Thread: F number
View Single Post
  #4  
Old November 20th 04, 06:08 AM
Mark Smith
external usenet poster
 
Posts: n/a
Default

As a long time photographer, I had a hard time with this. In my mind,
lower F numbers meant a faster lens (you get more light to the film
plane). Unfortunately, that only kind of works with telescopes.

If you are going to use your telescope as a camera lens, you CAN use
the F number the same way as you do for cameras. However, that
doesn't really tell the whole number.

The real key for telescopes is aperature. Aperature is what allows
you to suck in light and, generally speaking, more aperauter = dimmer
objects seen.

For two telescope with equal aperatures:

A Higher F number means longer Focal Length.

A Higher F number means narrower FOV.

A Higher F number means a less radically curved primary.

A lower F number (especially in reflectors) MAY mean better eyepieces
are required to combat coma problems near the edge of the FOV.

A Higher F number will mean more radical jumps in magnification per mm
reduction in eyepiece focal length. This is NOT necessarily good. It
allows you to get insane magnifications if you desire, but you often
do not desire large magnification unless you are looking at planets or
the moon.

Hope that helps a little.