The reason to use a focal reducer/corrector or 2" diagonal with an 8" f/10 SCT is primarily to allow wider true field of view than can be achieved in native uncorrected 1.25" mode. You aren't doing that with a 12mm eyepiece...producing roughly 0.56 deg FOV. Actual max TFOV in 1.25" is roughly 57.3*27/2032 = 0.76 deg. This latter is possible with a 40mm or 32mm Plossl, or a 24 Panoptic.
TFOV is determined by the field stop diameter of the eyepiece and effective focal length of the scope, not the apparent field of view which is an approximation since "apparent" includes distortion.
Before anyone heads down the rabbit hole, the first thing to note is that contrary to myth, an 8" SCT should be able to handle the widest full field stop of a 2" eyepiece in 2" visual mode without a reducer/corrector. The baffle diameter is less than this, but the visual vignetting is not really noticeable at night. That is the way I have used my 8" SCT since the early days.
There are some advantages and disadvantages of using a 2" diagonal vs. 1.25" diagonal with reducer/corrector.
- The longer overall length of a 2" diagonal set up might not be compatible with some mount arrangements.
- The longer optical length of a 2" diagonal arrangement changes the effective focal length of the scope (because of the moving primary focus system). For this and the reason above, an SCT threaded diagonal is preferable to minimize the extra distance--rather than a 2" visual back and 2" refractor diagonal. (However, my 25+ year old system on my 8" SCT is the latter with 2" visual back/refractor diagonal and it works well.) I do use this
- The 2" diagonal is more secure/less likely to rotate suddenly than the OEM 1.25".
- There are some very good 2" eyepieces for low/mid power views, so from a viewing comfort perspective I tend to favor them.
- The negative is that well corrected 2" eyepieces and nebula filters are pricey.
- 2" systems with heavy eyepieces can require some upgrades to balance. I use an adjustable dovetail weight on my OTA.
- The big advantage of reducer/corrector is that it corrects the coma and field curvature inherent to the basic SCT design. I have never used one so I can't speak from experience, I want to try one sometime--I just haven't gotten around to it.
- Using the reducer/corrector could make it more problematic to reach high magnification. Generally folks don't like to have to repeatedly remove and reattach this accessory while observing, because it means removing the diagonal and the SCT threads can be troublesome in the field--especially fumbling with them in dark and cold conditions.
At a given true field of view, the magnification level (and therefore apparent field) that is best usually depends on the target. At the low power/max true field end, more magnification generally provides more detail. However in dark sky with nebula filters, diffuse nebulosity is often better seen with lower magnification because the exit pupil is larger and the image therefore brighter (the Horsehead is perhaps the best example of this.) Also in dark sky some intermediate exit pupils work better to show extent of low surface brightness objects such as galaxy envelopes, rather than smaller exit pupils at higher magnification.
So the 1.25" cases of the max true field of view 40, 32, and 24mm eyepiece will produce 4, 3.2, and 2.4mm exit pupils and 0.76 TFOV at the native f/10. A .63 reducer will change that to 6.34, 5.1mm, and 3.8mm with roughly 1.2 deg TFOV. By comparison a 2" setup with a 55 Ploss or 41 Pan will produce ~5.5mm and 4.1mm exit pupil with about the same field of view--calculation says almost 1.3 deg, but the actual focal length of the scope makes it a little over 1.2 true based on drift timing I have done (an SCT diagonal will make it a little larger still.)
40 Plossls are a pain to use...the narrow apparent field of view is combined with very long eye relief, making eye placement more difficult. Consider that even in pristine sky, one will want to cup a hand around the eyepiece and face to reduce ambient glare, particularly because of the excessive eye relief.
The 55 Plossl (and 41 Pan) have long eye relief, but their adjustable eyepiece cups help in this regard. The 55 Plossl has a standard Plossl apparent FOV, unlike the 1.25" 40mm Plossl that is restricted considerably by the barrel.