Although I’m far from an expert in these matters, I believe your correct about the converging beam introducing astigmatism. To achieve the correct image scale, these ADCs are usually used behind a Barlow. Would this limit the astigmatism? And does the long resulting focal ratios achieved (>f20) limit the effect? I know that in practice I see improvement in image quality.
The attached image was taken at 17° altitude with a 7″ telescope in July. I can’t replicate this without the ADC without nasty colour fringing.