Polishing a giant telescope

According to CNN, the Subaru telescope is:

How do they polish something to this level of perfection? Obiously they don’t use an ordinary floor buffer. What do they do?

I heard this over thirty years ago at a club meeting in Cincinnati, OH. The speaker was talking about large lens manufactured in Lexington, KY. At first the company tried to use an automatic buffer, but that did not work. It needed to be done by hand. Everyone they hired to polish the lens would last one or two days and then would be caught using their elbows, legs and other parts of their bodies. Basically they got bored The problem was solved when the company tried mentally challenged people. They were told that the job was very important (which was very true). These people were able to concentrate on the job and were duly proud of their accomplishments. Since then it may have changed.

Cerium Oxide and Pine tar works for smaller mirrors. They probably use some fancy synthetic instead of pine tar on big mirrors, and computer controlled polishing arms, rather than a carefully randomized hand stroking, but the principals the same.

IANA optician, but I have ground and polished a telescope mirror.

These days large mirror blanks are almost all spun cast, which establishes the basic shape from the get go. The shape is refined by grinding with successivly finer grades of free abrasive grit, acting against “tools” which are sacrificial pieces of glass that themselves aquire a convex shape as part of the process. This leaves a finely ground (frosted) surface ready for polishing.

Initial polishing may be done with plastic surfaced tools containing polishing agent. This is adequate for eyeglasses, but astromnomical instruments demand much more precise surfaces.

The final polishing is done using one or more pitch surfaced laps. Pitch is made mostly of pine sap, and possibly other proprietary ingredients . Another thread about glass tries to distinguish if it is a solid or a liquid. Pitch IS certainly a liquid, as flowing is essential to it’s function in this case…however, in the short term it seems pretty solid. It’s ability to flow over time allows it to exactly conform to the mirrors surface.

The pitch lap is loaded with a polishing agent: Traditionally rouge (iron oxilate?) but nowadays more likely to be cerium oxide. The polishing is done wet.

Such a large mirror is likely to be pretty fast (low F#) so the exact curve will vary quite a bit between the center and the edges. For this reason the lap (s) will almost certainly much smaller than the mirror, allowing each to conform to the local curvature.

There is some pretty strong evidence that polishing of glass is due to microscopic flowing of the material in addition to material removal.

The laps are moved about with more of a translation movement than the rotary motion of a floor polisher, though rotation is allowed. Usually the mirror is rotating continuously, though fro such a large mirror, maybe not. (I know the Palomar mirror was spinning). Using lots of experience, and some trial and error (and probably computer modeling or at least SPC these days) the optician adjusts various parameters of the machine to change the “figure” (exact shape to fractions of a wavelength) of the mirror.

Thanks, keybo, that was very informitive.

Oh, is that what you call it?

The 200-inch (5.1 m) Hale Telescope (f/3.3) was the world’s largest effective telescope for 45 years (1948 - 1993). It is still a workhorse of modern astronomy. It is used nightly for a wide range of astronomical studies. On average the weather allows for at least some data collection about 290 nights a year.
200 Incher Workhores of Astronomers for 45 years!
IIRC it was cast from pyrex at Corning Glass in Corning NY.Grinding and polishing in CA.
Telescope mirrrors like mossst lenses are cast/and or ground to rough shape with secessively finer grades of abrasives.
They are then polished with iron oxide and other fine grained agent.
The Skylab telescope was not given the knife edge test and had to have some corrections done by the astronauts.
You would find “Amateur Telescope Making” interesting reading. Ask your library.


Are you sure you’re not an optician? :slight_smile:

Very interesting. Thanks for the info.

kniz, speaking as the brother of one who is mentally challenged, I’ve got to say that that story sounds like it has UL written all over it, but I can’t find anything on Google to confirm or deny it.

Sometimes an incredibly lucky sod gets to use it just have fun and look at whatever they want.

Can they be fixed if someone drops something on it, or are they screwed?

I don’t know about dropping things on them, but a large scope will still work if you shoot the mirror repeatedly with a 9mm handgun:
McDonald Observatory, University of Texas, Mt. Locke, Texas

Also, as the mirror nears its final figure repeated optical testing is used to identify the local areas on the surface that need to be worked on in order to bring them within the specification. When dealing with mirrors like that of the Hubble which was intended to work with near UV light exceedingly fine abrasives are needed. Jewlers rouge is almost too coarse to use.