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I suspect the bigger stars are due to the effect of atmospheric dispersion. Ha and Sii are both in the red end of the scale, Oiii is green/blue. I see this effect most strongly when using RGB - the B stars are larger - even when optimising focus for each colour (refractor). The only cure I know of is avoidance - that is shoot Oiii (or B) when your target is at maximum elevation. Would love to hear if anyone has alternate solutions.
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