Earth, because they are relatively rigid to changes
in orientation, and you only have one surface to control the quality of.
Lenses make more sense in space, I think, because they are no longer a
problem to support. Chromatics aside...

a perennial issue is that in addition to
centrifugal force, you have to keep a constant
"downward" force. so for instance you spin it
along one axis, and then rotate it around another
(e.g. a big counterweight on the antenna).

So you have two-axis rotation, which is inherently unstable.

but this means the this means the telescope is
now scanning and can't focus on a point.

Lots of spin-to-scan technologies exist, and conversion to raster images
is not a problem either.

but... if you could somehow solidify the
liquid lens - or at least the surface of it
(could be e.g. just a thin silver coating) -
in space, then you wouldn't need to maintain
a constant force. you'd only need to maintain
the force long enough for the surface to "dry".

You could make truly spherical lenses in space without spinning, or oblate
spheroids. And likely much less problem with dissolved gases when forming
them.

you could use a foldable tarp as the "casting
mold", so you could make a really really big
mirror.

But the inherent instability of two axis rotation (albeit with little
friction), might be a big problem.

it seems to me the biggest obstacle here is
finding the right materials and process
solidify the surface.

No, two-axis rotation becomes a problem too.

spin-casting the mirror in space. could
dramaticaly increase the size and decrease
both the weight and the cost.

Maybe.

David A. Smith

you could also use a constant linear acceleration for the "down" force, e.g.
through an ion thruster. that way you'd only have one-axis rotation.
more i think about it though you might need something more rigid as a tarp,
so as not to cause too much deformation with the "down" force. maybe
keeping it tight, maybe having support "spokes". i'm sure some compromise
can be found there between rigidity and size/weight.
but anyways, yeah... you can just use constant linear thrust for the down
force, that way you'd only need one-axis rotation.
another approach might be to blow a bubble, but that may be tricky with
almost zero external pressure. then if you solidify the bubble or something
inside the bubble, you can just cut the bubble in half (in the first case)
or pop it (in the second). though not sure how easy it'd be to get anything
close to parabolic, that way.

---

Dear Robert Clark:

On Sunday, May 15, 2016 at 7:54:35 AM UTC-7, Robert Clark wrote:
I wonder what the fluid would look like in a two-axis
rotation system.

That is how very large parabolic mirrors are produced on Earth, using gravity in place of he second spin axis.

That is, you would have the fluid rotating around a
vertical axis through the center.

The problem is, it is unstable, and unstable in the cooling time of most liquids. Additionally, with a short "major spin arm", you'd not end up with a section of a parabola, I wouldn't think.

But also have the entire spacecraft subjected to
a centrifugal rotation via a connection to a
tether with a counterweight at the other end,
as with proposals to simulate gravity with long
space missions.

To simulate this on Earth...

No point in trying, because on Earth, we already make mirrors and mirror panels this way.

David A. Smith

That is, you would have the fluid rotating around a
vertical axis through the center.

The problem is, it is unstable, and unstable in the cooling time of most liquids. Additionally, with a short "major spin arm", you'd not end up with a section of a parabola, I wouldn't think.

short major spin arm... i presume you're alluding to that centrifugal force is not uniform from center to edge, but increases ?linearly?... hence you want a large ratio between the mirror thickness and the major spin arm length, so that the low point of the mirror experiences the same force as the high point.

not sure how significant this would be or how much it could be compensated, but if you use constant linear acceleration in place of the major spin arm, you avoid the issue altogether.

Dear happy..."

On Sunday, May 15, 2016 at 3:55:17 PM UTC-7, wrote:
That is, you would have the fluid rotating around a
vertical axis through the center.

The problem is, it is unstable, and unstable in
the cooling time of most liquids. Additionally,
with a short "major spin arm", you'd not end up
with a section of a parabola, I wouldn't think.

short major spin arm... i presume you're alluding
to that centrifugal force is not uniform from center
to edge, but increases ?linearly?...

But, the rotational center of the "gravitational axis" (if you will) would be far from "infinite", or even 6300 km as it is here on Earth.

hence you want a large ratio between the mirror
thickness and the major spin arm length,

No, I think mirror *diameter*, and the radius of the major spin arm.

so that the low point of the mirror experiences
the same force as the high point.

Not worried about the force, worried about the local direction of that force.

not sure how significant this would be

.... it wouldn't be parabolic, I believe, so would not do what you'd hope without significant correction.

or how much it could be compensated, but if
you use constant linear acceleration in place
of the major spin arm, you avoid the issue
altogether.

That would be the best idea, unless we figure out how to build a space elevator, and then the anchor point would be an even better major spin arm. And you'd not have to have enough jets to turn it around and bring it back...

David A. Smith

Yes, that is how large mirrors are made on Earth. But the massive size of
the support equipment needed because of the Earth's gravity make them
impractical on Earth beyond a certain size for a single mirror, about 8
meters for a single mirror. Larger telescopes instead are made segmented.

The advantage of doing it in space though is you have zero gravity so you
would not need the massive support structures.

But you are correct about the instabilities. This is illustrated in this
video:

Torque free motion of a prolate axi-symmetric rigid body.
https://www.youtube.com/watch?v=s9wiRjUKctU

What's happening is when you have two rotations around axes both through a
common center, the result is a rotation around an axis at a diagonal line
between the two. It then has the appearance of gyrating wildly, while what
is really happening is it is rotating around an axis that is not on an axis
of symmetry of the body.

BTW, this is not exactly the same effect but it is cool:

Watch: WTF is going on with this object spinning in zero gravity?
Go home, physics. You are drunk.
BEC CREW 21 AUG 2015
http://www.sciencealert.com/watch-wt...n-zero-gravity

For our scenario, it probably could work to apply some restoring force to
maintain both rotations separately. But likely this force would be so large
that you might as well have applied a linear acceleration.

The reason why I was considering the centrifugal force case instead of using
linear acceleration is that this would require a rocket thruster operating
for a long period, perhaps weeks. But if this was by chemical propulsion the
propellant required would be prohibitive. On the other hand, if you used
electric propulsion such as ion thrusters, this would be an extremely small
thrust and acceleration.


Bob Clark

----------------------------------------------------------------------------------------------------------------------------------
Finally, nanotechnology can now fulfill its potential to revolutionize
21st-century technology, from the space elevator, to private, orbital
launchers, to 'flying cars'.
This crowdfunding campaign is to prove it:

Nanotech: from air to space.
https://www.indiegogo.com/projects/n...ce/x/13319568/
----------------------------------------------------------------------------------------------------------------------------------
"dlzc" wrote in message
...

Dear Robert Clark:

On Sunday, May 15, 2016 at 7:54:35 AM UTC-7, Robert Clark wrote:
I wonder what the fluid would look like in a two-axis
rotation system.
That is how very large parabolic mirrors are produced on Earth, using
gravity in place of he second spin axis.
That is, you would have the fluid rotating around a
vertical axis through the center.
The problem is, it is unstable, and unstable in the cooling time of most
liquids. Additionally, with a short "major spin arm", you'd not end up with
a section of a parabola, I wouldn't think.
But also have the entire spacecraft subjected to
a centrifugal rotation via a connection to a
tether with a counterweight at the other end,
as with proposals to simulate gravity with long
space missions.

To simulate this on Earth...
No point in trying, because on Earth, we already make mirrors and mirror
panels this way.

David A. Smith
---

On 5/16/2016 9:49 PM, Robert Clark wrote:
Yes, that is how large mirrors are made on Earth. But the massive size
of the support equipment needed because of the Earth's gravity make them
impractical on Earth beyond a certain size for a single mirror, about 8
meters for a single mirror. Larger telescopes instead are made segmented.

The advantage of doing it in space though is you have zero gravity so
you would not need the massive support structures.


with no gravity, no parabolic shape,
all your support stuff has to be in space too
and be enclosed to prevent.....

Found an interesting article after a Google search:

The shape of a liquid surface in a uniformly rotating cylinder in the
presence of surface tension.
http://maeresearch.ucsd.edu/~vlubard...s/Acta2013.pdf

This calculates the shape of the meniscus under both gravity and zero
gravity.

A topic I'm interested in is whether the method of making large parabolic
mirrors on Earth by rotating the glass in molten form to form a parabolic
meniscus then allowing it to solidify can also work in space.

This will have an advantage over transporting the already formed mirrors
into space because for large mirrors you have to concerned about the size of
the rocket fairing. But in fact in zero g you would have an advantage in
that you wouldn't have to worry about the mass and cost of the support
structures and of the mirror deforming under it's own weight.

You could emulate the Earth's gravity during the formation stage in space
by using either centrifugal force due to rotation around a second axis or by
using linear acceleration. Rotation around a second axis though could create
instabilities. On the other hand doing a linear acceleration for the weeks
of cooling time would require a prohibitive amount of propellant.

That is why I wondered if it is possible to do in zero gravity just using a
rotation around a single axis as on Earth. In the article ther were able to
only solve numerically the equations for the zero gravity case. So my
questions is, is it possible to set the starting parameters such that the
meniscus shape is a good approximation to parabolic? Note it would also be
sufficient to get a good approximation to a spherical surface since then you
can use a combination of spherical mirrors to cancel out the distortions due
to a non-parabolic surface:

Spherical Aberration.
https://starizona.com/acb/basics/equ...spherical.aspx

It may be possible to get it to work no matter the shape of the curved
meniscus by using a mirror of similar shape to cancel out the aberrations
due to the non-parabolic shape. For instance, the Hubble uses a combination
of hyperbolic mirrors to cancel out aberrations.



Bob Clark


----------------------------------------------------------------------------------------------------------------------------------
Finally, nanotechnology can now fulfill its potential to revolutionize
21st-century technology, from the space elevator, to private, orbital
launchers, to 'flying cars'.
This crowdfunding campaign is to prove it:

Nanotech: from air to space.
https://www.indiegogo.com/projects/n...ce/x/13319568/
----------------------------------------------------------------------------------------------------------------------------------
"Robert Clark" wrote in message ...

Yes, that is how large mirrors are made on Earth. But the massive size of
the support equipment needed because of the Earth's gravity make them
impractical on Earth beyond a certain size for a single mirror, about 8
meters for a single mirror. Larger telescopes instead are made segmented.

The advantage of doing it in space though is you have zero gravity so you
would not need the massive support structures.

But you are correct about the instabilities. This is illustrated in this
video:

Torque free motion of a prolate axi-symmetric rigid body.
https://www.youtube.com/watch?v=s9wiRjUKctU

What's happening is when you have two rotations around axes both through a
common center, the result is a rotation around an axis at a diagonal line
between the two. It then has the appearance of gyrating wildly, while what
is really happening is it is rotating around an axis that is not on an axis
of symmetry of the body.

BTW, this is not exactly the same effect but it is cool:

Watch: WTF is going on with this object spinning in zero gravity?
Go home, physics. You are drunk.
BEC CREW 21 AUG 2015
http://www.sciencealert.com/watch-wt...n-zero-gravity

For our scenario, it probably could work to apply some restoring force to
maintain both rotations separately. But likely this force would be so large
that you might as well have applied a linear acceleration.

The reason why I was considering the centrifugal force case instead of using
linear acceleration is that this would require a rocket thruster operating
for a long period, perhaps weeks. But if this was by chemical propulsion the
propellant required would be prohibitive. On the other hand, if you used
electric propulsion such as ion thrusters, this would be an extremely small
thrust and acceleration.


Bob Clark

----------------------------------------------------------------------------------------------------------------------------------
Finally, nanotechnology can now fulfill its potential to revolutionize
21st-century technology, from the space elevator, to private, orbital
launchers, to 'flying cars'.
This crowdfunding campaign is to prove it:

Nanotech: from air to space.
https://www.indiegogo.com/projects/n...ce/x/13319568/
----------------------------------------------------------------------------------------------------------------------------------
"dlzc" wrote in message
...

Dear Robert Clark:

On Sunday, May 15, 2016 at 7:54:35 AM UTC-7, Robert Clark wrote:
I wonder what the fluid would look like in a two-axis
rotation system.
That is how very large parabolic mirrors are produced on Earth, using
gravity in place of he second spin axis.
That is, you would have the fluid rotating around a
vertical axis through the center.
The problem is, it is unstable, and unstable in the cooling time of most
liquids. Additionally, with a short "major spin arm", you'd not end up with
a section of a parabola, I wouldn't think.
But also have the entire spacecraft subjected to
a centrifugal rotation via a connection to a
tether with a counterweight at the other end,
as with proposals to simulate gravity with long
space missions.

To simulate this on Earth...
No point in trying, because on Earth, we already make mirrors and mirror
panels this way.

David A. Smith
---


---
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