|
I'll add the problem with Amici prisms for astronomical use: the line down the middle. Though relatively invisible at low powers (binoculars), it is very visible at high powers, especially when a star drifts across the line.
The split and recombination of the light path can produce phase issues upon recombination as well.
As if that weren't enough, there is the issue of chromatic aberration with oblique angles of entry ("prism", remember) which confines the best use of a prism to long f/ratio telescopes, where the angle of incidence varies little.
As far as light transmission goes, I see little difference between a multi-coated prism and a mirror diagonal. However, at least in theory, a 1/10 wave mirror would result in a 1/7 wave error after passage, while a prism with 1/10 wave surfaces would result in a 1/3 wave error after passage. A prism would have to have more accurate surfaces to produce the same wavefront error after passage. Yet another reason why prisms are typically confined to low-power use.
The question of having the image not match star charts is one that typifies someone with a little less experience in the field [not that, from the list of scopes, this applies to the original poster]. I felt the same way for most of the first few months I observed (I had a reflector, where the image was upside down relative to my charts), but quickly adapted to the difference in orientation. Computer charts can be printed with any orientation, even the mirror-image orientation of refractors and catadioptrics, but pre-printed chart sets can be difficult to use if one has a variety of scopes with a variety of orientations.
Assuming that high-power use is planned, it is hard to beat a good mirror for use in a star diagonal, IMO.
|