Speaking of eye relief and comfort, this made me think of an eyepiece I have:
And described in detail in: https://www.cloudyni...epiece-sort-of/
Today, I compared it in a binoviewer with a 10mm Zeiss Jena ortho and their magnifications are identical, so it's a 10mm and not a 10.5mm, as I originally guessed.
But here's the interesting thing: This eyepiece is MUCH more comfortable than the 10mm Zeiss ortho or a 9mm VT ortho. It's as comfortable, more or less, as a 12.5mm UO VT ortho. It's also far more comfortable than a 10mm Plössl. And it has a wider apparent field than any of them, around 55°.
But the real shocker was that it's also SHARPER and BRIGHTER than the 10mm Zeiss Jena ortho in a direct comparison last night on Mu Bootis. The dim double companion (it's a triple star) was definitely easier to split in the 10mm König, than in the 10mm Zeiss! I did multiple swaps and the result was always the same. Sadly, clouds rolled in, before I could do some more comparisons.
Even more shocking is that the entire binocular that the 10mm König comes from cost HALF of what a single 10mm Zeiss Jena ortho did, when they were still produced. And the König is a better planetary eyepiece (at least on-axis). That makes absolutely no sense whatsoever, in light of recent claims that superpolished eyepieces are more expensive than others.
Off-axis, the König is showing some field curvature and astigmatism is starting to get a bit apparent near the edge, but in general use on my f/13 refractor, stars are quite tight over most of the field and performance is very pleasing. The center is absolutely dead sharp. I need to see how it performs at f/8 and at shorter focal lengths.
Now, I need to do more testing on several different telescopes, but my gut feeling is that this design should work eminently well down to at least f/8. Given how very comfortable it feels at 10mm, I'd say that this should work eminently well down to 4mm, just like an ortho. Given that it avoids the problematic triplet of the Abbe ortho and has much better eye relief than the symmetrical Plössl, I think the König should be given some serious consideration in a dedicated planetary eyepiece line.