Re: Urban Richest-Field Telescope?

Margo Schulter wrote:
Quadibloc <jsavard@xxxxxxxxx> 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.

Not sure myself, but this other thought I had might provide a clue. I thought that any local condition that permits you to see objects around you will affect your night vision. Hence the moon, and particularly the Full Moon, which often lights up a landscape brighter, it seems, than scattered artificial lighting.

It amounts to photons entering the eye regardless of the source.


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"?

That has to be another factor as we age. There is also the fact that younger eyes are more sensitive to light than older ones, and can see things sharper, in general.


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
mschulter@xxxxxxxxxx
Lat. 38.566 Long. -121.430
.