I've been looking for a way to get very low power and wide (true) fields of view with my Celestron 11" and 9.25" f/10 Schmidt-Cassegrain scopes. There are lots of excellent Cloudy Nights ratings for Russell Optics products in general including his eyepieces in the 56mm to 72mm range. The Russell Optics website states you should not go below 35x with a scope that has a central obstruction or you will not get the best results.. If I use his 72mm in my 11" it would generate 39x. If I were to use it in my 9.25", it would be a bit below the 35x limit at 33x.
So, does anyone have any experience using either his 65mm and/or his 72mm XL eyepieces in either the 9.25" or 11" Celestron SCT?
Thanks very much for your help.
The 40mm Pentax XWL has a 46.5mm field stop diameter, which is the largest possible with the 2 inch barrel diameter. This will give you the maximum TFOV at a 4mm exit pupil. I use this eyepiece in my C8. With the longer focal length due to the 2 inch diagonal, I get a TFOV of 1.2 degrees (in my C8).
Increasing eyepiece focal length will increase exit pupil, but you will not get a larger TFOV. The 55mm TeleVue Plossl, for example has the same 46.5mm field stop diameter, so will have the same TFOV, just at a lower magnification. This will lighten the background sky with the larger 5.5mm exit pupil, but have little impact on star brightness, which can be useful if you are having image brightness issues when using an OIII filter.
I also push the limits of my C8 by using a f6.3 focal reducer with these eyepieces in a 2 inch diagonal. How that relates to this discussion is that the 30mm Pentax XW in this configuration has a 5.5mm exit pupil and the view is fine. However, the 40mm Pentax XW with the focal reducer gives about a 7.3mm exit pupil and I see the shadow of the central obstruction. It's bad off axis, but just barely noticeable with good eye placement. I use it this way because it is my maximum possible TFOV.
Celestron recommends a maximum 35mm eyepiece focal length when using the focal reducer with the standard 1-1/4 visual back and diagonal. In this configuration, the OTA is close to f6.3; therefore, Celestron is recommending a maximum 5.6mm exit pupil to prevent the central obstruction from creating a shadow in the eyepiece view.
In my C8, I can get the TFOV to about 1.95 degrees good and clear, then quickly vignetting to dark at about 2.1 degrees (40mm Pentax XW with focal reducer). The C8 has a 37 to 38mm baffle tube diameter - much smaller than the larger OTAs. That might not matter, though, because the focal reducer has a clear optical path of only 41 to 42mm and that is what causes the vignetting in the 40mm XW eyepiece. I believe the 11 and 9.25 use the same focal reducer as the C8, but am not positive.
Again, this is my experience with a C8. But maybe it will give you some ideas for your SCTs.