Quote:
Originally Posted by glend
Well Greg it does have a thick front corrector plate, and while I am not sure exactly how that bends the light ( ala a refractor objective shape) it obviously is doing a good job.  |
Looks really good Glen - terrific results
Greg, the SW MN gets by with a small obstruction, because the designer did not insist on full illumination away from the field centre. This will produce some vignetting, but it looks to be quite manageable. This design will still produce round stars up to the edge of the field, because coma is controlled - which means that the atmosphere determines the star shape, not the scope.
The light does not bend around the secondary at all (apart from a little diffraction at the edges). The secondary obstruction produces a light cone that has a hollow core, but it still focuses to a geometrical point. The actual point spread function of an obstructed scope will have significant energy outside of the central peak, mainly due to the diffraction of the secondary - but for DSO imaging with the scopes we are considering, that does not matter, because the spot size due to the atmosphere is maybe 10x bigger than that due to the scope (in area). Provided aberrations are controlled, what the scope does is almost immaterial. That is why a scope with large central obstruction (eg RHA) can still produce high contrast images even though the design has very low theoretical contrast - it is simply that the contrast (and resolution for that matter) is all down to the atmosphere and not the scope. In any event, contrast can be adjusted in processing - it really is not a major issue for imaging (although it is for visual).
When we compare larger scopes by looking at their images, we are actually comparing atmospheres - above about 6 inches aperture, scope performance (assuming reasonable quality) has only a modest effect on the outcome, except in exceptional seeing. In typical Australian conditions, Glen's scope should do almost as well as anything, including the AAT (although not as quickly).