[ngcles]

Hi Rod, John & All,

Re the issues raised by John and exit pupil.

John, you might well be correct that either or both yourself and/or Rod can dilate to approaching 6.5mm given a dark environment but taking into account your age(s) is not particularly likely. 6.5mm is generally "teenage-territory".

Of course the only way to know for certain is to measure it -- and I believe based on what I've read elsewhere it is possible to do this. The procedure of course will re-enforce in your respective families a belief that you are completely nuts.

Start with a rainy or cloudy evening and get a camera (digital is best for instant results) with a flash, tripod and a suitable rule/measure and a mirror. Set the camera/mirror/rule set up so you can take a close-up photo of your face in good focus in the mirror with the rule/scale close to your eye and in focus -- in complete darkness. Make sure the red-eye reduction (pre-flash flash) is off. Get it all set up exactly right and then shut and the door and any blinds to the room turn the lights off and ... dilate.

After 10 mins your iris will be fully open. You won't be fully (chemically) dark adapted but it won't matter. Take the photo in complete darkness and compare the iris diameter to the rule/scale.

Simple.

I haven't tried this myself but I'm told it works -- though it will likely be several attempts to take a useful pic. You will look like a complete dill in all of the attempts.

Playing devils advocate, lets assume for a moment that you can physically dilate to about 6.5mm. Is it desirable then to use all of that? I reckon not. Optical aberrations are most likely to be found in the outermost edges or a lens and that includes your eyes.

Will you be able to tell when your iris is vignetting the light cone just by looking through the 'scope/eyepiece? No -- you won't have (can't have) any benchmark to compare it with.

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. Remember I said in my original comment that " the exit-pupil will be just on 6.1mm. _A bit on the marginal side_ I'd reckon"

But I've equally seen a lot of folks using 35mm and 41mm Panoptics and 40mm and 55mm Plossls to achieve that "Ultimate wide-field" in 'scopes that will make for 7 and 8mm exit-pupils -- sure its wide, but you may as well place an aperture stop over the front of the 'scope! Not marginal at all. You are throwing away between 15 and 30% of your light-gathering power!

Best,

Les D