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Ken is right, narrowband filters cut the band enough that bloating isn’t an issue as long as you refocus in between. As the focus shift is only on the order of maybe 100 microns or a bit more, registration isn’t an issue.
Where an APO really does well in narrowband imaging however is that most APOs have a considerably better optical figure than most achromats because they’re built to a much cheaper standard. THIS is where an APO will be better, smaller tighter stars and a better corrected field with less spherical aberration.
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