Quadibloc wrote:

After all, the dilation of the dark-adapted eye is 7mm (at least if
you're still only 30 years old) whether you are in the city or the
country, and that is what the definition of an RFT has been taken to
be.

Hi, John, and what you say raises an interesting question.

Based on a conversation with one experienced observer, I got the idea
that urban light pollution can limit dark adaption. Obviously direct
light trespass can do this at any site (thus star party etiquette!),
but somehow I drew the conclusion that urban skyglow, say magnitude 17
per square arcsecond, could prevent the pupil from reaching the dilation
it might attain at a dark site. Of course, I'd like to correct this
(mis)understanding if it's wrong.

If city skyglow doesn't substantially affect dark adaptation, then age
might be a main relevant factor: might we speak of 5mm exit pupil or so
as an "Elder's Richest Field Telescope"?

The popular and inexpensive 7x50 binocular is another one that meets
that criterion.

Yes, and curiously I'm still enjoying my pair from 21 years back,
although I realize that for someone my age, the effective aperture may
be a bit less than 50mm.

John Savard


Most appreciatively,

Margo Schulter

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