You seem to have me on that one. I use eyepieces so little these days that I missed it, as I mostly use the 24mm during initial alignment and visual use, and it does not have a lens in this component. I guess I always just assumed that this last element was a parfocalising corrector to compensate for focal length differences when using it as a 2", due to the eyepiece sitting back farther when in the 1.25" configuration.
According to Baader's info:
Quote:
Each Hyperion eyepiece in the series is actually two eyepieces in one! Simply by unscrewing the forward 1.25-inch diameter portion of barrel that houses the negative achromat lens group, will provide a wide-field eyepiece of much longer focal length. Color correction remains
excellent, and the field curvature that results is no greater than in a decent specimen of the 6-element Erfle design (and that’s what it is actually, when the negative achromat is removed).
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