Chris L Peterson wrote:
On Sun, 30 Sep 2007 09:23:02 -0700, wrote:

Do you have a reference for this? That the pupil diameter would be the
same in the urban night vs. a dark country night. I googled around a
bit but could find no experiments on this.

No, but I've measured my own and other people's pupil diameters on many
occasions. The pupil becomes fully dilated under moderately dim
conditions, such as inside a room with the lights out and blinds drawn,
even with enough light to still support fully photopic vision.


Hi, Chris, and all.

Thank you for this very important information which will help me in getting
my facts straight when discussing exit pupil and light pollution. Maybe it
should be a FAQ, although I'm not sure how often it's asked.

Today, when I had a routine eye exam, I asked my doctor after explaining
that I'm an amateur astronomer if he could check my usual pupil dilation
in the dark. He turned down the lights, pretty much as you describe, and
said it was somewhere around 5-6mm. I noticed that the room still wasn't
that terribly dark, maybe somewhere around nautical twilight (very
unscientific guess!).

So I guess a summary might be about like this:

(1) Under urban light pollution, a person's pupils will still dilate
to the same size as under dark-sky conditions, so that the
specific issue of match between the observer's eye dilation and
the exit pupil for a given telescope-eyepiece combination is
not affected.

(2) However, urban light pollution definitely _can_ affect the
degree of dark adaptation, which is mostly a biochemical
process occurring over 30 minutes or more.

Again, this is a very helpful distinction.

Most appreciatively,

Margo Schulter

Lat. 38.566 Long. -121.430