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tim53
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Re: Trends, future trends, and "should be dones"... new [Re: Jeff Morgan]
      #5328109 - 07/21/12 11:35 AM

Suspend a monolayer of aluminum in a tuned magnetic field?

-Tim.


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tim53
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Re: Trends, future trends, and "should be dones"... new [Re: Jeff Morgan]
      #5328110 - 07/21/12 11:36 AM

But my wish for a near-future trend would be inexpensive, intensified CCD cameras.

-Tim.


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careysub
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Re: Trends, future trends, and "should be dones"... new [Re: Jeff Morgan]
      #5328201 - 07/21/12 12:33 PM

Quote:

A substrate that can take a polish like glass, weighs as much as carbon fiber, and rigid enough to hold shape without an elaborate cell.

The mirror and the cell are to the two highest density/highest weight components in telescopes. Find a substrate to replace glass and then eliminate the cell - that would be game-changer.




Hmmm... not sure the material you describe would be as big a breakthrough as you might think. The key parameter here is "specific stiffness", the ratio of modulus to density. Using the units employed by Table 6 from "The design and construction of large optical telescopes", (p. 139, 2003), which are GPa/(kg/m^3), the stiffness of Pyrex is 0.032 and Zerodur is 0.036, while bi-directional carbon fiber/epoxy specimens range from about 0.040 to 0.055 depending on details of manufacture. This is an improvement on the order of 50% (Pyrex to mid-range CF, or Zerodur to top performing CF), which would help but not really be a radical game changer by itself. (The book has the two extremely costly exotics silicon carbide and beryllium holding pride of the show at 0.146 and 0.162).

Now what would be more radical is if it were economical to get away from the constraints of a homogeneous flat mirror - meniscus mirrors, hollow core, foam core, ribbed mirrors, or other designs that create hollow frames of some sort. Could a composite type of mirror substrate help there?

I would say the Hubble Optics has made a stab in that direction with their low-cost sandwich mirrors, though their sandwich technique is fairly crude. There is quite a thick forest of glass columns on the two halves of the mirror blank, which in my mirror don't match up very well - I think it is over-designed using extra material to substitute for finesse, a more careful process should be able to get much better weight savings.

And there is that StarStone project out there working on foamed mirrors. They say they are shipping 18" and 24" blanks and "light bucket" (worse than 1/4 wave) mirrors - has anyone seen one of these?

And on point 2 - the cell. What is a typical cell weight to mirror weight ratio? You can of course make a cell arbitrarily heavy if you like, but it seems to me that the cell need not be more than a fraction of the weight of the mirror since it is free to use its materials in a more efficiently stiff structure than the flat mirror (i.e. it should already have the structure we would like in an advanced mirror blank). An advanced mirror might do away with the need of a sophisticated cell, but the cell shouldn't be a big weight driver compared to the mirror itself.

What is the flexure tolerance in the cell anyway? It provides floating support points, which are designed to inherently adjust to distribute the load. The main problem would be collimation shift due to rotation of the support plane (and the mirror) relative to the secondary as the altitude changes, and only uneven changes in cell flex would cause this. Am I thinking about this correctly?


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Jon Isaacs
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Re: Trends, future trends, and "should be dones"... new [Re: careysub]
      #5328280 - 07/21/12 01:16 PM

Quote:

Hmmm... not sure the material you describe would be as big a breakthrough as you might think. The key parameter here is "specific stiffness", the ratio of modulus to density. Using the units employed by Table 6 from "The design and construction of large optical telescopes", (p. 139, 2003), which are GPa/(kg/m^3), the stiffness of Pyrex is 0.032 and Zerodur is 0.036, while bi-directional carbon fiber/epoxy specimens range from about 0.040 to 0.055 depending on details of manufacture. This is an improvement on the order of 50% (Pyrex to mid-range CF, or Zerodur to top performing CF), which would help but not really be a radical game changer by itself. (The book has the two extremely costly exotics silicon carbide and beryllium holding pride of the show at 0.146 and 0.162).





While the specific stiffness would allow for a lighter mirror, these are not the material properties which make glass and similar materials desirable as materials for mirrors. Composites inherently inhomogeneous and on a micro/nano scale have thermal CTE issues.

Jon


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careysub
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Re: Trends, future trends, and "should be dones"... new [Re: Jon Isaacs]
      #5328460 - 07/21/12 03:43 PM

Quote:

Quote:

Hmmm... not sure the material you describe would be as big a breakthrough as you might think. The key parameter here is "specific stiffness", the ratio of modulus to density. Using the units employed by Table 6 from "The design and construction of large optical telescopes", (p. 139, 2003), which are GPa/(kg/m^3), the stiffness of Pyrex is 0.032 and Zerodur is 0.036, while bi-directional carbon fiber/epoxy specimens range from about 0.040 to 0.055 depending on details of manufacture. This is an improvement on the order of 50% (Pyrex to mid-range CF, or Zerodur to top performing CF), which would help but not really be a radical game changer by itself. (The book has the two extremely costly exotics silicon carbide and beryllium holding pride of the show at 0.146 and 0.162).





While the specific stiffness would allow for a lighter mirror, these are not the material properties which make glass and similar materials desirable as materials for mirrors. Composites inherently inhomogeneous and on a micro/nano scale have thermal CTE issues.

Jon




Density and stiffness were the properties that Jeff Morgan cited though.

The "The design and construction of large optical telescopes" has a different figure of merit it uses to compare the best mirror materials - the thermal conductivity divided by the product of the specific heat, density (these two together are obviously the heat capacity per volume) and the coefficient of thermal expansion. By this figure of merit the best materials they rated were (in descending order): SiC, ULE (ultra-low expansion fused quartz), and Zerodur with everything else much farther back.

Curiously Zerodur (Astrosital, etc.) get their superior properties of zero thermal expansion because of nano-scale inhomogeneities, mixtures of different crystal structures, carefully tailored of course.


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Jon Isaacs
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Re: Trends, future trends, and "should be dones"... new [Re: careysub]
      #5330446 - 07/22/12 09:23 PM

Quote:

Density and stiffness were the properties that Jeff Morgan cited though.




This is what Jeff wrote:

"A substrate that can take a polish like glass, weighs as much as carbon fiber, and rigid enough to hold shape without an elaborate cell."

My point is simply that stiffness and density are important properties but are not sufficient for choosing a material. As Jeff said, "take polish like glass" and one would include thermal stability and homogeneity among others... That doesn't sound like any fiber composite I know of...

Jon


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tim53
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Re: Trends, future trends, and "should be dones"... new [Re: Jon Isaacs]
      #5330724 - 07/23/12 01:07 AM

Hm...

Why not spin cast "float glass" mirrors?: Float molten glass on molten tin, but spin it. When it cools, you've got a thin parabola.

-Tim.


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mark cowan
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Reged: 06/03/05

Loc: salem, OR
Re: Trends, future trends, and "should be dones"... new [Re: EyeSage]
      #5331602 - 07/23/12 04:02 PM

Quote:

Amazing an idea that's been around this long is a new idea to astronomers, appearing on a list of future trends alongside adaptive optics .






Indeed it is, using a dark shroud over your head in the dark - who'da thunk it?

Well, not many people, 'cause it caught on rapidly once first "observed".

Quote:

Why not spin cast "float glass" mirrors?: Float molten glass on molten tin, but spin it. When it cools, you've got a thin parabola.




I catch enough grief for molten pitch as it is. But though this sounds good and might get you in the outfield, the difference in density between the tin and the glass is going to result in a different shape on the glass...

Plus, well, the tin isn't hot enough to keep the glass molten, in making float glass the oven extrudes a layer onto the tin bath, where it floats, cools, and hardens.

Quote:

- Lightweight Ethos-like eyepieces.




+1. I would also like to see lightweight optical finders.

The use of carbon fiber and composites in telescope design will only grow.

Quote:

The mirror and the cell are to the two highest density/highest weight components in telescopes. Find a substrate to replace glass and then eliminate the cell - that would be game-changer.




Yes, but I'm now exploring ultra-thin fast mirrors (meniscus). And I have a very lightweight "fractal" cell design to support them (which I may patent).

But once you take that much weight away (talking a sub 4 lb 14.7" f/3 quartz mirror) even a recognizable ultralight dob design will never balance without adding counterweights (which unless they serve some other function like batteries I'd like to avoid). Fast f/ratios help here, but I'm still trying to crack a design barrier (and yes, double truss is nice...).

Quote:

...foamed mirrors...18"...has anyone seen one of these?




Have worked with one such blank.

Best,
Mark


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Trends, future trends, and "should be dones"... new [Re: mark cowan]
      #5331653 - 07/23/12 04:27 PM

Carbon fiber EQ and AZ mounts with carbon fiber tripods. Lightness of weight, yet super-high stiffness if constructed correctly for the material used. They may not look the same as the mounts we currently use.

Dobsonians with honeycomb-core carbon fiber rocker boxes and mirror boxes, carbon fiber poles and carbon fiber UTAs.

Secondary holders fine-adjustable with thumb screws in multiple dimensions: back-and-forth away from the focuser, up and down the tube, rotation along a center axis, and tilt in at least two axes.

Secondary spiders capable of accepting such high tension, and stiff enough, that 4-6" secondaries won't even move 0.001" over a full 90 degrees of altitude motion in the scope. We may have to re-think the way spiders are built and attached to do this.

Ultralight scopes designed to have the same amount of stiffness as a regular-weight scope.


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mark cowan
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Re: Trends, future trends, and "should be dones"... new [Re: Starman1]
      #5331668 - 07/23/12 04:34 PM


Quote:

Ultralight scopes designed to have the same amount of stiffness as a regular-weight scope.




Or simply better geometry (design) so forces are carried over shortest possible distances, allowing stiffness to be put to best use.

Ditto for spiders, I have a wire design that doesn't rely on tension for its stability. Hasn't been tested at large sizes yet, but a few people have built this so far.

Best,
Mark


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Pinbout
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Re: Trends, future trends, and "should be dones"... new [Re: mark cowan]
      #5331711 - 07/23/12 04:50 PM

i'd rather see my scope self-collimate.

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tim53
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Re: Trends, future trends, and "should be dones"... new [Re: mark cowan]
      #5331971 - 07/23/12 07:37 PM

Quote:


Quote:

Why not spin cast "float glass" mirrors?: Float molten glass on molten tin, but spin it. When it cools, you've got a thin parabola.




I catch enough grief for molten pitch as it is. But though this sounds good and might get you in the outfield, the difference in density between the tin and the glass is going to result in a different shape on the glass...




I don't think the density has anything to do with the shape: rotating furnaces link

Quote:

Plus, well, the tin isn't hot enough to keep the glass molten,




Precisely why it works - the tin is still molten when the glass has hardened.

Quote:

in making float glass the oven extrudes a layer onto the tin bath, where it floats, cools, and hardens.




Not so. The glass is melted at 1600C and floats, as a liquid, on the tin bath: Float glass manufacture




So, I'm envisioning something of a cross between the float glass manufacturing process and the Mirror Lab's spin casting process, whereby a rotating furnace with glass floating on a tin bath is rotated to produce a disk that has matching parabolas on its top and bottom surfaces and is, say, 1" thick and a meter in diameter, and only requires figuring. Well, that and mounting in a suitable cell! ...which is then mounted in a computer-controlled telescope in my observatory!

-Tim.


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Jeff Morgan
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Re: Trends, future trends, and "should be dones"... new [Re: mark cowan]
      #5332070 - 07/23/12 08:49 PM

Quote:

But once you take that much weight away (talking a sub 4 lb 14.7" f/3 quartz mirror) even a recognizable ultralight dob design will never balance without adding counterweights (which unless they serve some other function like batteries I'd like to avoid). Fast f/ratios help here, but I'm still trying to crack a design barrier (and yes, double truss is nice...).





Well if you could get the mirror that light ... why not add the counterweights forward of the diagonal mirror (as in a Springfield Newtonian) to raise the center of gravity coincident with the diagonal mirror. Say, 36"-40" off the ground. That would allow for a fixed height eyepiece at a normal sitting position. The only moving the observer would have to do would be in azimuth, and eyepiece weight would be negated.


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RobDob
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Loc: Brentwood (East Bay Area), CA
Re: Trends, future trends, and "should be dones"... new [Re: Jeff Morgan]
      #5332175 - 07/23/12 09:49 PM

Like the VLA, A network of CCD imagers spread around thousands of miles all taking an image at the same time, a central computer would use software to interpolate (fill in the gaps without the need for optics), thus generating the images from spaces between the CCD imagers, then combine some zillion images into one (accounting for location, parallax, distance, etc). You now have ~3000 mile aperture telescope! Think of the 3D effect from something like that!...

Edit: Big flaw here, would need a large portion of the earth to not have cloudy skies. Yikes! Oh well, maybe scale it down to 100 miles, or better yet, postion the CCD array around the clouds?

Another edit: Nevermind, I think this exceeds the realm of ATM

Rob

Edited by RobDob (07/23/12 10:30 PM)


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tim53
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Re: Trends, future trends, and "should be dones"... new [Re: RobDob]
      #5332354 - 07/23/12 11:50 PM

You could do that with just 2 telescopes, one on either side of the globe, pointing at the same object at the same time and looking at the same wavefront at the same time. Of course, it'd likely be day at one telescope if it's night at the other!

-Tim.


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RobDob
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Re: Trends, future trends, and "should be dones"... new [Re: tim53]
      #5332423 - 07/24/12 12:44 AM

Yeah Tim, it would be a logistical nightmare.

Thinking more about this, the sum of a gillion small aperture CCD's would still not yield a very large (in the order of many miles across) aperture. It would just be a collective of small aperture CCD's added together...

Oh well, it was fun to think about...

Rob


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tezster
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Re: Trends, future trends, and "should be dones"... new [Re: Starman1]
      #5332670 - 07/24/12 08:26 AM

Quote:

Carbon fiber EQ and AZ mounts with carbon fiber tripods. Lightness of weight, yet super-high stiffness if constructed correctly for the material used. They may not look the same as the mounts we currently use.

[snip]

Dobsonians with honeycomb-core carbon fiber rocker boxes and mirror boxes, carbon fiber poles and carbon fiber UTAs.





Being someone who's obsessed with telescope weight, these ideas sound fantastic to me, although it sounds quite expensive to implement.


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Dick Jacobson
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Loc: Plymouth, Minnesota, USA
Re: Trends, future trends, and "should be dones"... new [Re: Starman1]
      #5332725 - 07/24/12 09:06 AM

Quote:

Secondary spiders capable of accepting such high tension, and stiff enough, that 4-6" secondaries won't even move 0.001" over a full 90 degrees of altitude motion in the scope. We may have to re-think the way spiders are built and attached to do this.



How about using two spiders side-by-side, with a sufficiently strong rod connecting them and the secondary holder?


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Dick Jacobson
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Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Trends, future trends, and "should be dones"... new [Re: RobDob]
      #5332738 - 07/24/12 09:15 AM

See Hypertelescope for a proposal for an optical telescope larger than the Earth. Not exactly an ATM project, at least in this century.

Another mega-telescope idea is to send an array of detectors out to 550 A.U. and use the Sun's gravitational field as a lens.


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PierreDesvaux
Vendor (Dobsonian Factory)


Reged: 11/06/06

Loc: Paris, France
Re: Trends, future trends, and "should be dones"... new [Re: Dick Jacobson]
      #5333215 - 07/24/12 02:35 PM

Serge Vieillard made a few years ago an ultra-light 16" dobson with carbon composite: The weight of the structure is around 8 pounds, and the primary mirror 22 pounds.
He goes to Chile, Namibia, or Sahara desert with it.


http://www.astrosurf.com/magnitude78/T400/index.html

Concerning the question of balancing the scope when using an ultralight primary mirror, I wonder what coud be done using powerfull magnets between the sidebearings and the rocker. The magnets should not be in direct contact with the side-bearings, in order to avoid any stinction effect ... Did anybody try this ?

Concerning the ultralight mirrors, there are researches by amateurs on the principle of a thin layer of glass glued to a thick carbon foam. The carbon foam is very light and very rigid.


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