The *real* value of eyepieces for serious deep sky observing...

[nicoscy]

Don,

The scope was the Classic 18" f4.5 Obsession.

- The TV yields 65x magnification, TFOV 1.19d and exit pupil of 6.89mm.

- The ES yields 69x magnification, TFOV 1.22d (due to slightly larger field stop by 1mm VS the TV) and exit pupil of 6.67mm.

- The ages of the observers in that test referred to are 40+ so I am not sure they were taking full advantage of the exit pupils involved.

Questions:

1. Could partial loss of illumination occur as a result of potential aperture reduction lead to this difference?

2. Would a difference of 0.22mm of exit pupil justify such a difference (we are at the realm of 6.5mm + exit pupils on both eyepieces)?

Just trying to learn a bit more here so feedback is most appreciated!

[Don Pensack]

Don,

The scope was the Classic 18" f4.5 Obsession.

- The TV yields 65x magnification, TFOV 1.19d and exit pupil of 6.89mm.

- The ES yields 69x magnification, TFOV 1.22d (due to slightly larger field stop by 1mm VS the TV) and exit pupil of 6.67mm.

- The ages of the observers in that test referred to are 40+ so I am not sure they were taking full advantage of the exit pupils involved.

Questions:

1. Could partial loss of illumination occur as a result of potential aperture reduction lead to this difference?

2. Would a difference of 0.22mm of exit pupil justify such a difference (we are at the realm of 6.5mm + exit pupils on both eyepieces)?

Just trying to learn a bit more here so feedback is most appreciated!

Well, I was thinking a much longer focal length instrument with smaller exit pupils, and you're right to assume that probably most observers weren't using the entire exit pupil of either eyepiece.
The difference, now that i think of it, would simply be in the two primary differences I've noticed between the two brands:
--scattered light control
--induced field astigmatism
In the case of the first, the sky background in the ES would likely be a little greyer, reducing contrast--definitely a no-no for a marginal object.
In the second case, this slight blurring of point sources would tend to drop the visibility of smaller objects near the limit.
Add to that the slight difference in magnification (lower surface brightness in the 30 than in the 31) and a small difference in light transmission (maybe 3-4%),
and you have a combination of factors that would conspire together to reduce the visibility of a faint spot.
Normally, this difference would take one object from visible 50% of the time with averted vision only to 10% of the time with averted vision only, but you may have been lucky enough to find an object where the difference was visible directly versus visible mostly averted.
I've seen N7775 in my 12.5", though, and I would not think it's a limit object at all in an 18". So I suspect Ivan's explanation has some merit. If it was simply harder to hold the exit pupil on one versus the other, this would explain a lot.