Re: [APML]: Bright stars bigger?

[cvedeler@bio2.edu (Chris Vedeler)]

I'm not an optics expert but I will take a shot at this one.  There are two
reasons I can think of. 

First, the intensity of the light of a brighter star actually bleeds to the
adjacent granules of photosensitive layers on the film.

Second, all stars create defraction rings even when perfectly focused.  Even
though the stars are really essential points in the sky, these defraction
rings make the brighter stars look bigger.  The smaller the telescope the
bigger the defraction rings (in arc seconds).  This is why small telescopes
can't resolve as well as big telescopes.  Under ideal conditions under very
high power, this pattern is obvious.  With a bright star the first few rings
are actually pretty bright.  This would have the effect of making the star
look bigger.

Make sense?

Chris Vedeler

At 02:35 PM 2/5/98 -0600, you wrote:
>
>I had a hard time sleeping last night (too bad it was cloudy here or I
>would have spent some time under the stars).  For some reason I started
>thinking about stars, light, photography and I formulated a question
>that I could not answer ( limited experience with photography).
>
>Here it is:
>
>Why do brighter stars look bigger on film than dimmer ones?  At first it
>seemed obvious; more photons hence bigger spot.  But as I thought
>through the question I started thinking that a point of light is a point
>of light.  Stars are so far away from us that any photons reaching the
>film pane would essentially be coming in from a single point.  It seems
>to me that the brighter stars would show up sooner on film but how were
>they showing up bigger?
>
>Admittedly, I have thought it may have to do with the optics but I was
>wondering if anyone knew for sure what caused this result.
>
>Sorry if I am missing the obvious here ( the question was brought on by
>lack of sleep).
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Mike Rogers                         E-Mail: mike.rogers@born.com
>Born Information Services
>Englewood, Colorado
>http://www.born.com
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>