Dark Adaptation

It is an obvious fact that the eyes need time to adjust to the dark after coming out g of a brightly-lit room - a phenomenon known as dark adaptation. And two factors 3

are actually at play here. One is the dilation or opening of the pupils themselves, which begins immediately upon entering the dark and continues for several minutes. The other involves the actual chemistry of the eye, as the hormone rhodopsin -Q ^ (often called "visual purple") stimulates the sensitivity of the rods to low levels of O I— illumination. The combined result is that night vision improves noticeably for perhaps half an hour or so (and then continues to do so very slowly for many hours following this initial period). This is why the sky typically looks black on first

Fig. 4.1. Dark adaptation times for both the color-sensitive cones (open circles) at the center of the eye, and the light-sensitive rods (black dots) around the outer part of the eye. At the "rod-cone break" some 10 min after being in the dark, the sensitivity of the cones levels off and remains unchanged. The rods, however, continue to increase their sensitivity to low light levels, with complete dark adaptation taking at least 4 hours! For all practical purposes, the eye is essentially dark-adapted in about 30-40 minutes. Good dark adaptation is essential for viewing the fainter Herschel wonders.

Fig. 4.1. Dark adaptation times for both the color-sensitive cones (open circles) at the center of the eye, and the light-sensitive rods (black dots) around the outer part of the eye. At the "rod-cone break" some 10 min after being in the dark, the sensitivity of the cones levels off and remains unchanged. The rods, however, continue to increase their sensitivity to low light levels, with complete dark adaptation taking at least 4 hours! For all practical purposes, the eye is essentially dark-adapted in about 30-40 minutes. Good dark adaptation is essential for viewing the fainter Herschel wonders.

going outside, but later appears gray as you fully adjust to the dark. In the first instance, it is a stark contrast effect and in the second the eye has become sensitive to surrounding illumination, light pollution, and the natural airglow of the night sky itself that were not seen initially (Fig. 4.1).

Stargazers typically begin their observing sessions by viewing bright objects like the Moon and planets first and moving to fainter ones afterwards, giving the eye time to gradually dark-adapt naturally. This procedure is of critical importance in observing deep-sky objects like those in the Herschel catalog. Stars themselves are generally bright enough that they can be seen to full advantage almost immediately upon looking into the telescope. (Exceptions are faint pairs of stars and dim companions to brighter stars, where the radiance of the primary often destroys the effect of dark adaptation.) White light causes the eye to lose its dark adaptation but red light preserves it, making it standard practice to use red illumination to read star charts and write notes at the eyepiece. Another helpful technique is to wear sunglasses (preferably polarized) whenever venturing outside on a sunny day if you plan to look for "faint fuzzies" that evening. It has been shown that bright sunlight - especially that reflected from an ocean beach, bodies of water, and from snow - can retard the eye's dark adaptation for as long as several days!

Herschel often placed a dark hood over his head to maintain his dark adaptation. This is also done by some observers today, especially those living in heavily light-polluted areas. Such "photographer's cloths" (as they are generally known)

are simply dark opaque pieces of fabric that are thrown over the observer's head and the eyepiece area of the telescope, effectively eliminating stay light and preserving dark adaptation. They are available from camera stores and some telescope dealers, and can also be easily fabricated. In actual practice, these hoods can prove a bit suffocating - especially on warm muggy nights - and are sure to raise the eyebrows of any neighbor who happens to see you lurking in the dark!

Was this article helpful?

0 0

Post a comment