Bakken Galaxy-Hercules Project Page 1

Hercules Amongst the Stars

The Hercules telescope project was begun in June 1994. It progressed off and on from then untill it became an operational telescope in July 1996.

This project is a further symptom of my aperture fever. I have an acute case, stemming from the time I upgraded from a 60mm telescope to an 80mm. My first "good" telescope was an old sand-cast C-8 by Celestron-Pacific. When a friend came along with a 17.5" telescope, I was really hooked. I had to have my own but, being a physics student, my budget didn't allow for one. Then I thought, "why not build my own?"... Why not indeed!

My first mirror made by hand was a 17.5" f4.6. Another friend, Jeff Baldwin, and I learned mirror making together. My 17.5" homemade telescope wasn't perfect, but gave pretty good views anyway. It got me through my Herschel 400. I am Herschel club #54.

The next major phase of aperture fever came when I convinced Mike Benz to go homebrew optics in a large Dobsonian. The result was "The Beast", a 33.4" f5 Newtonian. It was featured in Sky and Telescope magazine, September, 1992. When Mike moved from Spokane, I needed a big scope again. That's when the seed for Hercules was planted.

Hercules is 15' tall when at the zenith, and weighs in at about 650 pounds. 185 pounds are in the Pyrex mirror. A 12' ladder is required to observe with the telescope.

The telescope is portable, it is towed in an enclosed 12' Haulmark trailer. The Alt-Az mounting is motorized with stepper motors which are driven with either an analog joystick or my notebook computer. The Sky astronomy software will be able to slew the telescope from object to object, and the electronics will keep the scope tracking when not slewing.

The Mirror Blank and the 30" grinding tool

The Pyrex mirror blank was ordered on July 1, 1994 from Glass Mountain, in Austin Texas. After waiting only six months for a quoted lead time of four to six weeks, it was delivered to my shop here in Mead, WA. The blank was ordered with a curve generated face of f 4.0, and as large as practical from a 4'X8' sheet of Pyrex, 2 1/8" thick. The back was ground flat, edged and beveled. The blank was fine annealed at Glass Mountain, and fine ground.

Unfortunately, the mirror had horrible astigmatism. This meant regrinding. I, and some dedicated volunteers, began the regrinding process, however, the supplied 30" tool proved to be too thin and flexible to do the job right. A second grinding session was undertaken, this time with a 20" tool. This grinding did get rid of the astigmatism but, due to some contaminated grit, we had to regrind again. This third and final time, we used both tools resulting in an acceptable shape and polish.

This is me using the 20" tool during the second session

Figuring, the selective polishing and testing to ensure the correct paraboloid shape, took 8 months. The wire test was very useful, but I also used the Foucault, Ronchi, Ross null, and star tests. The finished optic proved itself to me when I was able to look at Saturn at 350X and it was tack sharp. The thin mirror occasionally shows tweaked stars due to flexure in the cell. This seems to be a common problem with these large thin mirrors, but it is usually corrected by shifting the mirror in the cell and recollimating.

The rest of the telescope was built in between optical sessions. I have about 6 months of part time work into engineering and constructing the platform, mirror box and cell, frame, diagonal cage and other parts of the whole assembly. Many of the details are explained and illustrated on the following pages.

E-mail me if you are interested in Large Thin Mirror making. I am part of a mail group on working on large diameter telescope mirrors. We will soon have an archive and homepage to share the experiences of working monster pieces of glass.

Back | Continue