Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site nmtvax.UUCP
Path: utzoo!linus!philabs!cmcl2!lanl!unm-la!unmvax!nmtvax!c20
From: c20@nmtvax.UUCP
Newsgroups: net.astro
Subject: Re: re how many photons
Message-ID: <910@nmtvax.UUCP>
Date: Thu, 19-Dec-85 15:42:18 EST
Article-I.D.: nmtvax.910
Posted: Thu Dec 19 15:42:18 1985
Date-Received: Sat, 28-Dec-85 12:24:59 EST
References: <469@npois.UUCP>
Organization: New Mexico Tech, Socorro
Lines: 28

> 
> 	A few weeks ago, someone asked how many photons per square meter
> per second did we receive for objects of various stellar magnitudes. ...
>  Mag. Photons/sq. m./sec.    Notes
> 	20	2,000				   visible on photographs and
> 						       image intensifiers only.
> 	25	20				       near the limit of what can
> 						       be photographed through
> 						       our atmosphere.

It is interesting to note in regard to these numbers that under the right
circumstances (full dark adaptation), the human brain will perceive a flash
when the eye is presented with as few as seven photons (I can find the
reference for this, if anyone wants).  There is a certain necessary dis-
tribution of photons on retinal rods in order for this minimum number to
be seen.  This is, as far as I know, the only case in which humans can
perceive things at the elementary particle level;  we are otherwise limited
to the macroscopic universe.

greg
-- 

Greg Titus                  ..!ucbvax!unmvax!nmtvax!c20     (uucp)
NM Tech Computer Center     ..!cmcl2!lanl!nmtvax!c20        (uucp)
Box W209 C/S                c20@nmt                         (CSnet)
Socorro, NM 87801           c20.nmt@csnet-relay             (arpa)
(505) 835-5735
======================================================================