Considering Imported Mirrors
Aug 26, 2013
I often get questions about the inexpensive, "imported" mirrors. People want to know if they are good or poor, of course, along with other questions about value and such. This is understandable, given that the import mirrors are one half to one third the price of a custom-made premium mirror. Before I dive in here, though, I have to explain my bias. My exposure to these imported mirrors is generally as candidates for refiguring. And that means than in general, the only mirrors that I ever get to test from a customer are suspect. Mirror owners very seldom send me good mirrors, asking just how good they are. No, the mirrors I examine are usually here because the owner isn't happy in the first place. So I would expect that I am far more likely to see sketchy mirrors coming into my shop.
That being said, the mirrors that I see from the foreign mass-market fabricators range from "oh, that's awful" through "this one is pretty bad" up to "that's a tolerably good mirror." I've never seen one approach what I would call "premium" and it's actually pretty seldom that I see one that passes the the old 1/4-wave, 0.80 Strehl test. And most of those don't pass the Millies-LaCroix tolerances either. But again, I wouldn't expect the owners of the great mirrors to approach me for refiguring services!
Recently a client sent me a mirror that was a candidate for refiguring. Upon testing, it came in at about 1/2-wave P-V on the wavefront, with a .71 Strehl. The mirror had bad zones, a deep central hole, and a lot of radial marks due to aggressive polishing. After reporting back, the owner had the vendor pick out another mirror and send that one in for testing. The second mirror was considerably better. It came in at slightly better than 1/4-wave, but it didn't pass the ML tornado test. The surface was also quite a bit smoother. Still not a premium mirror, but this one would probably produce pretty good images at the eyepiece.
After consideration of these things, my client decided he wanted a premium surface, and he decided to refigure the worst of the two mirrors. Long story short, I did the work and shipped the mirror back. The client was very happy with the changes in the mirror, and sent me a report of his "first light" experience. I really appreciate his kind words, and he has succinctly pointed out the kinds of improvements that are possible with one of these mirrors:
"Hi Gordon. Despite having a pretty turbulent atmosphere I wanted to let you know I did get a chance to do some observing last night. While Saturn had already sunk into the murk and the moon wasn't yet risen, I was able to observe various DSOs and clusters. The Lagoon, Trifid, and Omega nebulae were all visible through the light pollution and with the improved contrast of the mirror I was able to make them out much more clearly from the rest of the field of view. What caught me most while star hopping around those objects, though, were all the much fainter magnitude stars that previously I was unable to see. I found myself just meandering through Sagittarius and admiring all the tiny pinpoint resolved stars. While observing the Butterfly cluster I was able to discern some subtle colors of the various stars. I have to say the most telling view from the evening was the Hercules cluster... when I turned the mirror toward it, it definitely popped in the eyepiece and all the stars were sharp and contrasty. When I observed prior to the refigure I kept feeling like the cluster was never in focus. This time, though, all the stars were resolved to sharp pinpoints. To date, that is the best view I've had of the cluster. I really cannot wait until I get the chance to turn the mirror on some planetary objects and see what it can do. If the weather holds out next weekend, I'll give it another run and let you know how it goes. I can tell already from the limited viewing I had last night that this is going to be one special mirror, and cannot wait for my first view of Jupiter!"
I have attached below the before and after images showing the Ronchi and Foucault graphics. If you would like to see the actual FigureXP reports, please check on our Main Menu the "Project Space" area, under 12" F5.0 Refigure.