Weird Telescope Mirrors
 


                      [ It is likely an epidiascope was used for illustrations]

     I was going to talk about unusual telescopes, but this was such  a big subject I decided to deal with mirrors. Mirror, mirror on the wall, who is the brightest star of all ?. Apart from size, you may think that once you’ve seen one telescope mirror you’ve seen them all - wrong!

    Today glass is the traditional material for telescope and other mirrors, but other materials have been used, including granite and obsidian. These are tough and hard work to grind but originally, having cooled very slowly, they are stress free, and obsidian conducts heat better than glass. After grinding they were vacuum aluminised like any other mirror.

    An American firm, the Astro Met Corporation, has made open cell ceramic mirrors of a light firm material with an aluminium layer bonded to the curved front face, which is then figured and polished. Attempts have been made, using GLA 2 ED pottery, without much success. As many in evening classes have found, work often cracks in the kiln     

    In the early days of reflectors, before aluminising was possible, the standard mirror material was speculum, a copper tin alloy of variable composition, and extra trace materials to harden it. Chemical silvering was very fragile, and fell off if exposed to the air. Mirrors for dressing tables, bathrooms, etc. were back silvered and then painted for protection.

    The Earl of Ross made many experimental and ever larger speculum mirrors, even soldering many speculum plates onto a large brass back assembly. He then ground the whole thing, but unfortunately, even after polishing the joints caused diffraction problems. Large metal mirrors distort easily, and speculum tarnished over time. Today very lightweight metals are used in satellites.

    Other attempts to make large lightweight mirrors included a thin top glass plate for the optical surface, supported on metal spacers which were bonded to it, but the different expansions of glass and metal  made this unworkable. A similar system, but all glass, with glass ribs glued to the optical plate, failed after a couple of years when it fell apart at the glued joints. 

    Light strikes conventional mirrors at close to 90 degrees to the surface, then is reflected back, having done a U-turn. Higher energy photons, like high end ultra-violet or soft X-rays will penetrate the surface and be absorbed in the mirror itself rather than be reflected. However these high energy photons will reflect if they strike the surface at a very shallow, glancing angle, less than 5 degrees.   

   A reflecting X-ray mirror consists of a set of concentric nesting mirrors, each of which is an annular or ring section of a very deep parabola, similar to concentric pastry cutters, but angled.  (Diagram1).

 

       Liquid Mirrors.

   Flat static pans of mercury have been used in zenith telescopes. These instruments are permanently pointed exactly to the zenith, and consist of a lens and traditional angled flat at the top end and the ultra flat and horizontal mirror at the bottom end.

 These are used by the US Navy and others to observe and time the meridian passage of stars, for timing and geodetic location purposes. This type of telescope never moves.

   Perfectly flat reflecting surfaces bring us to the often forgotten, less glamorous mirror, namely, the Flat. If you are making a “home made” telescope make sure that any Flat that you obtain really is flat, is front aluminised, and intended for use as a Flat.  A piece of silvered glass scavenged from junk may not be optically flat. I remember using a Flat which had a minute crack, and although the optical surface appeared to be fine it created multiple images which were only sorted by changing the Flat. Very large Flats, as used in solar telescopes or heliostats are very difficult and expensive to make.   Stress and temperature variations combine to create problems in making them optically flat.    

   Paris 1900. 60 metre tube, 2 metre sideostat.

   Believe it or not, a lot of Flats are meant to be curved. Folded lightpath cassegrain type telescopes have a convex secondary. Very large convex mirrors are hard to make and test accurately. An extreme telescope I will mention later even has a concave Flat or secondary.

 

   Rotating Liquid Mirrors.

   By rotating a pan of mercury or any other liquid you can create a perfect parabolic surface from the centripetal centrifugal forces. Again such a telescope can only point to the zenith, but we now have a parabolic optical surface. Early experiments with this fell victim to vibrations causing ripples in the mercury or changes in rotation speed which altered the focal length. ( Diagram 2 ).

   Apart from pure zenith work such telescopes are of no use for general astronomy. Consequently they are rarely advertised in the mainstream astronomy journals.

   More usefully, when molten, glass blanks can be poured into a revolving mould, and the upper surface forms a parabola of any desired focus by keeping its rotation at an appropriate constant speed as it cools and solidifies. It then requires only minor figuring and aluminising to go into any telescope.

   For those who missed the Guy Consolmango talk a week or two back, an extreme example of a mirror made by this centrifugal casting method is the Vatican Observatory advanced technology telescope with a 1.8 metre F1 mirror. This is 10,500 feet up on Mount Graham, Arizona. Such a large mirror, with an aperture equal to its focal length, would be very hard and slow to grind by traditional methods, and as the curvature is so steep it has a concave Flat, mentioned earlier.

   Going to the low technology extreme, a mirror was devised by stretching aluminised mylar across the bottom of a cylinder and topping it up with water. The water made the mylar sag into a somewhat parabolic form. It even worked - after a fashion! But enough of liquid mirrors…. 

 

   Small scale convex all sky Fisheye mirrors have been made by ingenious amateurs, combined with a camera, including the Hub Cap telescope ( See pictures 3 and 4), using a silvered plastic egg.  ( Modern cars - no hub caps ).

   The well known 200 inch Palomar telescope continues with the front line work it has done since it was opened in 1948.

   The record breaker for a single mirror is, of course, the 1973 Russian telescope 6000 feet up in the Caucasus Mountains near the Georgian border. Its mirror is 6 metres in diameter and weighs 70 tons. It is altazimuth mounted and computer driven, but has not been the optical breakthrough its size predicted. The optics are sound but being F 4 means a huge tube, nearly 80 feet long, and an enormous dome, both of which have resulted in a long history of thermal problems. The Palomar telescope mirror, at ‘only’ 15.5 tons, adjusts and settles down to temperature changes far better. It may be that the Russians were unable to afford the state of the art add on extras and gizmos to enhance performance that Western astronomers take for granted.

   Multiple Mirror Collectors are light buckets rather than designed to give high definition images. An example ( Picture 5 ) is a steerable array of some 300 mirrors, the whole dish being 10 metres across, and it is a cosmic ray detector. As cosmic rays hit the upper atmosphere each ray creates an extremely brief pulse of light as it hits the atmosphere. This instrument, based at the Fred Lawrence Whipple observatory, is designed for the task of observing these extremely faint flashes. The individual mirrors can be manually adjusted, but this is not adaptive optics.

   Other multiple mirror collectors are thermal test facilities (Picture 6 ). Anyone at the sharp end of this ( Picture 8 ) when it is trained on the Sun will become toast , but it has been used as a ‘telescope’ at night. This, ( Picture 7 ) shows what is seen when the whole array is turned onto Vega. As you might imagine, there is plenty of starlight but it is untamed.

 
             True Adaptive Multi - Mirror Optics.

Going back over 100 years attempts were made to produce a large quality mirror by putting it together in sections, and until about 1970 all attempts failed due to the complex 3-dimensional flexing of the support structure, not just when the telescope moves up and down but as it rotates when equatorially mounted. These stresses and flexes were too complicated to engineer out at a sensible cost. 

The breakthrough in true adaptive optics came in the 1970’s at the Whipple Observatory with the building of this telescope ( Picture 8 ). The altazimuth mount meant that stress and bending was in one direction only instead of an infinite number of directions. For the first time computers were available to correct the distortion, as were new lightweight yet very strong materials for the construction. The so-called Serrurier Truss Tube used here made one directional sags predictable and correctable.

This 6 mirror set-up was just the start. Technology has forged ahead, and there seems no limit to what is possible. Come to Chile for about 364 clear nights a year, and see the very large telescope array, 2,635 metres, up in the hills. Each telescope has a main mirror of many adaptive optics segments. The 4 telescopes are linked, and called Antu, Kueyen, Melipel and Yepun.

Similar style mirrors are in the Twin Teck telescopes in Hawaii, each with a mirror 9.8 metres in diameter, made up of 36 hexagonal segments.

Finally, the Hubble is still going well, but for how long ? And will it have a successor ?

[The talk ended by producing of a pair of binoculars, with the comments and presumably a photograph, as follows] 

This is a pair of binoculars, about as big as they get, and also very unusual in that they are 8 inch reflecting binoculars. Each tube has an 8 inch mirror.  The motorised mount allowed the observer to sit in it, but it was very difficult to get in sand out of the house (or garage), so this instrument is now on a more conventional Dobsonian.    Thank You.