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I posted these images in my ED80 thread but I'll put em up here again.
The Jewel box and Mimosa in Crux comparison. Pic4 is from a 71mm achromat and pic5 is from an Ekinox ED80. As Bojan mentioned, achromatic scopes can't focus all colours at the same focal plane. You can focus red, green and blue individually but not all at the same time when shooting one shot colour. What happens is one or more of the colours will be out of focus in the achromat and bloat, giving a red, blue or violet fringe.
On the other hand ED or APO scopes do a better job at focussing all the colours at the same focal plane.
I've included a close up of halos caused by the red plane out of focus using a 4 inch achromat.
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