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Construction Of A 220 mm Reflector

My aims in writing this article are to show how easy it is to construct a telescope and to encourage others to do likewise. It's a fun activity, extremely interesting and very worthwhile. If I can do it, so can you! I am now in my 89th year and am still trying to figure out ways of improving - both the telescope and myself!

My first telescope was a Vixen refractor of 90 mm aperture. I experimented with a variety of mounts for the Vixen, three of which are shown below. It was a very good telescope, but after a while I felt a need for something larger. Eventually, I decided to build a reflector of around 200 mm aperture. I thought that a focal length of f/6 would be satisfactory and would give a convenient height for viewing objects near the meridian. In view of my age (approaching 85 years at the time construction began) I decided to purchase the mirrors, flat support, rack and pinion focusser and finderscope (complete with mounting brackets) and to construct only the mirror cell. Although capable of constructing some of the other items myself, my reading of the experiences of others warned me that the process could be lengthy and not necessarily a first-time success. At my age I could not afford to wait! The following photographs and sketch illustrate the telescope and how I developed it in the light of practical experience. The evolution spans a period of some three and a half years from starting construction.

Figure 1. The Vixen refractor with an equatorial mount supported on an adjustable pillar.

Figure 2. The Vixen refractor with an altazimuth mount supported on a metal tripod.

Figure 3. The Vixen refractor with an altazimuth mount supported on a wooden tripod.

Figure 4. Tube assembly of the 220 mm reflector in its first configuration. For ease of construction, I made the tube from 9 mm plywood of square section. I glued and screwed quadrant moulding in the corners. The resulting flat sides made fitting the declination bearings easier. I made the declination bearings from 180 mm diameter blue plastic water pipe which a member of OASI kindly machined for me. (Incidentally, the pipe was a waste off-cut found by the roadside!) I cut the base from an odd piece of kitchen worktop. (No, I didn't take it from my wife's kitchen!) I fashioned the side pieces from discarded shop fittings, with an old 300 mm vinyl LP between the two base sections to serve as a bearing surface. The same member of OASI who machined the declination bearings also supplied a pivot bolt and self-locking nut to join the two base sections. I fitted two handles to make it easier to lift and stow the telescope in the garden shed. I placed a stiffener across the front of the bearing trunnions, thus making the assembly more rigid. I fitted the base with adjustable feet for setting up on uneven ground.

Figure 5. A modification made by cutting the tube into two sections making it easier for lifting and transport. The sections fit together and were secured by four bolts.

Figure 6. A small hood fitted over the tube end to shield the optics from a street lamp and to act as a protection from dew. This modification was not successful and I quickly dispensed with it. The image also shows a removable storage box containing lenses and other bits and pieces.

Figure 7. Further modifications: a pair of small wheels and a bolt registering in the base which, when located, prevents the tube revolving in its pivot when the whole assembly is being moved.

Figure 8. The counter balance weights at the lower part of the tube, and the three adjusting screws for the 220 mm main mirror. (See also figure 15.)

Figure 9. The first open tube version, using wooden struts, adopted for reduced weight. Other features assisting with ease of movement are a larger pair of wheels and two former pram handles.

Figure 10. The second open tube, constructed from aluminium rods rather than the wooden struts of the first open version. The image also shows the declination adjustment (provides both fine and course adjustment) which proved very useful.

Figure 11. A closer view of the fine adjustment. There is a length of threaded rod running in an outer length of tube. A plastic clevis and a plastic split collar, both salvaged from a child's buggy, serve to pivot and lock the component parts. I turned the handwheel using an electric drill to hold the mandrel.

Figure 12. A pair of screwed jacks, useful for uneven ground and also for taking the weight off the tyres. (The tyres tended to promote unsteadiness when left in hard contact with the ground.)

Figure 13. The reconstructed tube. Unfortunately, under open tube conditions, the mirror was directly exposed to damp in the atmosphere, and the aluminium coating rapidly deteriorated. It was therefore necessary to revert to an enclosed tube design. The figure also shows, in the background, a movable ramp into the shed, avoiding the need to lift the telescope in and out. Only one handle is now in use for moving the telescope around ‑ it comprises the handle from a child's buggy fitting over a short length of smaller tubing and secured by means of a cross pin through drilled holes.

Figure 14. My masterpiece! I originally purchased two gimbals to mount the finderscope, but these were always unsatisfactory as the finderscope was frequently out of alignment with the main mirror, necessitating constant adjustment. I made the mounting shown here mainly from wood: it was originally experimental but has since become permanent! There is a pivot pin at the eyepiece end allowing lateral movement of the base. Two set screws at the forward end permit adjustment by easing one screw off and tightening the opposite screw, so locking the base in position. A second plate carrying the finderscope is hinged using a good quality brass door hinge, thus permitting adjustment in declination. A screw and wing nut pressurises the coiled spring so giving vertical motion. The spring illustrated has since been replaced with one having fewer coils. Once set, the position remains unchanged for weeks, except when it gets accidentally knocked in the small garden shed: this is a vast improvement on the original set‑up!

Figure 15. The box housing the 220 mm mirror. This provides a three-point fixing and a separate, adjustable, three-point adjustment via spring-loaded allen screws. The adjustment nuts protrude through the end of the telescope (see figure 8).

Figure 16. I obtained many of the materials for my telescope from "low-cost" sources as the cartoon illustrates!

Fig01.jpg Fig. 1.

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Les Lamb (1913-2011)