What is the lower bound for converting a classical Cassegrain to a faster f-ratio?

Doug,
The lower bound on the F/#  for a Cassegrain type system occurs when the size of the secondary equals the size of the primary.  When you tackle this kind of project there a few basic things to remember.  First, everything gets harder as the focal ratio gets smaller.  Aspheric surface shapes become steeper and more difficult to make.  Second, the field correction of a classical Cassegrain is not all that good.  Achieving and F/4 or F/5 speed at the output will indeed provide a wider field but you'll also have a lot more coma and more field curvature than an equivalent Newtonian.  As the optics get faster, it becomes necessary to introduce refractive elements to control the field and to minimize the obscuration ratio.  The other soft underbelly of these fast two-mirror systems is the difficulty of achieving and maintaining the alignment tolerances required to get good field performance.  You can certainly design some field optics to flatten and correct the field but if you are going to do that, it's easier (and more realistic) to start with with an RC or CDK configuration.  The biggest advantage of a Cassegrain type system is that they are compact; but in my opinion, once you get faster than around F/6, the difficulty of achieving good optical performance becomes very challenging.  It's not impossible to go faster but the manufacturing and alignment requirements become VERY challenging.

Another big reason that few Cassegrain type systems aren't much faster than about F/6 is the size of the secondary.  At F/6 (or even F/7) the size of the baffles needed for a large sensor, produce a system with an obscuration ratio between 40% and 50%.  I did a quick F/2 - F/3 first order layout for a classical Cassegrain to illustrate the problem.  Without baffles, the obscuration is 58%.  If you add baffles, the obscuration will almost certainly be well over 60%.




In my view, the primary reason for using a faster system shouldn't be about signal strength; it's mostly about field size -- and 2-mirror Cassegrain type systems are not well suited to that task.  I believe that the modern replacement for the Schmidt camera is either a Hyperstar or a RASA system.  The next best way to achieve a fast, well corrected wide-field  telescope might be a fast Newtonian with a coma corrector such as a Tak 160/180.  After that, refractors do pretty well.  Remember that with a properly matched sensor, the only thing that you get with a larger aperture is fainter stars.  Under sampling a fast system certainly gathers signal faster and that's a valid approach with a wide field system, but you lose a lot of that advantage when you reduce the optical throughput with a huge secondary.

John

Hey John,  I found a couple online Cass design calculators that produce metrics for the secondary size, distances, and final aberrations.   I was able to use those to see the points you made clearly.   If you haven't already read & acted on my 2nd request above, it's unnecessary.   If you have, I'll use to backup/confirm the calculator info I entered.   In any case, thanks for the original feedback.   Doug

Hi John,   The scope was acquired by TAAA, disassembled in Longmont CO, and is currently stored.   It's headed for the Chiricahua Astronomy Complex in AZ early next year.   Once reassembled, it will be checked out at its original f/13.6 config to be sure the move didn't damage anything.   Afterward, it will be assessed for better use potential at another optical speed.  The CAC is 40 miles southeast of Wilcox AZ in a nice Bortle 1-2 sky.  

Hopefully by late 2024, it might be worth a visit.       For grins, here's an image of it up in CO before disassembly.  CS   Doug