24" ZeroDur Mirror

By Dave Gamble

The telescope mirror we are about to discuss has not led an ordinary life, nor is it an ordinary optic. The bouncing baby 24" ZeroDur blank came out of the annealing oven in the early 1970's weighing 120 pounds. It was four inches thick and sported a ten inch central perforation.

This singular mirror was fabricated for a Dall-Kirkham telescope being built by Lorenz Scientific Ltd. of Downsview, Ontario, and after a tumultuous life it was freed from its original OTA bonds to become a free agent. In 2006 the Okanagan RASC Centre was able to adopt it for use in the main telescope of its new Okanagan Observatory. Now, over two years later, fate has again stepped in. The short of it is that the mirror is again looking for a new home.

It is understood that the original telescope was manufactured by Lorenz Scientific for installation in a private observatory on Highway 89 by the family which owned Spartan Scientific. From there the history becomes somewhat hazy. Family changes required moving the instrument from its original location and as it was being disassembled the secondary was apparently damaged. It is understood that it was reworked but not re-aluminized, and this suggests that the telescope was never again made operational.

Years later Spartan Scientific closed down and its buildings were emptied of equipment and vacated. Some of the Lorenz telescope parts were saved from recycling and these included the primary mirror and the sophisticated secondary mounting with its actuating motors. These were made available through the RASC National Council and several Centres expressed interest in acquiring the ZeroDur mirror. In the end it was released to the Okanagan Centre and transported across the country to optician Barry Arnold's shop in Edmonton by veteran member Alan Whitman.

At this point the writer came into the mirror's life as chair of the Okanagan Observatory project's telescope committee. So, what does one do with a 120 lb., four inch thick 24" ZeroDur mirror with a 10" hole in it? After the committee of five considered the options, it was decided that we would adopt a folded Newtonian approach similar to JMI's NTT design. The 10" secondary flat effectively made constructive use of the light loss from the primary's 10" perforation and the lower, more accessible eyepiece height would be perfect for our needs.

Barry suggested refiguring the f5.6 ellipsoid to an f4.1 paraboloid, but since the mirror had acquired a small fracture about ¼" below the surface on one edge, Barry proposed regrinding to below this level as the first step of the operation.

The telescope was designed around this plan. We began purchasing accessories and the frame of head end of the telescope was built. Since the 24" mirror had to wait for Barry to complete an earlier low expansion project, nothing was actually done on it. Then, early in 2008 Barry decided to close his optical shop and the committee was faced with a dilemma. It seemed impractical to find another skilled optician to undertake the formidable regrinding operation so we turned to locating a different primary. In the end a 25" f5 mirror was chosen to replace the 24", and we were fortunate that our earlier construction and purchases could be worked into the same general design.

And this left us with the hapless 24" ZeroDur mirror which my wife and I went up to Edmonton to reclaim last summer. Returning home, the heavy plywood box with its re-orphaned contents was carefully unloaded onto the basement floor of my shop.

There were echoes of Howard Carter in King Tut's tomb about this. The legendary mirror and myself were alone in the subdued light coming through a small upper window. One by one, the nails were eased from the lid of the coffin, er, plywood box, and finally the lid creaked open. The breath hissed out of me when only a blue styrofoam cover was revealed. Again tension built as this was lifted off, and there it was! Under a layer of protective paper the huge object lay.

Uncovering it, I was face to face with a beautiful aluminized surface, a bit blotched in places, but not abraded or sleeked. On one edge was an area where Barry had removed the aluminizing to explore the fracture. I'm not sure what I had expected, but the ZeroDur material was as clear as water… just beautiful. The fracture seemed inconsequential, a good ¼" below the surface, about 1 1/2" long and no more than half an inch wide.

The large ten inch hole in the centre combined with the enormous 4" thickness gave the mirror a surreal appearance compared to any mirror I had ever used or worked with.

Still in King Tut mode I carefully placed the mirror on its ample edge and with a flashlight backed off until the surface was flooded with light. I halved the centre of curvature and found the mirror was an f5.6. Ellipsoid or not, it was tempting to somehow jury-rig a setup to try it on the sky.

I soon settled down, but not before exploring the subject of small correcting lenses which would be one way for a future owner to get around the ellipsoidal figure. I further concluded that if no one else was interested in acquiring the 24" mirror, Okanagan Centre might hold on to it for a future project. In this case I would suggest simply refiguring the ellipsoid to a paraboloid with a subdiameter polisher, or tool if the correction was too large to polish out. If the small fracture area showed up in the final Foucault test, it could simply be masked off.

The resulting f5.6 mirror could be configured as a traditional Newtonian and mounted as a Dobsonian, providing the mirror's 4" thickness and 10" perforation could be overlooked. It would in effect have the light gathering power a 22" telescope.

Such a plan would be years off for Okanagan Centre, and if another club wished to proceed with a plan of their own for the 24" we would be glad to cooperate. Both the mirror and the secondary mounting unit would be available for $1,500 (FOB the Okanagan) which is the amount the club invested in transporting the mirror out from Toronto.

If another RASC Centre is interested, please contact Okanagan Observatory chair Guy Mackie at guy.m@shaw.ca.

Dave Gamble is a member of the RASC Okanagan Centre and has been making and using telescopes since his childhood.