CN FORUMS ARCHIVE Telescope Reviews: Omni XLT 127

Patrick,
The illumination at the edge of the field, especially when viewing a dark sky, isn't real critical.
No one can easily see a 30% drop off, and most peple can easily tolerate a 40% drop off (0.4 magnitudes). Some people can even tolerate a 50% drop off at the edge of the field.
In your case, your 38mm eyepiece has a 46mm field stop, so the illumination at the edge of the field, given a 27mm opening in the visual back, is below 50%. That's no big deal--it just means that illumination drop off isn't something you're particularly sensitive to (neither am I, by the way--a 60% illumination seems fine). Though you might notice it in daytime use, you obviously aren't bothered by it at night.
Here's an article on vignetting:
http://www.mailbag.com/users/ragreiner/vignetting.html
Remember, in this case a 50mm baffle is described, not a 27mm baffle.
I agree the 2" visual back is stronger, if somewhat longer. It's a good match for even a 1-1/4" diagonal.
At which point, I can tell you of my experiences:
In my 8" SCT, with a 38mm baffle, an eyepiece of 46mm field stop did not produce visible vignetting. When a focal reducer was used (the same illumination reduced to a 24mm field), I was able to successfully use an eyepiece with a 31mm field stop, but an eyepiece with a 38mm field stop showed visible vignetting. As I calculate it, I was just able to see vignetting when the light drop off exceeded about 0.6 magnitude.
On my 5" Mak, with a baffle of 27mm, an eyepiece with a 29mm field stop produces noticeable vignetting. Why? Because the secondary size chosen didn't produce the same edge illumination as the secondary in my SCT. In that scope, I feel confident in telling people to stick with 1-1/4" eyepieces (though some users have reported using 2" eyepieces with 46mm field stops--I have no clue how this doesn't produce visible vignetting for them).
On the C5, I would not be SURE that a 2" eyepiece cannot be used, but I would not steer someone to this scope if they really had a desire for a field of view larger than 1.2 degrees.
[Especially when a 6" with a much wider field (over 2 degrees with 1-1/4" eyepieces) is available for LESS.] Also because neither you nor I can be sure the owner won't be sensitive to vignetting.
Given the wide range of accessories available and desirable for this range of scope, I would argue a 2" visual back/diagonal/eyepiece is pretty far down the list of expenditures.
And with the 6", it isn't even on the list.

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Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member