|Pupil Size Gauge
Maximizing the aperture of your telescope requires that you consider your personal optical equipment!
|When you are choosing the
use with your telescope, do you
take your eyes into account? Many folks don't realize that the maximum
size of your eye pupil is what determines what your optimum low power
eyepiece should be. By using a lower power eyepiece than the optimum,
one effectively shrinks the aperture of the scope. Since pupils tend
not to open as widely as we get older, it is clear that one needs to
adjust to get the maximum performance out of our equipment (personal
You can search the net and find many references on the importance of pupil size in determining your optimum low power eyepiece, here are two good ones:
Sky & Telescope, "A Pupil Primer"
When does your 8" telescope deliver less light than a 3"? This title gives an indication of how important paying attention to this can be! This article by Jim Sapp has apparently become unavailable on the internet and the link to it has failed. It is such a good article that I had printed it out, and convinced my wife to retype it in. I scanned in the diagrams and converted it to a PDF of about 9 pages and 250K in size. If you send a request to firstname.lastname@example.org, we would be pleased to email you a copy.
It also has a nice chart of pupil size vs scope speed (F/?) if you don't like the math. I have reproduced the table in the article below.
Find your largest acceptable exit pupil in the left column, and your telescope’s focal ratio across the top. At the intersection of the row and column is the longest usable eyepiece (in millimeters) for conserving the light collected by your telescope.
Give this program your maximum pupil size, telescope aperture and speed (F/ratio). It will tell which of 284 eyepieces will have exit pupils your size or less. Delivered by email attachment, $5.00 Runs on Windows 2000 and later. If your eyepiece of interest is not in our database, just email its name, size, and Apparent Field of View and I will send you back a copy of the program with your eyepiece(s) included. Program is available in Windows and in Linux. For $5 registration, you get either or both by email. Upgrades with current eyepiece database are free for registered users by email request.
(c) John R. Duchek, 2006-2010
|Note: On first run,
you may get a text file error. This is because when you quit it
writes a data file to save your latest pupil size, aperture and
Simply clear the error and the program will run. Press quit and the file will be written. Run again and it will find the file it wrote. Calculations are done whenever you click on an eyepiece.