Exit Pupil and Visual Acuity

I often hear or read that with a magnification yielding an exit pupil of 2mm, the image resolution is matched to the eye's resolution and all the detail available in the image should be visible. Added to this is usually the comment that the detail is easier to see with a magnification yielding a 1mm exit pupil. Texereau recommends an exit pupil of 1.0 to 0.8mm for planetary work. Some folks like to go even smaller (but floaters become an increasing problem).

After a bit of a wait, I managed to get a copy of "The Human Eye - Structure and Function," by Clyde W. Oyster through interlibrary loan. It has far, far more information about the structure and workings of the eye than most of us would ever need, but there are quite a few tidbits of interest to folks using their eyes for critical observations.

One very interesting graph shows the MTF of the eye as a function of pupil opening. The eye is not diffraction limited even at its mimimum 2mm opening, and gets worse rather rapidly with larger pupils. With a telescope yielding a 1mm exit pupil you are really stopping down your eye to 1mm, and almost certainly reducing the eye's aberrations and improving the MTF. This may partly explain why a 1mm exit pupil provides more detail than a 2mm exit pupil, although the inceased image scale would also help. (Another point in the discussion of various types of eyes is that larger eyes with larger images on the retina work better.)

Clear skies, Alan

Hi Alan,

This is a great topic. I push the exit pupil a little smaller toward the .8 range. I do find what you say to works in real life..., at least for me.

Sol Robbins

Hi Alan,I agree with Sol great topic.Sept.2005 issue of Sky& Telescope has special report on Night Myopia and on page 34 there is picture at the bottom of why stars look spiky and it shows that the star looks spiky from 7-mm pupil down to 2-mm pupil and doesn't look pinpoint until 1-mm pupil and this would agree with Texereau.Alan,I'm sure you saw this issue.
Steve