What's the Strangest Mirrored Telescope You've Ever Seen?

Try at least a 6 inch reflecting binocular. You can fold the light path so as to fit in between the tubes. I hear it's awesome. There are plans around. Check it out. The depth of field is great I hear. I haven't done it or even seen it done but would like to. good luck.

I had thought of making a pair of binoculars by cutting a 6 or 8 inch mirror in half. That way one gets identical primary focal lengths. Holding the mirrors apart one can get them to reflect forward onto an off axis flat in the middle of the tube (or pair of cassegrain convex mirrors) and back into the eye pieces

Hi! I've looked through a 13" reflecting binocular. Amazing! but it does have some inherent problems. Collimation (adjustment) of the 2 light paths is many times harder than with a conventional mono scope, and the observers position between the tubes with his back towards the object under observation isn't the best.

Finders are difficult, high powers also and the observers body heat can interfere with the seeing.

Have you tried one of Russell Porters "Springfield" mounts from the Scientific American?

Years ago a colleague of mine had one but while I knew him he never got it assembled. Nice observing position but has the problem of field rotation and image reversal because of 3 mirrors.

Binoculars won't work for me. I have monocular vision. Hasn't hindered me none. I've a second class medical from the FAA which qualifies me medically to fly wide bodied cargo jets. No depth perception related problems either.

But. . . . I would have a tough time justifying the design and testing of a device that requires binocular vision.

Thanks for the suggestion though.

Although I haven't seen nor built this type of telescope, a spinning liquid mirror looks interesting.

I know that they used to use a pool of mercury for a flat mirror pointing to the zenith, and it is possible to spin the bowl of mercury to shape it to approximately a parabola. You can change the focus by spinning it at different rotational velocity.

Mercury is not a nice substance to handle, and lately when I had an application to align a direction indication laser on a medical linear accelerator to the vertical, I became interested in mirrors made by floating silver nano-particles on oil or water. You can get 80% reflectivity from such a mirror.

By spinning the liquid it will form a concave surface which once again can be a primary mirror for a zenith telescope.

Currently research is being done on ways to distort the mirror surface in real time by use of dynamic magnetic fields acting on magnetic particles in the liquid. It would be possible to use such methods to tilt the mirror away from the zenith slightly, or it may be possible to compensate for atmospheric inhomogeneities on a large telescope made this way.

I suppose that if you had a large flat moveable solid mirror that you can move to reflect stars from all directions onto the horizntal spinning mirror. you would be able to make a Newtonian Telescope that can look everywhere in the sky, but you dont have to move, because the focus stays fixed

I cant give you links, but I used google to look up liquid mirrors, silver nano particles and found my way into all sorts of way out stuff

Good Luck

Lleros

I have heard of the spinning liquad mercury mirror before too, but I also heard that they need a anti-vibration base to isolate them to stop the ripples in the surface tension, and affecting your image quality.

A telescope I saw once was made from a couple of PVC pipes, quite wide, with home made mirrors and lenses, the tube was about 2 foot wide and about 7 foot long, a bit aquad for general mobility ;o)

Mercury mirrors are subject to vibration and the reflected image shimmers unless something is done to stop the vibration. A more viscous liquid like an oil with silver nano particles in far more stable

If mercury based spinning mirrors are not practical for the likes of NASA or other research groups, they are hardly likely to be adopted by amateur astronomers. Besides, you can't aim them.

Any large aperture mirror made from mercury would be a nightmare to balance and to insulate from vibrations.

I've already experimented with spin forming mirrors using clear casting resin. It too is a problem material. Too soft for use as an optical matrix. Thermal expansion and lack of geometric stability didn't help matters.

In addition, the slightest vibration in my mirror bench was sufficient to create standing waves in the liquid which defeated the whole idea.

"If mercury based spinning mirrors are not practical for the likes of NASA or other research groups, they are hardly likely to be adopted by amateur astronomers."

Amateur scientists will play with anything. They dont have to adopt whatever NASA does, just experiment with them. They may very well invent something useful.

I started with mercury to align lasers vertically downwards (shimmered and was dangerous), progressed to water (shimmered and reflectivity not high enough) progressed to silver nanoparticles on engine oil ( no shimmering) and then found that engine oil was not only viscous enough to dampen vibration but had sufficient reflectivity for the job.

In fact I could make a rotating oil telecope to look at the sun near the Zenith!!!!

I've already experimented with spin forming mirrors using clear casting resin.

So have I experimented with polyester resin. When I was 14 years old I cast a mirror against the convex tool from making my 6" mirror. Used very small amount of accelerator in the resin to reduce stresses from exotherm, and when it had cured (a week) baked the casting and cooled it very slowly. The plastic was soft enough that fine grinding with 1000 powder for 20 mins was sufficient, and it took only an hour to polish and a further hour to figure. NASA hadn't been thought of, I enjoyed the experiment for the sake of "playing" and got a second telescope out of it. Could have made a pair of identical mirrors quite easilt to make a giant pair of binoculars

I understand that there is now a ceramic slurry containing silver which is spun to get its primary shape, dried, and during the baking process the silver comes to the surface. It is then fine ground and figured

"I understand that there is now a ceramic slurry containing silver which is spun to get its primary shape, dried, and during the baking process the silver comes to the surface. It is then fine ground and figured"

That IS of interest! I'm experimenting now with ceramic substrates that contain aluminum! Those, when figured, would be coated in a vacuum deposition chamber as is done now with glass.

Can you provide links or name a resource for more specific information?

Thanks!

The material I was referring to is called Precious Metal Clay patented by Mitsubishi in 1990 for Jewellery.

I was going from memory when I posted, and I apologise for any inaccuracy against the report I was Quoting (mis quoting) . An article appeared in the April 1997 edition of "Night Sky" a newletter of the BinTel company in Victoria/ and NSW Australia.

Fine particles of silver are mixed with a binder to create a material with the feel of clay. It can be shaped or moulded to make objects eg jewellery. When dried it is fired and the silverparticlesfuse together making a metal like oject.

By substituting powered Zerodur for 80% of the silver, test mirrors have been made using the same process. Astronomical mirrors produced in this way are self reflective coated and only require a very thin dielectric coating to give 98% reflectivity.

By pressure moulding the PMC/Zerodur/silver slurry in a high perssure high temperature mould very accurate thin walledhoney combed mirrors can be made....The thermal stability is nearly as good as Zerodur..

PMC galss manufacturers have made a number of 1.45m mirrors available to amateur astronomical societies for trial....

You may have heard of this. It is possible to eliminate secondary mirrors altogether and get a focal plane to one side of the primary by shaping the primary mirror appropriately. I don't remember the details, but if you want I can dig it out from my archives. This was described in a Sky & Telescope (I think) quiet a few years ago.

You have described an off axis telescope. Image in central field suffers from Coma, but if the primary focal length to diameter is > than f10 this is minimal

You may be right. I have never actually seen this instrument. But as I remember it the shape of the primary (and only) mirror was very special (not part of a paraboloid) and it was claimed as a breakthrough. They used special purpose machinery to achieve the shape. You know how these opthalmic glasses with complex surfaces are achieved routinely.

"It is possible to eliminate secondary mirrors altogether and get a focal plane to one side of the primary by shaping the primary mirror appropriately."

I don't remember which of the scope companies built this design (Meade? Orion?) but the project was a commercial flop and the product discontinued.

Their method for making the primary mirror was to grind and polish a 16" mirror and then treppan four inch diameter mirrors from the disk. The result placed the mirror's origin asymmetrically .

The problems were many. For one you had the coma of the big mirror without the advantage of it's light gathering.

There are any number of successful telescopes that are obstruction free. My 6 Inch Schiefspiegler is but one. There is the Herschel which like the Schief, folds the light path and is obstruction free. Both give fantastic contrast and are completely free from the spikes and diffraction caused by the secondary spider needed in Newtonians. The Schief is easier to make however as it does not need corrective lenses, both mirrors are spherical and are identical in focal length.

I have seen only telescopes (reflecting type) with secondaries which obstruct the light path to the main mirror. I have not heard of either the Schiefspiegler or the Herschel optics. Will look it up. Thank you.

Dear Laughing Jaguar,

please post a rough sketch of what you have.

Did you ever think about star interferometers?

I have a bad (at buying) Chinese made Maksutov (100mm) which I converted to a beautiful one (at the real theoretical limits) by dumping the eyepiece and adopting an old and badly used Zeiss eyepiece from a military range finder (scrapped as laser ranging proved better results.)

I have a really good Takahashi Mevlon 180 on a Losmandy G11 mounting that exceeds our visibility conditions.

I wanted to build a "simple" adaptive optics system (7x7matrix of deformable mirror) for this but then my technician quit and the project was dead.

The future will be remotely operated (private) scopes at optimum locations may be in space.

RHABE

"Please post a rough sketch of what you have."

Not sure how to do that or what format to use. Even if I did, I'm so busy that I simply don't have the time needed to do that properly.

=======================

"Did you ever think about star interferometers?"

No I do not. I know little about them.

This is a rendering of two engineering models. The top one is a 4" Schiefspiegler. The lower one is what my 6" Schiefspiegler will look like after I complete an all new composite cradle.

Here is the existing Schief which has focal length of 4700mm!

There was a good thread last year on telescope mirrors:-

http://cr4.globalspec.com/thread/6516

I hope that Mr. Truman Brain won't be too offended if I suggest that you are probably more of an expert than him. However I liked his idea of forming the basic shape of the mirror by bending a flat piece of glass and then holding the shape with thermally stable resin.

Don't they (didn't they) make flat glass by floating molten glass on a bath of hot mercury. So would it be possible to spin up a dish of mercury; float the glass on the surface and allow to solidify slowly. As the glass became less fluid it would gradually eliminate the standing waves. Of course I realise it would still need to be ground for the final surface.

I think they make float glass on molten tin.

The tin is more dense and rotating it would see the tin take on a concave shape and the glass convex

If you had a ceramic (even unfired clay) of convex shape you can get a sheet of glass (albeit quite thick) to slump into the concave shape in an oven. That might be a better starting point

Having built a 6 inch telescope in my early teens I wanted to build a bigger one in my early 60s, but found that I could buy a 12 inch diameter newtonian for less than AU$ 1000 including quite good eyepieces. So I am saving my energy to make a clock drive for it (hence my previous posting on constant velocity gearing

Lleros

The intention was really to use the concave surface of the glass after it's removed from the "mould":-

On reflection (no pun intended) perhaps the tin (or mercury) is not needed at all: just leave the glass in the vessel it was formed in.

But, perhaps following your thoughts you could get a more accurate surface if you put another slightly less dense liquid on top of the glass. There would be a lot of scope for choosing this top liquid: it could either set along with the glass as long as it could be removed easily, or, it could be poured off.

Incidentally this method does (theoretically with perfect liquids, no surface wind drag effects et.c.) form a parabolic surface, not spherical as I've drawn it.

Hi Randall,

Not offended in the slightest, quite the opposite really! If my posts in the past have been of any use then I'm !

I am indeed a very large stride away from being remotely connected to the word expert in astronomy! But I did once go to see 'The Great Patrick Moore' talking about going into space! I sat front row, middle seat! Unfortunately for me, I only understood about 30% of what he was saying and about 50 of what he was actually explaining! (Check out link if you need more of an explanation!)

Anyway, back to the telescope building thread!

I can't add to the discussion about constructing telescopes but I'm lucky enough to live near to Herstmonceux Castle which was the site of the Royal Greenwich Observatory for some time before it moved to Cambridge. The telescopes are available to the public for viewing.

My company has an association with the observatory having supplied image intensifiers for use with their telescopes. When the company was first formed a couple of it's founders were astronomers from Herstmonceux, they are still directors of the company although one is semi-retired & the other mostly runs his farm in Sussex.

I used to service their Mini Computers around 30 years ago!!!!!

Hi,

Schiefspiegler scopes are made by Lichtenknecker (I thin in Belgium).

Why shall this design produce coma?

Glass melting and rotating has been tried by the "Optical Science Center", Tucson, Arizona in the late 80ies. They wanted to cast a 14m mirror from borosilicate glass.

The mirror should include big hexagonal inserts that are to be washed out after cooling so that nowhere the thickness exceeded 1".

There was a very big slip-ring (to transmit 1.5MW heating power to the radiators in the cover of the oven.

This approach was abandoned after the success of the first Keck multi-mirror-telescopes with its earlier training assemblies used for evaluation.

See for more information in the SPIE proceedings, SPIE.org has a magnificent website and so many buried information that anything is sometimes difficult to retrieve.

The above information on casting float glass on tin is correct, never use lead as having a high evaporation rate of very toxic metal and oxides.

Tin is much lower in evaporation, Gallium would be best but who can afford this?

The slope of a centrifugal shape is independent of gravity as it is composed of two vectors: force from earth gravity and centrifugal force, both are proportional to density, so if you double the density both forces will double and the resulting vector has the same direction.

Any plastic lenses or mirrors are small because shape is instable by humidity, best is COC second best Plexiglas, injection molding is used for making aspherical mirrors of up to 20cm longer side-length. Injection molding is artwork until today!

Elastic deformation to near ideal shape is possible and was used to polish the final shape of big mirrors (aspherisation of 8m blanks at university of Texas).

This process is used today to produce the aspherical correction plates for Schmidt telescopes, suck the plate by a very defined vacuum onto a big pot, polish (spherically) and release.

RHABE

Actually, the Steward Observatory mirror lab under the University of Arizona football stadium, is still in operation and routinely turns out mirrors 3.5 meters across.

http://www.thepepper.com/tucson_steward_mirror_lab.html

I seem to recall one non-pointable mirror that was built that is a spinning tank of molten mercury. It was a 2.7 meter f/1.9 deep sky survey instrument.

http://www.europa.com/~telscope/LMT.txt

The latest twist on this is a ferrofluid adaptive optics version that uses modulated magnetic fields to distort the parabola.

http://arxiv.org/ftp/astro-ph/papers/0212/0212189.pdf

I once saw a show on TV where a telescope was made to project/reflect the image of the sun down into the basement of the maker. The image apeared on a large flat surface like a table with the sun's image about 3' in diameter. The detail was beautiful showing sun spots and solar flares. I forgot if the transit across the sun's face by mercury or venus was shown but of course that would also be possible.

You could create a time lapsed film of such events or the just the changing surface of the sun.

I think what your talking about are "Solar Observatories"

Where a tracking (heliostat I think its called) mirror tracks the sun and takes the light to a objective surface, then that surface is observed by people and cameras ;o)

If you look at my posting #2, you would realize that we have come the full circle. By the way thanks for the off topic vote for that posting.

I suppose that if you had a large flat moveable solid mirror that you can move to reflect stars from all directions onto the horizntal spinning mirror. you would be able to make a Newtonian Telescope that can look everywhere in the sky, but you don't have to move, because the focus stays fixed

Whether it is is harder to make a larger (that is larger in the oblong direction than the diameter of the primary parabolic mirror) elliptical optically flat mirror and mount it in such a way that it tracks the stars (or the sun) to beam into the fixed horizontal parabolic mirror (be it a spinning liquid or a solid) or alternatively a solid parabolic mirror mounted in a conventional star trackable telescope is a moot point.

The advantage of a heliostat ("stariostat" for stars) into a fixed position parabolic mirror is that the observer sits in the same position no matter where the system points. Of course with a spinning liquid telescope the focal length may be changed easily and the observation position would only move vertically.

Lleros

What negative vote? did I make that? I just replied to your posting AFAIK.

Sorry snaketails, didn't mean it personally, meant to say "thanks to whomever voted my comment off topic"

tHERE YOU GO, GAVE YOU A ga TO NEUTRALISE YOUR POSTING