I know we've discussed this subject before, from time to time, but I've been reading a lot of threads about planetary eyepieces lately and this made me think of it again, since people often see extremely subtle differences between what is often considered very high-end eyepiece lines of excellent quality, yet they rarely mention a crucial part of the light path:
I'm mostly thinking of the kind used on refractors and cassegrain-type telescopes, but the ones used on newtonians obviously apply as well, though I'll primarily discuss the former type.
I really do wonder, if all the people who discuss high-end planetary eyepieces (or just eyepieces in general) here with such vigor, have all considered how large an impact the diagonal can have on the view. I'm quite sure that some haven't and are just using one, based on the assumption that since it was from a well-known maker, it should be good enough.
But is it?
My own experience tells me that one can't be sure about that. My first 2" Baader dielectric diagonal certainly wasn't so hot optically, but this only became apparent, when I tested it against another diagonal. And even my 2" Baader Zeiss prism diagonal is not quite as good as my Baader Zeiss T2 diagonal. And it is a fact that even the Baader/Zeiss T2 diagonal isn't invisible. It logically follows that if the diagonal obviously (even if slightly) degrades the view, then this is going to completely overwhelm any subtle differences between eyepieces and will limit what the telescope is ultimately capable of. It doesn't help that even some relatively expensive diagonals have shiny parts in their barrels, are out of collimation, etc.
We're currently seeing almost a kind of war between several companies about who can make the very best planetary eyepieces. Is it time we saw a similar war between who could make the most "invisible" diagonal for planetary observing?