Observation About the Eye's Cones and Rods in Terms of Deep Sky Objets

Physiology of the eyes is important in terms of vision thence of astronomical observation. Bottom of the eye's retina is coated with two types of photorecepting cells allowing vision. 5 to 7 million cones by eye located at the center of it are used with strong lights and colors. By day, the eye's peak of sensibility is in the yellow part of the electromagnetic spectrum. Then 100 to 120 million rods by eye mostly located at the periphery of the retina are dedicated to weak lighting conditions as they are not sensitive to colors. The peak of sensibility by night is shifted towards blue-green

What consequences in terms of astronomical observation such that physiology of the eye is bringing? First, except for Moon and planets which are bright objects necessitating the use of cones, night is the domain of rods. As those are needing a protein called rhodopsin to convert light into a electric signal and that protein needing about 20 minutes to be activated, there is the explanation of why the eyes need a adaptative time of that duration for night observation. The eye's maximal sensitivity is even reached after about 45 minutes. The spectral sensitivity of the eye, like said above, makes that, by night, a observer has to use, when necessary, light sources in wavelengths to which the eye is few sensitive, like red, for example (which is the complementary color to blue-green). A other important consequence of that the eye is not sensitive to colors in weak light conditions is that the eye, by night, is seeing only in black and white. In terms of deep sky objects, that brings the disappointment that galaxies and nebulae are not seen with the fine colors they are displaying on astronomical photographs. Astrophotography thus is the only way to accede to such colors as deep sky objects are only seen in shades of black, grey and white by a observer's eye. As the observer's eye further does not have time enough to accumulate deep sky objects' feeble light like a photographic sensor does, those are few constrasted, thence fuzzy. A amateur astronomer will have to train one's eyes to the observation of DSOs at the effect of being able to surpass, in terms of visual observation, that early approach. Like observing a long time, scanning the objet and 'shifted vision.' As night vision rods are located at the retina's periphery, the observer will shift its vision axis towards the ocular's edge bringing to that the DSO is -better- seen with the peripheral area of the eye. As the eye, at last, is mostly seeing in the green part of the spectrum at night, a telescope with a large aperture will allow to see some DSOs with that color, like the Orion Nebula (which also is seen with some more colors). Some rare nebulae, like M17, M27, etc. are seen with some colors even with small aperture telescopes