Popular Post acey Posted April 2, 2015 Popular Post Share Posted April 2, 2015 (edited) A true dark site is not "pitch black". Once your eyes are fully dark adapted the sky is markedly bright with stars, Milky Way and natural airglow. Moving around without any artificial light is easy. A lamp is only needed for seeing small objects, reading etc. Foreground objects (trees etc) look truly black against the bright sky. In a telescope at high power, the sky background looks truly black (you can't see the eyepiece field stop). After viewing for some time, when you look up at the sky again it's dazzling (you need to shield your eye from it while looking through the eyepiece).At a light polluted site it's very different. The eye adapts to the ambient light level (dictated by the pervasive glow of streetlights etc, even if not directly visible). Under those conditions a clear sky can look "pitch black", but only because the eye can't adapt fully.So apparent blackness is not a good test of sky quality, it only tests dark adaptation. The test is limiting magnitude. If you can see stars down to 4 mag then you should manage a few bright DSOs in a telescope (e.g. M31, M42, M57, M13, M81/82). If you can see to 5 mag then you'll see many more, and the Milky Way may be visible. If you can see to 6 mag then the MW will be clearly visible and you'll be able to see all the Messiers above your horizon with a 100mm scope (or smaller), and all (or nearly all) above-horizon NGCs with a 12". If you can see stars fainter than 6 mag then you have very good eyesight.Light pollution is of three types. One is direct glare from steeetlights etc. You need to shield yourself from that, e.g. by choosing your viewing spot, putting up barriers etc. Two is ground light reflected off walls etc; you can't see lights directly but your garden is indirectly lit up. Shield yourself by putting a hood over your head at the eyepiece and give your eye time to adapt. Three is skyglow caused by ground light reflecting off water vapour in the air. This limits the faintest stars you can see, and there's nothing you can do about it (unless you find that a "light pollution filter" works for you). "Nebula" filters (OIII, UHC etc) are effective on emission nebulae (e.g. M42, M1) but have no effect on other types of objects, i.e. clusters, galaxies (note for pedants: there can be some slight effect on a handful of large galaxies at a dark site, e.g. M33).At a light polluted site the easiest DSO types are open clusters, bright globular clusters (e.g. M13), bright planetary nebulae (e.g. M57, Eskimo, Cat's Eye). Diffuse nebulae (emission or reflection) are generally more difficult, though with a few bright exceptions (e.g. M42), and galaxies are generally very difficult, again with a few bright exceptions. The reason for all this is that light pollution hurts the limiting surface brightness of a scope more than limiting stellar magnitude. For example see Figure 18 of this paper:http://arxiv.org/pdf/1405.4209v1.pdfA telescope can't improve the surface brightness of a target, and most galaxies are of about the same surface brightness as the Milky Way. So if you want to see galaxies well, you need to be able to see the Milky Way with the naked eye. Edited April 2, 2015 by acey 86 2 Link to comment Share on other sites More sharing options...
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