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An Explanation of the Resolving Power of Telescopes

Resolving power is not as well understood as the magnification of telescopes. Yet it is more important. This article explains resolution, what it is, and how to calculate it.

Magnification of a telescope is easy to understand. The higher the power the closer the look we get at images in the night sky (the bigger they look).  Resolution is another critical component of telescopes and it is very easy to understand if you think of it like television sets. Older televisions have a certain number of lines of resolution and the new high definition sets have many more lines of resolution so you get a sharper and more detailed image.  The same applies to telescopes; the higher the resolution the more lines of information we get so the better the image.  And this resolution has nothing to do with magnification. If you get closer to your television does the image get sharper? No. It just gets bigger. The resolution remains the same.

The factors that affect resolution

The wavelength of the light being observed is a factor in resolution. The higher the wave length of the light the more waves a telescope sees so the more information it gets, and the better the resolution.  A second factor is the size of the telescope objective.  The bigger the lens or mirror the more wavelengths it will gather. And more wavelengths mean better resolution.

Formula for resolution

This relationship between the wavelength of the light and the diameter of the telescope objective gives us, along with a constant, a formula for calculating the resolution in arc seconds.  Divide the diameter of the telescope objective by the wavelength of the light being observed then multiply this by 252,000 (which is our constant). This gives you the resolution in arc seconds.  (Resolution = 252,000 X  (wavelength) /(diameter)). You can see that the larger the telescope the smaller the resolution is in arc seconds.  And the smaller the resolution in arc seconds the better. It is like smaller lines on your high definition television.  Smaller lines means more lines per inch or centimeter and a crisper, sharper image.  

Other Factors in Telescope Resolution

Atmospheric conditions are a big factor in resolution. A turbulent, thick, or unsteady atmosphere will distort the wavelengths coming into the telescope and distort the quality of the light and information. This will reduce the resolving power.

The quality of the telescope is another factor.  High quality scopes with good optics will correctly gather good information and give you good resolution. Poor quality optics will distort the images and give you a lesser resolution.

ResolutionFree Web Content, while lesser known than magnification is much more important. It is the factor that determines the quality and sharpness of what you see through the telescope and it is the reason why bigger telescopes are better than smaller ones.

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You can read a more comprehensive explanation of telescope resolution, with drawings, on the authors website :

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