Should the amount of light pollution in my area influence my choice of telescope?

As you might guess from looking at the sky from a suburban setting, the effect of light pollution is to mask the light from faint stars. In a typical situation, if you were to look at, say, the constellation of Orion, you might see the belt stars and the 4 stars making up the body of Orion, but not the Orion nebula or stars fainter than a visual magnitude of +4. The following table gives the limiting magnitude under perfect conditions for a variety of aperture sizes in inches:

1.0 inch.......9.0m

3.0 inch.......11.5m

6.0 inch.......12.8m

8.0 inch.......13.5m

10.0 inch......14.0m

12.0 inch......14.3m

Clearly, the fact that light pollution might limit you to +4.0m or brighter stars with the naked eye, does not place any constraint on telescope aperture because virtually every popular telescope size from refractors to reflectors, will exceed the light pollution threshold significantly. If you want to study the bright planets and the Moon from your suburban setting, go ahead and get the largest aperture you can comfortably afford, and feel like lugging around. If you expect to take trips into the country on occasion, your choice will automatically let you see stars down to the magnitude limit for your instrument.

One question I have always had, but never have had the time to check out, is whether you might still see stars below the sky pollution limit if you used a telescope of large enough aperture. It seems to me that a star which is, say, 5 magnitudes below the naked eye limit ( example = +4m above), would be seen as a 1 percent enhancement in light over a milky background in the telescope eyepiece. This 1 percent enhancement would, it seems to me, be hard to make out in a small-aperture telescope with its limited light gathering power, but in a large-aperture instrument, it ought to be observable. It would be like looking for a bright star or a planet in the daytime, which can be done if you know exactly where to look. The impact that this would have on your telescope selection is, again, to argue in favor of a large aperture over a smaller one.


Copyright 1997 Dr. Sten Odenwald
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