This was posted today in Yuri's fb group.
I thought I would share this here as well, to all those afflicted or think they will become afflicted...
Tyson - thanks for posting... I saw this a few days back when Yuri was discussing it and I was a little suprised to see it. I have owned three TEC OTA's purchased from Yuri and was fortunate enough to tour the TEC "factory" in Golden, CO some years back. From the slightly less then ½ day I spent at TEC I came away knowing that TEC is the "real-deal" when it comes to making telescopes. They control almost every single part of the process from optical manufacturing to OTA fabrication. At one point, TEC was even producing focusers for their OTA's but have switched back to the FT3545. So, it may be better to outsource some of the production to SME (subject matter expert) type of companies on expensive, hard to produce parts.
When the conversation with Yuri, Alex and Eduard got around to optical fabrication Yuri told me that they do not use interferomtery on every serialized lens. They did use test plates and double pass auto collimation among some other methods.
In any event, seeing Yuri's post surprised me (in a good way) that interferometry has become part of the standard production process. As good as the star test, double pass, foucault etc. is - these tests are qualitative. Sure, they can help discern a great optic from an average optic from a poor optic; but, only an interferometer is able to assign wavefront ratings and truly measure the lens or mirror under test. When an interferometer is used in a production environment two things happen: (1) It tells the optician when the lens meets the minimum spec and is, therefore, finished; (2) It certifies the lens.
In the TEC io.group Yuri mentioned that he can test up to a 100 lenses per day. And, while I am not privy to the TEC testing process, interferomtery is very sensitive to temperature deltas in the glass. Meaning that after you touch a lens to move it onto the test stand, the interferometer will give wildly inaccurate readings because the surface of the glass changed shape from the heat of your hands touching it while moving it into place on the stand. Sometimes you need to wait 15 minutes or more depending upon the size of the lens under test.
It would be very good to know what wavelength (color) Yuri is testing at. The most common is red (635nm) but most refractive optics are nulled in green. If testing in red, the results when testing in green will be worse. There is no reliable way in which you can use simple math to convert the PtV/RMS/Strehl in red to what the equivalent would be in green because of the effects of spherochromatism.
Edited by peleuba, 01 May 2020 - 09:47 AM.