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the makig of either a 2" (say 50mm) or 1.25"(30mm) ep depends on the field stop (basically the edge of fov of the eyepiece)
a 9mm eyepiece will have a field stop close to 9mm, therefore there is no reason to fit it into a 2" housing -the 1.25" size is more than enough
a 40mm will benefit as if you put it into a 30mm housing you are clipping the fov, so it will benefit from the larger 2" housing -you can effectively use a arger fov in the longer f/l eyepieces
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