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Re: The Effect Of Aperture On Light Polution

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Posted by Dianne/">Dianne on August 10, 1999 15:17:05 UTC

: Please consider the following two statements:

: 1. There is an astronomers old wives tale that you can't use a 'scope over 3" aperture in light polluted areas without loosing significant contrast. (This is supported by statements in the latest Orion catalog as well as throughout website comments.) : 2. S&T's City Astronomy states that "No telescope can make an extended object appear any brighter: all it can do is provide magnification for you to see it better."

: Since the two statements are contradictory, one is correct and one is incorrect.

: I suspect that since "general seeing " conditions usually limit magnification, and since smaller 'scopes don't have extended magnification, they're by definition "unlimited." Maybe this is the basis for statement #1.

: Since "extended objects include planets, the moon, and nebula, I expect they also include the atmosphere through which we are attempting to observe. Therefore, my particular belief is that larger apertures don't improve the contrast of planets, etc., but they don't hurt either. The f ratio of the 'scope seems to have no bearing on the outcome. Increased magnification will darken the sky as well as the extended objects, but will not darken stars. But going from 75X to 150X on a 10" scope will have the same effect as with a 3" scope.

: Anyone out there care to agree or disagree?

I am just a neophite in astronomy but a professional in engineering, so this is my personal observation and experience:

Since all the light collected by a scope is focused on an area of fixed size (your retina), the collecting area is directly proportional to the brightness of the image - double the area (A=pi X r*r) and you double the brightness (or very nearly so).

On the down side, you are "looking through" twice as much atmoshpere and will double the effects of atmoshperic distortions.

When you increase magnification, you are reducing you field of view and thereby reducing the total number of photos being focused on your retina, therefore the image brightness drops.

Like everything in life, appature and magnification are compromises.

I hope this helps explain some of the physics.

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