Quote:
Originally Posted by ngcles
Hi Argonavis & All,
It isn't easy at all to measure at home (but it can be done). Therefore I proceed on the assumption that I have just less than average and then subtract a fudge-factor -- hence my self-imposed limit of 5.5mm.
Best,
Les D
Contributing Editor
AS&T
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Hi Les,
It is in fact quite easy to measure your pupil dilation at home, or at least somewhere that you are properly dark adapted.
Take a set of tools of known diameter which increase in diameter in set known increments. I in fact use a set of metric "Twist drills" that increase in .5 mm increments. You could also use for instance, a set of "allen keys".
Get yourself "dark adapted". Look at a star of about 2nd or 3rd magnitude. Get one of the drills. Start at say 8mm and hold the drill at arms length and introduce it between the star and your eye. If it blocks out all of the light from the star, your pupil dilation is smaller than the diameter of the drill. Continue the process with progressively smaller sized drills. When you come to the first drill that allows you to glimpse the starlight around its edges, you know your pupil dilation exceeds the diameter of that drill and lies between that drill and the previous larger sized drill.
My own pupil dilation lies between 6mm and 6.5mm as you correctly predicted.
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. The Pentax XW's have marginally better contrast and light throughput as compared to the equivalent focal length Naglers and the contrast gain is further improved by using increased magnification. Indeed if I wanted to maximise optical performance at the 30mm focal length end, I would be using a 30mm Pentax XW and not a 31mm Nagler. That isn't the goal however at this end of things, it's all about field of view. It is also worth making mention of the fact that the optical effects of those issues you raise, are clearly less significant when observing from truly dark skies.
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.
Cheers,
John B