I use a Sky & Telescope Pupil Gauge. They sold them many years ago.
It is similar to this one:
You hold it up to your eye and slide it up and down until you see the edges of the holes *just* touch.
My dark-adapted pupil: 4.5mm.
Now, that is looking at the sky at night because you need to have some light to see the edges of the holes.
I can feel my dark adapted pupils contract when I look at the sky because the sky is so much brighter than the ground.
I try to only look at the ground and through the eyepiece so my night vision stays good.
(and that is at a dark site)
So I suspect it is more like 5mm when I am looking through the scope.
My daylight pupil is about 1.5mm (I am VERY sensitive to light due to having light blue eye color), so my night vision pupil diameter is, if 5mm, is about 11x as large as my daylight pupil.
The average increase in sensitivity of the eye is many thousands of times at the retina (I've read up to 90,000 times as sensitive as day vision when you have achieved the
scotopic limit). As you can see, pupil diameter dilation is simply a tiny figure in comparison. I'd say it's largely irrelevant.
Reaching maximal retinal sensitivity is the name of the game, and that means:
--no PC, tablet, or phone at the eyepiece
--no red light or any light if possible, or only an extremely dim red light if required, and never directly into the eye.
--no looking at the sky before a really dim object search, for at least 5 minutes, even though you are already dark-adapted.
--full dark adaptation--no dim object viewed until at least an hour and a half after sunset at mid-northern latitudes, with you by the scope, dark-adapting as it gets darker.
--no Moon in the sky and preferably no Venus or Jupiter, either.
--a black cloth over the head at the eyepiece, or hands cupped around the eyepiece to block ALL peripheral light.
You can experiment if you observe near some woods. Walk into the woods in the middle of the night and continue to stare at the ground for at least 5-10 minutes. Then turn around and look at the clearing you just walked out of. Bright, isn't it? Now, look down. You can no longer see what you were just looking at and the only light that damaged this night vision was the light in the clearing from the sky!
The sky itself is much brighter than that, and can really knock down your night vision capabilities. Imagine how much damage to your night vision the use of a bright red flashlight or tablet does.
And, through all of that, your pupil probably stayed the same.
So what is the value of determining your pupil diameter? To pick a lowest power eyepiece, I guess; to match your pupil size with the exit pupil of your lowest power eyepiece.
other than that, I can't see any purpose to knowing your pupil diameter because it has so little to do with your scotopic vision.
Edited by Starman1, 12 April 2020 - 05:10 PM.