Robert7980

Optics matter the same regardless if you’re mono or OSC, there’s simply no substitute for premium optics. Refocus per filter can help disguise CA, but the overall quality is just far superior with superb optics. My Esprit 120ED absolutely destroys the performance of my budget triplet with FPL51 glass even though it has no CA, that’s with filter offsets in mono. The question is when is good enough good enough and the cost starts to outweigh the benefits. So there’s much more to it than just CA.

The Canon spec is 44mm, but with any optic it’s whatever spacing is required to flatten the feild. For terrestrial photography it’s not critical, for the extreme demands we place on optics for astrophotography even tiny differences can matter. With my Samyang 135 I’ve found adding about 0.7mm to the ZWO Canon filter drawer gives better corners with the 2600MC and factory infinity focus is much closer to dead on. I’m not sure what the exact spacing of the stack actually is though. This is without a filter, would be slightly less than the 0.7mm with filter, closer to 0.5mm. The lens needs to be stopped down some from f/2 to achieve perfect corners as well. F/2.8 is decent and f/4 is nearly perfect. This is best done with stop-down rings rather than with the lenses iris due to the diffraction the iris causes. I’ve only played with the one lens so I’m not sure how much they vary copy to copy, I doubt it’s much though.

It’s been the same here in North Carolina USA… Just one or two partial nights per month for more than a year now. Had a good week with a few days in a row of clear, but wildfire smoke from Canada made imaging impossible. Been frustrating.