Hi Joe: Not a dumb question at all, but a little clarification may be in order. Are you asking about star diagonals? See the current issue of S&T (August) for a great description of the wedge prism and it's use in eliminating atmospheric dispersion. I've got the germ of an idea for a home-brewed cell incorporating the wedge prism, slide the whole works into the focuser, stay tuned for that. Other projects ahead of it, though. An eyepiece class would be nice, for instance... As far as a mirror vs. prism when used as a star diagonal or Newtonian secondary, you have to remember that a with a prism, the light path is through the glass. Glass has a refractive index, meaning that as light travels through it, it is dispersed into it's various wavelengths. Left uncorrected in a telescope, this is called chromatic aberration. Bright objects can display a colored halo, with the color or colors depending on what wavelengths are just out-of-focus. The greater the angle of incidence, the greater the distance the light travels in the glass, and the more it is refracted or dispersed. In a telescope what this boils down to is that a prism should probably not be used as a star diagonal or Newtonian secondary in fast optical systems because it will introduce chromatic aberration. My preference would be about f/10 as the cut-off point of prism use if we are talking about the main instrument. The amount of chromatic aberration introduced is small and usually only seen at higher powers unless the f/ratio of the system is very fast. If you have an f/10 SCT, a prism should be fine. But if you use a focal reducer and change the effective focal ratio to say, f/6.6, perhaps a mirror would be better. Most binoculars have quite a bit of this color fringing but we don't really notice it unless specifically looking for it. At powers under 20x, it isn't a problem at all. I use prisms on many of my finderscopes, all 'fast' systems. -BUT- If you want top performance out of your system, especially at high powers on bright objects, or to discern low-contrast details of similar hue, this is something to consider. Modern coatings render reflectivity issues moot. Does this help? Chuck --- Joe Bauman <bau@desnews.com> wrote:

OK Chuck, here comes a dumb question -- but it's been bothering me a little. Can you elucidate the pros and cons of mirrors vs. prisms? Thanks, Joe

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