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I have 36mm Astronomiks with my 2600MM and while there is vignetting visible in a stretched flat with my 10"F4, unstretched you would never see it and flats correct lights nicely. I have only ever run across a noticeable Oiii halo the once and that was on an extremely bright star. I can't comment about any cheaper or price comparable filters to the AStronomiks but the Chroma's and 'Dons would want to be pretty flash when you compare the money.
They are close enough to parfocal to live with the difference, at one point I had a clear and HA-Oiii with my 2600mC, focusing through the clear and they looked fine. There is a trend of focus difference visible between the filters but it was not much more than the variation you see focus run by focus run with the same filter. Focusing through the individual filters did produce measurably sharper results, subjectively there was not much in it.
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