would this work?

[tonybegg]

I do not own one of those ray tracing lens design programs. There is a reflecting (Cassegrain) configuration dating back to the 17th century by Mersenne that consists of a concave paraboloid focusing (star) light onto a convex paraboloid of the same focal length and producing a narrower parallel bundle than that reaching the objective. Some people have then put a small apo after this "field compressor" to make it into a telescope with a larger effective aperture. IStar are concerned with large refractors (no obstructions). If you replace the mirrors with lenses and say have a singlet BK7 convex objective of a given optical power focusing (star) light onto a smaller singlet BK7 concave lens of the same power but of opposite sign, we would end up with a field compressor and since the whole thing has zero power and uses the same glass there would be no chromatic aberration. If you then followed it with (say) a 90mm apo refractor would you end up with a well corrected system of potentially large (250 mm?) aperture? Or are other aberrations introduced (off axis perhaps) and since you don't have many degrees of freedom you cannot fix them? Thanks.

[tonybegg]

Sorry I mis-characterized the Mersenne. The two paraboloids have the same focus - the convex secondary has its virtual focus behind it and this is arranged to be where the primary's real focus is. The focal ratios are the same.

[astrotrailer]

I saw an article in Sky and Telescope a couple of years ago describing the above setup. Solar observers use the setup for refractors to achieve a collimated beam through an Ha solar filter which then uses a positive lens after the filter. This allows the use of a large objective with a small filter.

[tonybegg]

Thanks astrotrailer. Problem is that solar observers in H alpha do not need chromatism. I realize that what I described as a "field compressor" (elsewhere I have found it is called a "beam compressor" or more commonly in reverse as a "beam expander" - they sell them as Galilean arrangements for lasers but that again is monochromatic) is very similar to the Schupmann telescope. This uses a singlet objective and a Mangin mirror (back silvered mirror) of the same glass. The glass part of the mirror does the inverse of the objective so together they have zero power and any longitudinal chromatic aberration cancels, the mirror part then focuses the light. Things need to be tilted somewhat and a field lens is needed to counter lateral chromatic aberration. So my idea was a straight through version of this, with the lens part only of the Mangin mirror, relying on a small apo to bring to focus. The Schupmann has perfect apochromatic behavior (better than a triplet apo) on axis. I was wondering if such an arrangement could be used instead of achro + Raycorr to allow large aperture refractors with apo performance. I expect like the Raycorr it is the lateral color (off axis color) that is the difficult thing to minimize. Wish I had a lens design program and the smarts to use it.