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I'm a skeptic when it comes to these super flat flats in amateur telescopes. I think if you have a genuine 1/12 or 1/15 wave flat it is amply good. Most mass produced flats polished on a planetary polishing machine the error is a pure concavity or convexity. Zemax ray- trace simulations suggest that a 1/2 wave spherical flat causes a 4 or 5 % reduction in the Strehl ratio due to theoretically causing primary astigmatism in the image which still makes a high Strehl ( 0.98 to 0.99 ) mirror come in the mid nineties close to perfection in the image.
I don't believe , particularly in the light of the real thermal conditions a diagonal is subjected to that you will ever realise the difference visually between a true 1/12 wave and a 1/30 wave flat. I understand that there are anecdotal reports ( my fire proof suit is donned ! ) as such but I would like to see the results of some double - blind tests in identical instruments under superb conditions. Comparing instruments , even the same one with tweaks to the optics is a very difficult thing to do given constantly changing seeing and thermal gradients on different nights.
I have my own anecdotal experience of comparing view through a very fine 20" binocular telescope in good seeing conditions. One of the 5" diagonal flats was no better than 2 waves and we really were hard pushed to see the resulting astigmatism caused by the bad flat - the image was still very good in both sides . That night brought home to me the very different effects in the instrument that errors can produce in both primary and secondary. A 2 wave error in the primary in the form of spherical aberration would have rendered the instrument completely useless !
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