In my opinion no. The baffle tube is too small to take real advantage of 2" eyepieces. A less costly and more effective way to coax a wider field out of a C8 is the f/6.3 reducer/corrector.
If the baffle tube is too small to take advantage of 2" eyepieces, it is also too small to take advantage real advantage of an F/6.3 reducer/corrector. In either case, the primary limiting factor is the diameter of the rear baffle tube. The R/C turns an eyepiece with a 27mm diameter fieldstop (i.e., the widest TFoV 1.25" eye[oeces_ into the the virtual equivalent of an eyepiece with a 43mm fieldstop. Once you consider the degree to which the typical SCT 2" diagonal extends the focal length of the SCT, it makes it rather close to the equivalent of a 45-46mm fieldstop (approaching the widest 2" eyepieces).
Well I think many will disagree on that one,as thay cause unwanted vignetting and only really should be used for AP,imaging.the less glass as possible in the optical train the better the image and unwanted distortion.
The Celestron F/6.3 R/C has a clear aperture of 41mm-- 10% larger than the rear baffle of of the C8. With a C8 or smaller it does not add vignetting. Furthermore, it provides some mild field flattening (albeit at the cost of a slight reduction in transmission and an increase in scatter). It was designed to be used both visually and photographically. Quite frankly, I consider more useful for visual use than photography (there are better, albeit more expensive, photographic options). Visually, I consider the flattening vs scatter to be about a wash. YMMV.
With that out of the way...
2" diagonal advantages with an 8" SCT:
- Convenience-- don't have to put on and take of the R/C
- Stronger/Sturdier, which can be very important if you are using physically large eyepieces (like UWAs and wider). Even longer 1.25" focal-length ultra/mega wide eyepieces are big and heavy enough that a 2" diagonal may be preferable.
- Potentially better contrast due to elimination of R/C.
- Big eye lenses with many 2" eyepieces make for an impressive presentation.
- If you already have 2" eyepieces (e.g., because they are needed to take full advantage of that telescope's capabilities), you can use them with the SCT.
2" diagonal disadvantages:
- Cost: good 2" eyepieces are expensive
- Weight: almost all 2" eyepieces are very heavy, and can create balance issues, particularly when switching to short focal length eyepieces.
- Most 2" diagonal setups more back the focal plane beyond the designed optimal backfocus. This increases focal length and causes an over-correction of spherical aberration (very slightly degrading optical performance). In extreme cases it can cause loss of effective aperture.
- At F/10, a prism diagonal will outperform a mirror diagonal of comparable optical quality (prisms have CA, mirrors have scatter-- at higher focal ratios the scatter outweighs the the CA).
1.25" + R/C advantages:
- Significant cost savings: The R/C costs about the same as a decent 2" SCT diagonal. If you already have a 32mm plossl and/or a 24mm Pan/ES68/SWA, you don't need to buy additional eyepieces to max out your field of view. It's like getting a 55mm-ish plossl and a 40mm-ish Pan/ES68/SWA for free! You get double duty out of your medium to to long FL 1.25" eyepieces.
- The R/C+1.25" diagonal and eyepiece generally weighs substantially than 2" equivalent: it's easier on your mount AND your wallet, and you have fewer scope balancing issues.
- SCTs have significant field curvature, and the R/C provides some modest field flattening
- The C8 was designed around the use of a 1.25" prism diagonal, and you get optimal performance with the setup.
1.25" + R/C disadvantages:
- Some people find putting on and taking off the R/C to be hassle.
- The R/C adds some scatter
My primary C8 setup uses a SCT 2" diagonal, but I could be quite happy with 1.25" eyepieces and the R/C, if my only telescope were an 8" or smaller SCT. If I'm trying to squeeze out every last bit of planetary performance, I'll go back to a good 1.25" prism, though. For my C6 and C5, I stick to 1.25" and use the R/C when I want wider fields. Quite frankly, I'm surprised how well it works on these smaller scopes with their even more constrictive rear baffles.
One more word on the C8's 37-38mm rear baffle tube... the baffle is far enough away from the focal plane that it does not cause any abrupt vignetting-- just gradual falloff. People sense brightness logarithmically, and most people are pretty good at ignoring this sort of falloff. And it's not like there isn't already measurable falloff before you get to the 37mm image circle (or even the 27mm circle) point. This whole idea that it's a waste of time to use an eyepiece with a fieldstop larger than 37mm with a C8 is nonsense as far as I'm concerned: I'd much rather have a flawed 1.2-degree-ish field than NOT have the option. And FWIW, it's not like that field would not be flawed if we eliminated the constraint of the rear baffle: it would still have falloff (just not as much) and still have just as much field curvature and coma. The latter two are much more noticeable to me than the former, anyway.
Edited by jallbery, 01 April 2021 - 07:32 PM.