1. See: http://www.popastro.com.
2. Dickinson, T., and Dyer, A. The Backyard Astronomer's Guide. Ontario: Camden House Publishing, 1991, p. 26; Harrington, Philip S. Touring the Universe through Binoculars. New York: John Wiley, 1990, p. 2; Salmon, T. http://arapaho. nsuok.edu/~salmonto/VSIII/Lecture11.pdf.
- Binocular Optics
There are three main parts to a binocular's optical system:
• Objective lens assembly. Its function is to gather light from the object and form an image at the image plane.
• Eyepiece lens assembly. Its function is to examine the image at the image plane, rendering it visible to the observer's eyes.
• Image orientation correction. In modern binoculars this is usually a prism assembly. In large binoculars this may also require the eyepieces to be at 45- or 90-degree angles to the main optical tube. Binoculars are usually classified by the type of prism assembly they use (e.g., "Porro prism binocular" or "roof prism binocular") (Figure 2.1).
Astronomical observation is exceptionally demanding of optical quality; this applies equally to binoculars as to telescopes, despite the much lower magnification usually used in the former. Hence, binoculars used for anything other than casual scanning of the sky as a preliminary to using another instrument need to be of the highest optical quality you can afford. Once you have used a high-quality astronomical binocular it is very difficult to use one of lesser quality without being dissatisfied, even irritated, by it.
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