Posted 09 October 2012 - 06:42 PM
Vignetting with focal reducers has two primary sources.
1) The far field obstructor. Examples include the inner opening on SCT primary baffles, or Newtonian secondary mirrors. Due to their considerable distance from the focus, any vignetting they introduce is seen as a more gradual fall-off in illumination, beginning immediately from the edge of the circle of full illumination.
2) The near field obstructor, which here is the reducer's clear aperture. The vignetting introduced here is of a more rapid fall-off somewhere near the field edge, the 'sharpness' of the shadowing being due to the relative nearness of the the reducer to the focus.
A reducer in a cell of 2" OD does *not* require a 3" rear opening on the scope. Even a 2" opening is good. In fact, said rear aperture need be hardly larger than the reducer's clear aperture. Remember, the only light getting through to the focus is that which enters the reducer's front aperture; light outside this is of no account.
The Universe's 28.4mm diagonal demands of a reducer working at 0.5X a clear aperture of *at least* 2" if near field vignetting is to be minimal. This is impractical for most scopes. And so there is a sharp fall-off near the edges on the long dimension. But the component of far field vignetting, upon which the near field vignetting is superimposed, is not itself affected by the reducer, beyond the expected reduction in scale of the illumination profile. That is, whether a 0.5X reducer has a small or large clear aperture, the far field vignetting is the same, but the too-small reducer aperture superimposes its own component of sharper fall-off inside the field edge.