April 1, 2011:  Unfortunately NOT an April Fool's Joke

The old adage goes - "You get what you pay for."  In this case, the client found it out the hard way.

He bought a piece of "quartz" from a Chinese glass company.  Keep in mind that at this point, Chinese glass is not exactly the greatest in quality.  The blank didn't look too bad after we generated it, so I agreed to proceed and see how it ground and polished.

Immediately into grind I noticed parts of the glass contained signifant impurities and bubbles.  Clearly, it was not going to be up to the client's standards when polished, and I let him know.  I also said I would continue and polish it out, to see how the surface looked.  Then, hopefully it would be useful for an application where lots of bubbles didn't really matter, such as LIDAR, etc.

Here's what it looked like as it was being polished.  At this point all of the "stuff" in the glass could be seen, and where they met the surface some marks could be seen.  Still, I felt it might be optically useful, and I kept the polishing machine spinning.

Weird "quartz"

After putting a good polish on it, it was time to put another mirror on that machine, and I decided to give the mirror a quick Foucault test.

Here are photos of the test - the first photo is at 0.000",  the second photo is 0.025" farther away, and the third photo is 0.025" farther away.

Inside ROCAt ROC
Outside ROC

The rotation of the knife edge shadow is clear and dramatic, an indicator of severe figure of revolution problems.  In fact, I have not seen a figure this asymmetric since I tested Bob Holmes' 32" mirror after it was returned from an incompetent optician.  (See these images of 32" mirror before I reworked it, and note the asymmetry of the shadows.)

Also note the "fingerprints" of the "features" in the glass where they intersect with the surface.  These form dark and light areas across the surface.

In this case, the substrate is at fault.  No material should polish out with this much astigmatism.  It's simply BAD GLASS.

To add a quantitative aspect to the measurement, I set up my interferometer and snapped a photo of the image on the monitor.

Interferogram of mirror

The fringes clearly show a "saddle" shape, where two quadrants on one axis are higher than two quadrants that lie on an axis perpendicular to the first.  In other words, using a clock face as a reference, the mirror at 10:30 and 4:30 is either higher or lower than the mirror at 1:30 and 7:30.

To quantify the error, simply count the fringes between neighboring quadrants.  My count gives 15 - 16 fringes, or 7.5 - 8 waves on the glass.

So, this piece of glass is not very useful.  Not sure what we'll do with it.  Maybe it would make a good bird bath, or possibly a table top.

So what is the moral of the story?  Certain sources of glass are not to be trusted, and you should trust your optician to select a good supplier to provide the substrate for your next telescope project.  Life is short, but you can be stuck with a bad mirror for a long time if your money is not spent wisely.

My sources for glass have a track record of good annealing, good machining, and timely delivery.  That's why I have chosen them.

Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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