The human eye has an enormous range of light accomendation…one million times that of a camera. Light accommendation to varying light level occurs in 1/10 of a second by the iris but the eye takes about 30 minutes to adapt . This is due to a lag in the photochemical response. For this reason it is reccommended to allow at least this time for adequate adaptation to the dark during observational astronomy.
The eye can cover an angle of about 120 degrees, actually 60 degrees on either side of the fovea or center of the eye but limited on the nose side of the fovea to 45 degrees. Therefore it requires both eyes to obtain a 120 field of view. ( If you take into account that the eye can move in its socket 45 degrees then the effective field of view would be 210 degrees for both eyes together. However the view is clear only in 0.2mm center of the eye… the fovea, representing 2 degrees in the field of view. We know that the center of an eyepiece lens has the clearest optics. Therefore knowing this it would make sense to center the object to be observered in the fov of the eyepiece and hence in the eye and to look straight ahead, for the best views.
The fovea of the eye has a higher number of cones whereas the rods are more numerous outside this area. Rods allow one to see in the dark and for this reason using averted vision (looking from the side of the eye not directly in front) is the best strategy when searching for a faint object in the eyepiece.
Light pollution strongly affects what we can see in the night sky. In very light polluted cities, stars of the order of 4 magnitudes can be seen, moderately dark skies reveal 6 magnitude and the darkest of skies may reveal stars of 8 magnitude. The visibility of star clusters and galaxies are far more affected than stars and planets by light pollution. The higher in declination the object in the sky, the less the viewing is affected by light pollution and atmospheric turbulance and this is why we generally can see more stars higher up in the sky. The best time to view the sky is very late at night when some of the city lights are turned out. The fainter objects can be better seen and imaged when the Moon is not visible.
A clear sky will be indicated by the fact that the stars are not twinkling (due to atomospheric turbulance), the visibility of The Milky Way and a relatively dark horizon compared to the zenith or sky overhead.
The angular resolution of the eye is about 0.02 degrees or 1 arc-minute. Which means theoretically, that the naked eye is able to see as 2 distinct objects, stars having an angular separation of at least 1 arc-minute … for examples see 20 Fun Naked Eye Stars by Jerry Lodrigues.
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