wpo - flux collector - Sky & Telescope :May 1979

   astro-optics page   New Scientist 1985 article  "making telescope mirrors from silvered plastic" 
text & images [c]Maurice Gavin 1978/2001

Aluminised Mylar as a Flux Collector
The accompanying pictures show  a novel telescope of 21-inches aperture that I designed and built for about $1 during July, 1978. It incorporates some of my recent experiments with concave and convex mirrors induced by partial vacuum or increased pressure behind aluminized polyester film [ICI Melinex/Dupont Mylar]. The image produced is not suitable for viewing, but such a mirror serves nicely as a flux collector for infrared observations.  My mirror weighs a mere half ounce, the telescope as a whole 12 pounds, less than 1% of the weight for a conventional telescope of this aperture. The only expense was for the mirror - $1.00 for a yard of 30"- wide material from a local plastics store. The balance of the parts came from the scrap heap! These include five pressed-steel bicycle wheel rims, two skate wheels, some 1/8" plywood,  TV antenna, and a vacuum cleaner.

Two of the bike wheels were joined together to form the cell, with plywood as  the backing and the mirror material held in place with adhesive tape.  The other three rims were used to form the mount.  For stability only one ring was cut, and the section removed plus the two skate wheels forms the split-ring bearing.  The legs for the base ring and the struts for the flux collector were made of aluminium tubing from the TV antenna. The mount is designed to make the polar and declination axes pass through the center of the mirror membrane, yet only a small amount of obstruction (6.8 percent maximum) by the polar ring occurs at declinations + 22o to + 90o at my latitude of 51o north. The spider was offset slightly to the south to avoid hitting the polar ring at high declinations.

To stretch the membrane to shape for observing I use the vacuum cleaner to draw out air from the cell.  A tire pump with its washer reversed would serve or even lung power !  Since the membrane is permeable a constant suction has to be applied.  In the case of my mirror the sagitta [depth of curve as measured from the edge] is 0.72".   I have made mirrors as fast as f/0.25, with membranes 6 to 36 microns thick, but the spherical aberration below f / l causes the image to be unacceptably diffuse.  Experiments show that the circle of confusion for point-source images with my 21-inch f/1.9 is typically 5 mm.  This is still small enough for my photodetector.  There are many possible causes of this diffuse image.  Among them are defects in the membrane material, variations in its thickness, differential stress under tension [the effect of warp and weft], uneven tension of the edge support, astigmatism due to warped cell rim, and many more.  Thus the results are surprisingly good.

The telescope was first shown at an August 5, 1978, exhibition meeting of the Southeast Group of Astronomical Societies, and again on August 26-27 at the Epsom Borough Show, in the booth of the Ewell Astronomical Society (of which I am vice-chairman).

Maurice Gavin - Worcester Park - Surrey - England