Herbert, Another interesting post.... some comments....
You are making too much of the F/3.5 light cone & eyepiece business. The F/20 Schief light cone, while 7 degrees off axis, is still F/20. It does not have the same eyepiece aberrations as a filled F/3.5 light cone, since only a tiny portion of the F/3.5 is illuminated. Below I will include a spot plot for an F/20 Schief with an old fashioned 6-element Erfle. All of the light is well within the Airy disk shown by the circle. Whats the problem? There is no problem.
In my next message I will post the same Erfle in an F/5 Dob. The performance of the Erfle in the F/5 Dob is similar (maybe a little worse) than the Schief. 1980's eyepieces killed the Schief no more than they killed the Dob.
I can report that Naglers work fine in Schiefs. But Erfles were fine too.
Next lets talk about size and weight. Whats your comparison scope? Lets compare apples and apples -- un-obstructed, apochromatic systems for planetary observing. An 8" F20 Schief is about 60 pounds and 68" long. A nice 8" F9 TMB APO refractor is 68 pounds and 65" long. Pretty similar.
Three mirrors? No more than your average SCT, Mak-Cass, Classical Cass + diagonal. I don't see this as a big deal.
So... bring out your Schiefs.... the waters fine....
You are making too much of the F/3.5 light cone & eyepiece business. The F/20 Schief light cone, while 7 degrees off axis, is still F/20. It does not have the same eyepiece aberrations as a filled F/3.5 light cone, since only a tiny portion of the F/3.5 is illuminated. Below I will include a spot plot for an F/20 Schief with an old fashioned 6-element Erfle. All of the light is well within the Airy disk shown by the circle. Whats the problem? There is no problem.
In my next message I will post the same Erfle in an F/5 Dob. The performance of the Erfle in the F/5 Dob is similar (maybe a little worse) than the Schief. 1980's eyepieces killed the Schief no more than they killed the Dob.
I can report that Naglers work fine in Schiefs. But Erfles were fine too.
Next lets talk about size and weight. Whats your comparison scope? Lets compare apples and apples -- un-obstructed, apochromatic systems for planetary observing. An 8" F20 Schief is about 60 pounds and 68" long. A nice 8" F9 TMB APO refractor is 68 pounds and 65" long. Pretty similar.
Three mirrors? No more than your average SCT, Mak-Cass, Classical Cass + diagonal. I don't see this as a big deal.
So... bring out your Schiefs.... the waters fine....
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