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Large Dobsonian ATMing

By John Vogt of Huntington, New York   2004-Oct-04

My ATM career began in 1977 when I decided to replace my 16" Cave telescope with a 20" of my own design. At that time Coulter Optical was selling thin mirrors up to 20" and I placed an order. After waiting almost a year, Coulter informed me that they could no longer obtain glass blanks and could not fill the order. A friend of mine recommended I make my own and offered to help with the process. I was able to locate a 21" blank and set about building a grinding machine.

After being instructed on grinding, polishing and lap making I was able to get the mirror almost finished when disaster struck and I broke a large chunk out of the glass while trying to set it upright on the grinding table. Angry but undeterred I called around to locate another blank and was rewarded when a glass supplier, sympathetic to my plight, gave me a 22" x 2 1/8"(Fig. 1). I started the process all over again and after a year was able to finish the mirror.

The mount is a massive German equatorial(Fig. 2). The RA assembly is a 24" diameter sheave from a paper shredder with a 4" shaft. The sheave rides on stainless steel rollers with one being driven for tracking. The DEC housing is a 10" steam pipe and contains large tapered roller bearings carrying a 5" solid steel shaft. The tube is rolled aluminum, cork lined, and was made at a tank truck fabricator.

The performance was good although at times hampered by tube currents, even with fans, and some astigmatism due to the thin mirror and its changing position from being mounted equatorially.

I used this instrument through 1982 , but with light pollution increasing it was time to become portable to travel to darker skies. The increasing popularity of the Dobsonian telescope provided the perfect avenue to accomplish this. A 24" cellular blank (Fig. 3)was obtained from Newport Glass and was finished in time (Fig. 4) for the 1983 convention where it took 2nd place for optical performance. (Fig. 5)The mount was standard Dob. fare but did include a slide out mirror holder and rolled plywood upper cage.

A year later I decided to try something larger and obtained a 30" cellular blank from Newport. The supplier ran into problems making this size and after 3 attempts finally delivered a usable blank.

This mirror was completed in 1986 and provided usable images but suffered from some print-through and a damaged rib from the coating lab.(Fig. 7)

I wanted a scope that was functionally perfect and decided to find another blank. This led me to Hextek Corp. located in Tucson,Az. Hextek makes lightweight cellular blanks by a gas fusion process and are generally expensive to produce. They offered me special pricing on the blank if I was willing to increase the size to 32" as they were producing a quantity of these for a client and could use the existing tooling. I of course said yes, and in 1989 took delivery of the blank.(Fig. 8)

Making this mirror took much longer than anticipated due to problem with print-through, astigmatism and lack of time due to building an extension on my home. The mirror was finally completed in 1992 and has an excellent figure.(Fig. 9) Unfortunately the coater broke a rib and damaged the figure.

Although very disappointed, I decided to repeat the process and obtained another blank from Hextek although at twice the price of the original. I did receive some satisfaction by submitting an insurance claim and having my insurance company successfully litigate this against the coater, Evaporated Metal Films.

The replacement blank arrived in 1993 and took 3 years to finish.(Fig. 10) I again experienced problems with print-through and astigmatism. When finished the second mirror has a figure a little better than the first.(Fig. 11) The mirror was entered in the optical competition in 1998 and took 1st prize.(Fig. 12)

The mounting for this mirror is essentially an Obsession clone but with a number of enhancements. A computer drive for tracking was installed in 1996 and goto functionality was added in August of 2002.

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John Vogt: Figure 1
Figure 1
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John Vogt: Figure 2
Figure 2
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John Vogt: Figure 3
Figure 3
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John Vogt: Figure 4
Figure 4
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John Vogt: Figure 5
Figure 5
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John Vogt: Figure 7
Figure 7
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John Vogt: Figure 8
Figure 8
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John Vogt: Figure 9
Figure 9
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John Vogt: Figure 10
Figure 10
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John Vogt: Figure 11
Figure 11
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John Vogt: Figure 12
Figure 12

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