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Genesis SDF
The 4" f5 Genesis SDF is not an APO; it's better!!
The SDF stands for Special Dispersion Fluorite. The Objective is a two element lens with Fluorite elements and further back in the tube there is a special two element Nagler "corrector" which give it a tremendous colour corrected flat field. So it is a four element system.
Re the blue halos etc I've never had this on any photos I've taken with the Genesis and if you check out the web you'll find many conventional film and digital astro images from the Genesis which are 100%
I can only assume an artifact of the processing of the image???
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