August 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - August 1999 Issue

The Telescope Nut
by Jeff Baldwin

Optical Flats for Secondary Mirrors

I recently made a couple of secondary mirrors. The project was successful...with some errors. Learned a few tricks along the way.

I bought three 9" Pyrex blanks 3/4" thick. I labeled the sides of the first one A and a, the next one B and b , and the last one C and g. I then ground A with B, B with C, and C with A. I cycled around this way with rough grit, then fine grit. Doing three like this keeps them flat. I checked between steps with a spherometer to keep as honest as possible. I then polished all six sides. I chose sides a, b and g as the best sides of the three plates.

To test how well they mated with each other, as far as being concave or convex with respect to each other, I used a fringe interferometer. It is a Hg. vapor light source with deep green filters, allowing only 546.1 nm wavelength light to pass through. This goes out onto two glasses spaced atop each other, then back up to a reflecting glass that allows me to see the glasses as they are illuminated. Cellophane spacings are placed between the glasses to tilt them very minutely. As constructive interference and destructive interference occurs between the glasses, bright and dark bars are seen. If they are straight, then the glasses' surfaces match, if not, they are concave, convex, or some other goofy mismatch.

I tested a with b, b with g, and g with a. I then determined what the shape of the glasses were and which way to correct them. If the bars are curved upward (smiling) when the thin spacing is toward me and the thick spacing is away from me, then the glasses are concave with respect to each other. If they curve the other way (frowning), then they are convex. I used a little algebra to determine which way and how much the glasses need to be corrected, and worked them until they were flat. (Holler at me if you are going to do this and want the arithmetic.)

The glasses were finally made fairly flat. They deviated by as much as 1/10 wave at 546.1 nm. My commercial secondary that was on Black* tested spherical, being about 3/4 wave across the minor axis, and over a wave along the major axis. These new flats would be much flatter, and therefore better as secondary mirrors.

I waxed b against g and placed them at a 45° angle in a wooden box. I filled the box with plaster. When dried, I drilled a 4" hole through the whole thing. The core that came out contained two secondary mirrors, ready to be aluminized and then glued into the telescope.

Here's where I goofed:

I should have used blocking pitch instead of wax. The glasses came loose and clunked around, chipping the outer edges of the secondary mirrors.

I used rolled sheet metal around a round pipe end to drill the holes. I should have used pipe.

Originally, I tested the mirrors in the garage. The temperature variation was great enough for the glass to change shape during testing, so between polishing episodes I would take the glasses into the constant temperature house, wash them in room temperature water, fan them for half an hour using room temperature air, then set them onto the fringe tester for half an hour while they relaxed into their least restrictive condition. This took the most time in making them. I would work the glass for a few minutes, then spend a few hours waiting for them to relax, then video tape their fringe images for one minute. I would get about 10 minutes of work done for every couple of hours of waiting time!

On the following page [below] are some images. The oval is the original secondary mirror from Black, very spherical. The round one is glasses b with g, which both eventually got sawed out.

The new ones aren't perfect, but much flatter.

I star-tested Black using the original secondary mirror and there is an astigmatism. I tested the mirror in the shop and there is no astigmatism in the primary mirror, so I assumed that it was due to the spherical nature of the secondary combined with its 45° tilt. When I sawed out the new secondary mirror and glued it in, most of the astigmatism went away. There was a slight amount left over, but it was probably due either to the mirror not being brought to air temperature, or the slight spherical error left in the flat was enough to see the astigmatism. At f/3.7, it doesn't take much. When the mirror comes back from the coating services, I'll star-test under well-controlled conditions and see what became of the image.

The next secondary mirror I'll be making will be for the 40" scope. It will be 7" wide and 10" long. I'll use 12" glass to make it, and I will strive for error free flatness and no chips on the edge.

*For the benefit of new readers, "Black" is Jeff's 24" Dobsonian.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.