Light pollution, airglow, diffuse light, and viewing faint objects.

[Markopolo]

On page 5 of his Night Sky Now website, in the post dated 2/4/2013, Gregg raises the issue of the variability of observing conditions, and the ability to view faint objects.  Excellent subject for us amateur astronomers to learn about, because these things really do affect the ability to see the things that you're looking for, and they are rather complex and variable.  But by learning about these conditions, you can get the most out of your equipment and your observations.

The night sky, even in the spaces between the stars, isn't perfectly black anywhere on Earth, even though it may appear to be.  Stray light or scattered light is fairly obvious when you look up at the night sky in a large metropolitan area like Los Angeles, Tokyo, Paris, or Mumbai.  It's also pretty obvious when you look up at the night sky when there is a full Moon above the horizon, even if you're in a remote location which does not ordinarily have much light pollution.  Light pollution is normally described by expressing the magnitude of the faintest star visible.  At a very dark, remote location, people with good vision and who are dark-adapted (explanation to follow), can see stars as faint a magnitude 6.  At the same location with a full Moon, the visibility is reduced to perhaps magnitude 3 or 4.  In extremely light-polluted areas, such as the middle of Los Angeles or Tokyo, only 1st magnitude stars may be seen.  This is something that amateur astronomers should be aware of, and plan their observations around.  You can monitor the phases of the Moon, and when it is above the horizon, and you can also travel away from a metropolitan area to a remote location to do your observing.

Professional astronomers (and photographers) have a tool to measure light, called a "photometer".  If you measure a blank part of the night sky from a remote location under excellent viewing conditions and no Moon whatsoever, you'll still register light, although at a very low level.  Besides scattered light from the stars and the Milky Way, some sunlight is scattered through the atmosphere around the Earth on it's dark side (your night sky).  Excluding even those characteristics, there is a condition known as "airglow", which is the result of chemical reactions occurring high in the atmosphere.  Airglow (and light pollution in general) can be measured using a photometer to calculate the limiting magnitude, expressed per unit area, such as per square arc second.  Airglow is typically on the order of 20 to 22 magnitude per sq. arcsec.  This doesn't really affect amateur astronomers, but it does explain (in part) why space observatories like Hubble, Spitzer, and Webb are much more effective than groun-based observatories.

The next thing to consider on this subject is the characteristics of the objects which are being observed.  Stars, theoretically, should present themselves as point-like objects (I'm not going to get into Airy disks here), and in amateur telescopes, do in fact appear point-like.  Other objects, with a viewable, extended surface area or shape, have variable surface-area brightness, which depends on the type of object, and which can be measured using the photometer.  The surface of the Moon is perhaps the brightest object in the night sky, both in absolute terms, but also in terms of surface area brightness.  The planets, which also reflect sunlight, are next in brightness per unit area.  This explains why some surface features on Mars and the belts of Jupiter are visible even with relatively small amateur telescopes. 

Other extended objects, such as nebulae and galaxies, present a diffuse light, much fainter than the Moon or planets, obviously.  These extended objects have a surface area brightness which can make observing them more difficult.  There are ways, however, for amateurs to maximize their ability to see diffuse, extended objects.  Most importantly, and perhaps the easiest, is to plan the time and location for your observing to minimize the amount of light pollution.  The phases of the Moon are easily tracked, and even the time during which it is above the horizon at your location.  It's not too difficult, either, to hop in the car and drive an hour or so out of the town that you live in to get to a relatively dark countryside or mountain location.  The next way to maximize your observing ability is by getting your eyes dark-adapted.  Without getting into the technicalities of how the human eyeball works, you should know that it takes at least 10 minutes for your eyes to become adapted to seeing in the dark, and up to 30 to reach full adaptation.  What this means is that it is not a good idea to get up from your well-lit house where you've been staring at a relatively bright computer or television screen, then walk out into your backyard and immediately try to start observing.  I'd suggest using that time to either set up your telescope (you should be able to do it in the dark, or using a faint RED flashlight directed away from your eyes), or to learn the constellations or plan the sequence of observing for the objects you want to look for, or watch for meteorites, or just simply sit quietly, enjoying the view.  The next way to maximize your viewing pleasure with faint, extended objects is to use "averted vision", or off-axis viewing.  Again, without getting into technical medical terms about your eyeball, you should simply be aware that the center of your vision is actually not as sensitive to faint light as your peripheral vision.  So, once you've found the faint nebula or galaxy with your telescope which you want to observe, center the object up in the eyepiece, but don't look directly at it.  Look slightly off to one side or the other, and you can sometimes detect fainter features than you can see by looking directly at the object.

Lastly, you should know that the telescope manufacturers make filters of various sorts which screw onto your eyepieces.  Some of these can enhance the contrast between the object being observed and the surrounding night sky, by filtering out unwanted wavelengths.  But you should know, too, that all filters reduce the light which passes through them (by definition), so that if the object you're observing is extremely faint, near the limits of your eye and your hardware, adding a filter may not really help.

Hope this all helps.  If you have any questions, don't hesitate to ask.  I'll look the answer up for you if I don't know it.

Enjoy!