Hi John,
Quote:
Originally Posted by ausastronomer
Everything you have mentioned about "lost aperture and wasted light" and "ones own eye aberrations manifesting at larger exit pupils" is 100% correct. Not disputing any of it. However, the practical reality of the situation is that it doesn't really matter it is all academic.
When I use a 31mm Nagler in my F4.5 scope, I am not doing so to make critical observations of targets on the verge of visibility. I am using that eyepiece solely to maximise the FOV an an extended target or an extended starfield. If I wanted to critically observe the target in intricate detail or indeed observe threshhold targets on the verge of visibility, I wouldn't be doing it with a 31mm Nagler in an 18"/F4.5 scope. I will be using one of my higher powered Pentax XW's, in the 5mm to 20mm range ...
It is also worth noting that I am not talking about going ridiculously overboard with oversized exit pupils. I am talking about an exit pupil less than 10% over sized as compared to the observers own pupil dilation. Clearly, 7.5mm and 8mm exit pupils for people over 40 years of age are taking things past the extreme and are useless IMO.
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For many, many practical purposes, you are right. Looking at very bright high-surface-brightness thngs like M42 or Eta Carinae (NGC 3372) or Omega Centauri (NGC 5139) etc etc it won't make any practical difference at all, except that you are bringing the outer edges of your cornea into play -- relevant, but not critical at low-power
Remember I said earlier:
"Will it make _that_ much difference in the end? Again no, not a great deal of difference. If the exit pupil is 6.1mm (assuming 20" f/5 with 31mm T5) and you can only dilate to 5.9mm you are loosing 2/3rds of 5/8ths of ... not a lot.
But, I think even a seemingly small variation (lets say, 0.4mm) would make a _small_ practical difference when looking at an extremely faint but extremely large object (like the California Nebula NGC 1499 or the Witches Head Nebula IC 2118, or outlying segments and wisps of the Vela SNR -- yum!) -- particularly when it might be a threshold object at a particular aperture.
As you note, the over-the-top exit-pupils are, well, over-the-top!
For fine detail/ close things again you are certainly correct about using a higher power.
Which brings me back to the reason why I made my original comment. I think 6.2mm is starting to get into the "marginal" territory as an exit pupil. In fact, with Rod's 'scope assuming a 504mm aperture (reduced 4mm by bevel on mirror) at f/5 and the 31mm gives 6.25mm exit pupil -- certainly I think in the "marginal" territory. If it were more than 0.5mm over maximum dilation I'd say "clearly too big". From my brief experience with it in my 18" f/4.9, the 28mm UWAN is a very, very good eyepiece and would, for the purposes of this exercise reduce three things:
(1) The true field (slightly -- a few arc-mins) -- undesirable but we are not talking about too much.
(2) The exit pupil (slightly) to take it away from "marginal" to perfectly comfortable. Desirable but not critical for many purposes.
(3) Price (markedly) -- Very desirable.
In the end, it is up to the preference of the individual. Whichever way you go Rod, I certainly hope it provides many, many years of very enjoyable observing!
Best,
Les D