Mercury Mirror Space Telescope

Michael Bloxham · 2006-09-17 01:02:51

Here's an idea I came up with a few years ago after reading an article on mercury mirrors. I haven't seen this anywhere else yet so I'll release the idea into the public domain. Don't forget where you read it first!

Tethered Mercury Mirror Space Telescope:

By using artificial gravity, the telescope is free to view any location in space with respect to earth.

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Spinning the telescope on a long tether creates a centripetal force which holds the liquid mercury against a circular dish.

The tether is very long so that the rate of spin is extremely slow, thus allowing longer exposures before seeing the effects of motion blur. Also, the tether system will have to be designed so that it does not intefere with the telescopes line of sight.

The circular dish containing the liquid mercury is then spun axially (by either spinning it on air bearings or spinning the whole telescope assembly) at a rate required to attain the desired concave mirror shape.

The wole assembly can be moved about slowly with a small reaction control system to allow it to image any location in space at any time.

noosfractal · 2006-09-17 02:07:27

The whole assembly can be moved about slowly with a small reaction control system to allow it to image any location in space at any time.

The "any position at any time" claim seems to be contradicted by the requirement that the "slow" rotation in the plane containing the tethers remain constant. Say I want to look at something between the two counterweights, don't I have to wait for a full rotation before I can see it again?

Also, won't the repositioning movements cause ripples on the mirror due to acceleration and deceleration?

Michael Bloxham · 2006-09-17 03:00:49

It will be slow, your right, but unless your hunting for supernovae this probably isn't much of an issue. I imagine the rotation could be slowly tweaked over time to coincide with your desired image points. In any case its a whole lot better than pointing straight up all the time. I wonder about any potential rippling effect, but you could probably just wait a while for the mercury to settle before imaging.

Michael Bloxham · 2006-10-11 00:30:05

Has anyone thought of this before, or is it a piece of genious?

noosfractal · 2006-10-11 01:41:46

Has anyone thought of this before, or is it a piece of genious?

Well now, novelty need not be a prerequisite for genius.

You might be interested in this sci.astro thread ...

http://groups.google.com/group/sci.astr … d048ba6c4a

Michael Bloxham · 2006-10-18 04:05:27

Thats cool that you found that. Those guys do seem to argue a lot, but reading through it has showed me a few potential problems.

First let me say that the guy who actually mentioned something similar to my proposal above was virtually ignored, or at least misunderstood. Most of the posts appeared to argue whether it is possible to produce a suitable parabola shape of mercury within a satellite subjected to axially spin only, which would be pointless as the problem of lack of gravity can be solved by replacing it with centripetal acceleration.

One guy mentions that the resulting shape of mercury produced in a dish subjected to two axes of rotation (like the one I propose) would be 'astigmatic' in the direction of rotation. This would best be imagined by visuallizing a flat dish of mercury, without axial spin: The mercury would assume a curve with a radius equal to the centre of rotation, i.e. somewhere in space between the dish and the counterweight. However, I suggest that if the tether is made very long (kilometers, maybe hundreds), the radius of curvature would be so large that the mercury would be very nearly flat. Flat enough to produce a good image?

A very long tether would simultaneously slow down your rate of rotation, allowing longer exposures. Also, the centripetal acceleration would only need to be very small; just enough to hold the mercury against the dish.

P.S.: When I read how it would be impossible to produce a parabolic shape when only axial spin is considered, as the mercury would quickly assume a cylindrical shape, I quickly thought of this solution: How about continuosly pumping the mercury over a slowly spinning parabolic shape, so that the mercury enters in the centre, naturally flowing outwards over the parabola and towards its periphery. The mercury would be collected from the sides of the cylindrical container, and pumped behind the parabola so that it enters in the centre again. Pointless but possible? A clean flow would be required, but again because the rate of rotation need only be very small the mercury might only need to flow very slowly.

Michael Bloxham · 2006-10-18 04:31:32

Heh, one guy mentions moving the CCD across the focal point would minimize blurring due to motion. Cool. (although doing this would also reduce the viewing angle in one direction.)

Michael Bloxham · 2006-12-26 05:58:01

Perhaps a 'gravity-peak'-type Lagrange-point location can be utilized: If the tether is sufficiently long, a small amount of gravity will tug on the mercury dish, while the gravity that tugs on the counterweight will keep the dish from floating away. Thus the centre of gravity of the system will be kept balanced over the lagrange point. Using this method, rather than a rotating tether, will allow complete freedom to image any point in space (although perhaps not completely perpendicular to the gravity fields?), and allow extremely long exposure times.

Michael Bloxham · 2007-05-21 07:36:51

C'mon space dudes. Am I a genius or what?

noosfractal · 2007-05-21 08:05:54

I don't think you get the genius designation until you show a design that beats the LLMT by an order of magnitude or two ...

A Lunar Liquid Mirror Telescope (LLMT) for deep-field infrared observations near the lunar pole
http://www.optics.arizona.edu/loft/Publ … 206265.pdf

You might though. They've got dust issues.

Michael Bloxham · 2007-06-11 06:18:31

Im surprised that this idea (so far) appears to be new. If that is the case, the concept should be spread. I would have added it to wikipedia, but their policy does not allow it. Could someone please add it for me?

http://en.wikipedia.org/w/index.php?tit … ction=edit

Michael Bloxham · 2007-06-13 22:08:27

Well, I guess if no one else will do it, I'll just create a stub for it myself. If they don't like it, they can take it down.

Michael Bloxham · 2007-06-13 22:36:06

Here is the link for the page: http://en.wikipedia.org/wiki/Mercury_mi … _telescope

You can see that I've added a link to this thread as reference. I know I probably shouldn't add a link to my own work, but we'll see what happens.

Michael Bloxham · 2007-06-14 21:57:07

The other way to induce artificial gravity would be to accelerate the mercury dish forward (toward its image target) using small rockets. These would have to be very smooth though. Perhaps some sort of electric propulsion method would suffice (or perhaps even a solar sail?).

The acceleration period would only need to be as long as the desired exposure length, and the amount of acceleration would be very small; just enough to hold the liquid mercury in the dish.

The benefit to this method is that the exposure times can be much longer, as there is no relative motion between the telescope and its image point. However, I'm not sure if this would be the case if the telescope is in earth orbit. Perhaps corrections can be made to keep it pointing in the right direction, or would this cause ripples in the mercury? This might not be as much of a problem if the telescope is in solar orbit, or if it is situated in a suitable Lagrange point. Also, there should be no problem with coriolis forces using this method.

Michael Bloxham · 2007-06-14 22:21:59

I've made the assumption that if the tether is extremely long, and the rotation rate extremely slow, that any Coriolis effects acting on the spinning dish of mercury will be negligible.

Can anyone tell me if this is a sound assumption?

I'm also guessing that if the coriolis effects are small enough, the resulting blurring effects will also be small. Perhaps the secondary mirror or camera lenses can be shaped to correct for this. The slight astigmatism in the direction of rotation may also be corrected in the same way.

Perhaps there is a possibility (though unlikely) that some of these effects may actually work with each other to improve the accuracy of the telescope. For example, perhaps the astigmatism in the direction of rotation may work with the effects of motion blur and the effects of moving the CCD (or the secondary mirror), in such a way that these effects, taken together, increase the accuracy of the telescope? Perhaps the coriolis effects may fit in somewhere in the scheme of things as well?

Michael Bloxham · 2007-06-14 22:42:50

Getting back to the original problem of earth-based mercury telescopes only being able to point straight up: Perhaps the whole telescope can be briefly accelerated in a horizontal direction to allow the telescope to be tilted slightly? Its horizontal motion would have to be very smooth, and unless you had a very large area in which to operate it, these accelerations would have to be very small and very brief, resulting in short exposure opportunities.

However, for a very precise terrestrial mercury mirror telescope, perhaps such a brief ability to tilt would help to counteract the motion-blur effects of earths rotation? This could work in conjunction with moving the CCD across the focal point; further increasing the telescopes precision.

Michael Bloxham · 2007-06-14 23:42:08

Another mercury-mirror related idea would be to use a tethered mercury mirror space telescope in a "Tidal Stabilization" arrangement (http://en.wikipedia.org/wiki/Space_teth … bilization), where the tether is always held perpendicular to the earths surface.

In earth orbit, with the telescope pointing away from the earth, any point in space can be imaged by tweaking the orientation of the telescopes orbit (anywhere between equatorial and polar). If the orbital altitude is high enough, the effects of motion blur should be kept minimal; allowing long exposure times.

If the center of the tether system is in geosynchrous orbit, the time it takes for the same image point to return will be one day. If the orbit is lower, the time will be less, but the relative motion speed will be higher, and vice-versa. I'm not sure if it is the same for non-equatorial orbits.

If the telescope is pointed towards the earth, I can imagine that this same method would work extremely well for a spy telescope. One in geosynchrous orbit would be able to image the same location at super high resolutions in real-time for years.

Edit: For clarification, I believe the effect that allows this arrangement to work is also responsible for the "Tidal locking" effect that causes the moon to face the earth.

Also, reading through the Sci-Astro thread, one guy mentions that a mercury mirror telescope has the advantage that the focal length can be changed on the fly (assumably, by varying spin-rate of the mercury dish).

Michael Bloxham · 2007-06-15 00:30:52

Here's another idea I just thought of. Remember when I suggested that accelerating a terrestrial mercury mirror telescope horizontally would allow the telescope to be tilted briefly? That idea probably wouldn't work very well on earth, because the gravity is very high. For example, if I wanted to tilt my mercury telescope 45-degrees, it would require a horizontal acceleration of 1g to be sustained for as long as the required exposure time. Its obvious that that wouldn't work if the exposure time is anything more than a few tenths of second.

However, for a mercury mirror telescope situated on the moon, the horizontal acceleration required to tilt the telescope at 45-degrees is the same as the moons gravity (1.62 m/s2 instead of 9.81 m/s^2). This might be reduced enough to allow this method to be useful. Perhaps the telescope can be situated on an even lighter body, such as an asteroid.

Another thought is that a hybrid thrust-induced and rotation-induced acceleration system may be possible (i.e. using rocket thrust to allow a tether-based mercury mirror space telescope to point at an angle in a similar way to the system described above). Because a tethered mercury mirror telescope only induces a tiny centripetal force, only an equivalently tiny horizontal acceleration would be needed for it to tilt at an angle. The ability to tilt could also be used to reduce the effects of motion blur.

Sloshing effects might be a problem though...

RGClark · 2007-06-15 11:06:56

Michael, in that sci.astro thread I cited a reference you might want to consult for the different shapes the liquid would achieve during rotation in zero-g:

Fundamentals of Low Gravity Fluid Dynamics and Heat Transfer.
Antar, Basil and Vappu: Boca Raton: CRC Press Inc., 1993.

I don't remember though if it considered rotations aroung two different axes at once, which your original proposal would require.

Bob Clark

Michael Bloxham · 2007-06-15 14:27:02

Thanks Bob (can I call you Bob?). I wonder about liquid mirror space telescopes which don't use artificial gravity, but instead rely on surface tension or pressure effects. Do you know of any other sources of information besides the book and the sci.astro thread?

Was there any further discussion after that sci.astro thread?

RGClark · 2007-06-19 12:36:29

Bob is fine. I'm hardly an expert in this field. I looked though that book you mentioned when I had an idea I wanted to investigate the possibility. It was a few years ago. I haven't looked at it in awhile.
You might try a web search on "surface tension" , "zero-g" , and rotation.

Bob Clark

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#1 2006-09-17 01:02:51

Re: Mercury Mirror Space Telescope

#2 2006-09-17 02:07:27

Re: Mercury Mirror Space Telescope

#3 2006-09-17 03:00:49

Re: Mercury Mirror Space Telescope

#4 2006-10-11 00:30:05

Re: Mercury Mirror Space Telescope

#5 2006-10-11 01:41:46

Re: Mercury Mirror Space Telescope

#6 2006-10-18 04:05:27

Re: Mercury Mirror Space Telescope

#7 2006-10-18 04:31:32

Re: Mercury Mirror Space Telescope

#8 2006-12-26 05:58:01

Re: Mercury Mirror Space Telescope

#9 2007-05-21 07:36:51

Re: Mercury Mirror Space Telescope

#10 2007-05-21 08:05:54

Re: Mercury Mirror Space Telescope

#11 2007-06-11 06:18:31

Re: Mercury Mirror Space Telescope

#12 2007-06-13 22:08:27

Re: Mercury Mirror Space Telescope

#13 2007-06-13 22:36:06

Re: Mercury Mirror Space Telescope

#14 2007-06-14 21:57:07

Re: Mercury Mirror Space Telescope

#15 2007-06-14 22:21:59

Re: Mercury Mirror Space Telescope

#16 2007-06-14 22:42:50

Re: Mercury Mirror Space Telescope

#17 2007-06-14 23:42:08

Re: Mercury Mirror Space Telescope

#18 2007-06-15 00:30:52

Re: Mercury Mirror Space Telescope

#19 2007-06-15 11:06:56

Re: Mercury Mirror Space Telescope

#20 2007-06-15 14:27:02

Re: Mercury Mirror Space Telescope

#21 2007-06-19 12:36:29

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