Good question. The main reason to avoid a cylindrical primary baffle is to minimize diffuse scattering visibility at graze angles along the tube's exterior surface. That's also why the secondary baffle is tapered down away from the primary mirror.
If the primary baffle were straight and cylindrical, stray light reflections from the primary that graze off the baffle heading toward the secondary would then be reflected from the secondary back to the focal plane, but not be anywhere near focus. This would create fogging and contrast reduction at the focal plane.
If the primary baffle is conical but just fits inside the innermost light zone on the primary, it would also create stray graze angle scattering toward the secondary and on to focus.
To avoid either of these grazing conditions, I set the rear diameter of the primary baffle 4% smaller than the light volume. The light from the innermost zones of the primary does not intersect the baffle until its forward end toward the sky. This does not totally avoid graze angle fogging, but it reduces it.
If the primary baffle were tapered so that the rear diameter is halfway or so between being a cylinder and the innermost used zone on the primary, sharp-edged baffle rings could be installed on the exterior of the primary baffle to trap any grazing light. This is the same principle as interior baffling. The primary baffle would thus have two sets of sharp-edged baffle rings, one set on the outside and one set on the inside as in a refractor.
However, these exterior baffle rings are directly exposed to stray skylight and sources, and can act as annular sources of stray light as seen from the secondary mirror. Their profile needs to be as low as possible to just trap grazing stray light down the baffle tubes without presenting too much projected area to the secondary.
These additional rings might also be a little too much trouble to make (except for Preston and Jason!). I chose to taper the primary baffle just inside the inner zone volume and leave the tube exterior clean. A ring baffle can always be added later if a problem becomes apparent. As I did for Preston, the primary baffle can also be partly conical, then transition to cylindrical with at least one exterior stray light baffle ring. Interior baffle plates are still installed inside the primary baffle.
Stray light control is maddening because it always seems like a countermeasure/counter-countermeasure exercise. Stray light can come in from any angle, in contrast to the actual relatively narrow FOV of the telescope. You can account for nearly all possible light paths with enough multibounce raytracing, but you reach a point where what you're doing is just not reducing enough fog light to be worth messing with.
All these partial tapering and exterior baffling techniques would be interesting and easy options to add to CassDesign and this ZEMAX baffle optimization macro code.
Mike
-------------------- 56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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