Re: ATM Exit Pupils & Big Scopes

[Bob Luffel <bobl@gr.hp.com>]

> 
> 
> I was recently looking into this myself (how to measure your entrance pupil).
> I have the Sky & Tel pupil gauge.  According to this gauge, my pupils
> (at age 38) dialate to 5mm.  However, I also measured using my telescope and
> various eyepieces.
> 
> I have an f/7 scope (ratio pretty accurate).  Basically, I pointed my
> scope at a bright star and defocused the image so I could see the scope's
> exit pupil size.  With a 40mm eyepiece the exit pupil should be 5.7mm.  I could
> just fit the whole exit pupil (large image of the star) into my pupil without
> cutting any of it off.  By this method, I estimate my pupil to be ~ 6mm.
> 
> This disagrees with the Sky&Tel gauge by quite an amount.  I wonder if the
> method I used with my telescope has some flaw in it, or if the Sky&Tel
> approach is just too coarse to measure accurately.  I would be interested
> in hearing others' methods for measuring entrance pupils.
> 
> 

Some additional info. about my first reply, I noticed that my pupil size was 
very sensitive.  After staring at a very bright star (try vega), my pupil
reduced in size at least .5mm.  If I looked away for 30 seconds, or so, 
my pupil returned to the 6mm range (again, all based on whether the 
out-of-focus disc was cut down or not).



Bob