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Old 24-01-2018, 09:20 AM
Stefan Buda
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Stefan Buda is offline
 
Join Date: Jul 2013
Location: Melbourne, VIC
Posts: 401
Well, a lot has happened since my last post and not all good.

After reworking R4 and reversing the incorrectly installed lens, star testing showed that the coma was gone, but the Roddier test showed an increase of the astigmatism. Looks like the lens may not have been evenly supported during polishing. The instrument seems extremely sensitive to miscollimation, that shows up as astigmatism, and I don't know at this stage how much of the stig is due to what.

I also had an adventure with the AR coating of the corrector lenses.
After searching far and wide to find someone willing to do BBAR coatings, I gave up and decided to do my own single layer MgF2 coating. I had a larger than usual evaporation boat already installed, one that I had used for evaporating silicone monoxide, and decided to use it rather than swap it out for one that I was used to for MgF2. It turned out to be a big mistake. As I was adjusting the current, I managed to overheat the boat and the MgF2 started spattering and I ended up with two lens surfaces with tiny droplets of MgF2.
Doing the second sides of the lenses I was more careful and had no problem.
Since then, I reground, polished and recoated one of the spattered lenses.
The second one can wait as it is not important at this stage.
Last night I sent Bratislav another set of test images for Roddier analysis.

Here's Bratislav's comment: "Yup, close enough. If stig is taken out in software, it is diffraction limited (1/22 wave RMS). PSF with and without astigmatism included in the snapshot. With stig components "on", Strehl drops to about 0.5 (L/8 RMS)."
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