|by Stewart Beveridge of
In 1989, I bought Telescope Making issue #36 and found an article called "Constructing a
Low-Cost 6-inch f/21 Littrow Refractor". After reading this article, I decided that I would be able to afford to
have a 6-inch refractor if I bought the glass blanks and made the lens myself. I organized some letters and soon
had a piece of Crown and Flint glass to start my big project. The Crown was HC 518603, and the Flint DF 620364.
These glasses were procured by David Cartwright in England from "Chance Pilkington Glass Works" and sold by him
as kits for Littrow lens doublet projects.
I started by joining the "Astronomical Society of Victoria" and found a small group of people
who had made doublet lenses. I had never made a mirror before but the techniques are basically the same exept
that it takes a while longer as there are 4 surfaces. The big plus is that the first and second surface of the
crown are ground on the concave of the Flint as these surfaces are matching. The last surface of the Flint is
flat and can be ground on window glass. It took me approximately 2 years to go through all the grinding and
polishing and I completed correcting the lens by star testing with a Ronchi screen that I had fitted into a 35
mm film canister that fits nicely into a 1 and 1/4 inch focuser. I would estimate the amount of correcting and
do this during the day and test at night.
I wanted to take my completed project to my Astronomy clubs Star Parties so I ended up making
a tube out of thick wall cardboard that breaks into 3 parts. I recently finished a wooden tripod which is from
the directions out of Richard Berry’s book "Build Your Own Telescope" and is extremely steady. The whole project
took about 8 years of ‘on and off’ work with the lens being used in various tubes and tripods until the final
configuration as per the photographs.