ASTR 1230 (O'Connell) Lecture Notes


THE HUMAN EYE AS A DETECTOR


Eye Cross-Section

Structure and Function of the Eye

With or without optical aid, the only light detector most of you will use in this course is the human eye. Although we take its operation for granted, the eye is, in fact, a remarkably capable optical instrument (an auto-focus, auto-exposure, electrical camera), and it is important to understand some aspects of its behavior.

Dark Adaptation

Under the very low light conditions you will typically encounter when observing faint sources through a telescope, the rods slowly become more sensitive. It takes over 20 minutes to achieve highest sensitivity, or "full dark adaptation," but reasonable sensitivity occurs in about 5 minutes. You must avoid looking at bright lights to become and stay dark-adapted. Because the rods are less sensitive to red light, using a red flashlight helps preserve night vision.

Averted Vision

The rods are preferentially distributed toward the edge of the retina, which is therefore more sensitive to faint sources. This is the basis for the "averted vision" observing technique: stare at a point about 20 degrees away from the object of interest but focus your attention on the target.



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Last modified September 2011 by rwo

Copyright © 2008-2011 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.