Initial deviations from the Wilson & Miller design.
The finished product.
More innovations...
This telescope was inspired by my first visit to Riverside Telescope Maker's Conference (RTMC). When I left Michigan (where I worked as an optician) for Arizona I thought I would never have to work optics again, but the bug bit me at the 1980 RTMC. The original idea was to make a fast Newtonian for observing variable stars with the AAVSO, that could be portable and not necessarily be the highest quality, but at least binocular quality. When purchased at the RTMC swap meet the blank was a 10.5" f/1.3 with a 2.5" hole in the center. Cost was $5 so there was little to lose!
It was going to have to be carefully ground down (from the edge) such that none of the central 0.5" thickness would be lost. This resulted in the same volume of glass being removed as if one had started with a flat blank! The first step was to build a machine to grind it. Having done a few mirrors by hand (and worked optics professionally) I knew that time building a machine for optics over 8" is time well spent. Using an old cabinet, parts from a cooler and a washer, a machine was quickly constructed for $15.
An experiment was tried in roughing out this mirror that worked very well. Machine shops occasionally crack grinding wheels. They are usually tossed aside and forgotten but not disposed of. Such was found and gladly given to me. A spindle block was pitched to one side and it was used as a tool. First, a good bevel was put on the mirror to prevent any chipping. Then one charge of #80 grit started the disintegration process and the wheel provided it's own grit as it broke down! This worked beautifully for hogging out. As the desired f/4 curve was approached, a proper tile tool was made and the mirror was finished off to 5 micron grit with that.
The original grinding machine was a simple spindle and non-driven overarm. The main turntable rotated at 6 or 12 rpms depending on the setting. This slowness let me catch mistakes before the became serious and kept my grit on the job instead of the wall. Yes, it takes longer to grind but who cares, the machine is doing the work! Very soon a driven overarm was needed. Since a high torque motor could not be had a weaker motor was used with some mechanical advantage. A simple 1 rpm timing motor was used on a 3" cam. This with a slightly oversized tile tool helped control the sagitta quite nicely. A tent was added as a preventative for scratches caused by falling dust. Also, a drip system made from a hospital I.V. set up with a small bell on the end of the overarm finished the setup. The drip kept the tool moist so grinding and polishing could procede without interruption. The bell allowed me to know if there was any vibration starting from a dry tool (before it seized). This allowed me the freedom to be working on other things while this process was going on.
By the next summer the mirror was fully polished and ready for figuring. I have heard it said that a mirror is only half done when it is polished. I can tell you that this must be multiplied by at least two for a mirror so thin. It took two years of painstaking work to get this mirror to 1/6 wave. The final figuring technique was mirror on top, normal stroke and the figure controlled by trimming pitch squares.
The original mounting for this telescope was based on a german equatoral designed by Richard Berry and presented at RTMC '82. It served well for a number of years but the harsh Arizona desert climate eventually destroyed it. A move shortly thereafter caused the mirror to be stored in its custom box for a number of years. But inspiration blossomed forth in a book, "Making and Enjoying Telescopes" by Kenneth Wilson and Robert Miller. In that book was, among many excellent designs, one for an 8" f/5 Newtonian. I immediately saw several changes that would adapt this design to my situation quite nicely. I have an observatory at my home, but a rather unusual one. It's a 'roll-out' observatory. All telescopes are on wheels and brought out from an enclosure. This conserves space in an already too small yard. So a mount that incorporated wheels was a must. Clyde Tombaugh gave me some ideas on this when I visited him at his home in 1984. Several other changes were envisioned as well.
It took several months to construct the mounting in bits and pieces of time I could find between job, family, graduating daughter and hospital visits for ulcers. It was made from parts of a large packing crate! There were 6 sheets of 3/8" plywood and lots of 2x4s. The pipe was reclaim too (though not used for plumbing). So I was still into this for only $20. The counterwieghts were picked up for a dollar here dollar there at garage sales and the tube is a thin all cement form tube.
One thing I did here that was different. In my work with the Catalina Sky Survey and getting our new schmidt type telescope working (now the second largest in the US), I whas shown how to make a truly, flat black telescope interior cheaply. We lined the interior of the telescope with door screen that had been painted flat black. It's truly amazying how well this works. There are NO reflections, shine or light coming off the tube. I did the same to this telescope and am very please with the results. I highly recommend this to anyone building a telescope and may even incorporate this in my Celestrons.
I bought a 2.5" diagonal second hand for $10, and got one of the reject Celestron Comet Catcher focusers for $5. A little work on it and it was fine.
The mirror was originally floated on a thin sheet of foam rubber. This worked well but I did not like the way the clips damaged the coating. It is now foam but held in place with some of the new silicone adhesive, also used on the secondary.
The result is a very portable telescope with great sturdiness. The whole thing rolls out on the wheels by picking up the rear (long) strut and using it to push and steer. The mount breaks into 2 pieces and the tube unscrews from the mount for a total of 3 pices that can easily fit in my little Hyundai Accent.
So there you have it. A 10.5" f/3.95 Newtonian for under $50.
The above three photos show another feature of this telescope. The finder is in brackets made from two different sizes of PVC tubing, painted black. The front one is the traditional three screw adjustment but the rear of the 'scope sits on two screws and is nested into them with a strap under spring tension. The finder itself is a discarded 2.4" refractor with a plastic Celestron focuser (Surplus Shed for $5) and an old Unitron 40mm eyepiece for a nice 16x finderscope.
The whole tube rotates very (perhaps too easily) on discarded roller skate wheels in the saddle and is held in place with felt pads. No part of the sky is inaccessable, a requirement for this telescope.