About the Resolving Power of Telescopes



What is resolving power? It is the ability of a telescope to see really small details. In this article I give you a really good understanding of what the resolving power of a telescope is and I show you lots of pictures and drawings. A good explanation of this would be to look at a double star.






Double Star Resolution

Here is a view of a double star through a small telescope. Notice how the stars are rather fuzzy. And see how they blend together? This sharpness and definition is a good example of the resolving power of the telescope.

So you might say let's use a higher magnification and get it sharper.




more magnification

So here we have the same double star with a higher magnification. What happened? The image got bigger but we still have two overlapping fuzzy blobs as stars. The image is bigger (magnified) but the resolution is no better.

So magnification really has very little to do with the resolution of a telescope. Once you reach a telescopes resolution limit you can't magnify your way to better viewing.



with a bigger telescope

Now lets take a look at the same double star with a larger telescope. (Larger in terms of objective. It has either a larger primary mirror or lens.

Same two stars, same magnification as picture 1 above. You can see the stars are the same distance apart but they are now separated. the better resolution of this telescope has given us a sharper and more detailed view. Think of this as the same thing as with televisions. The older tv's have only so many lines of resolution on the screen and the new High Definition televisions have many more lines of resolution. Pretty much the same thing.


big telescope more magnifiedHere is another view with our larger telescope and at a higher magnification.

You can see that it is magnified but our double stars are still of course kind of fuzzy. This is why there is always the drive for larger and larger telescopes. The larger the telescope the better and better the resolution will be.




A Look at the technical aspects of Resolving power

You are probably familiar with wavelength and light colors. Different colors of light have different wavelengths and these wavelengths are a critical part of resolution. With a large wavelength a telescope will only gather a certain amount of light and this means that the resolution will be less. With a smaller wavelength more of the light will be sampled and you will get a sharper image.

wavelengthsThis image shows the objective lens of our telescope and below it are two wavelengths of light. This mirror captures 4 waves at the lower wavelength and eleven waves at the higher wavelength. It is easy to see how a larger mirror would capture more wavelengths and give us a better resolution. It is also easy to see how the telescope gathers much less information (waves) in the lower wavelengths so the resolution is not as good.

Summary: Lower wavelengths of light are not resolved as good as higher wavelengths.


This little drawing also gives us a real good look at why bigger telescopes have better resolution. You gather more wavelengths and you have more information which gives you a sharper and more accurate image.

More factors that affect resolution

The quality of the telescope - Not all telescopes are created equal! Some are simple composed of better optics than others. The better the optics the more accurate the information (light) that is gathered and the better the resolution of the telescope.

The Atmosphere - Also has an effect on the light gathered. A turbulent atmosphere will distort the light and give inaccurate information.

The Forumula:

For a perfectly made telescope the resolution is measured in something called the minimum resolvable angle in arc seconds. And the formula is composed of a constant and two variables. The constant is 252,000 and the two variables, which we have talked about, are the wavelength of the light and the diameter of the telescope (in centimeters)

Here is the formula:

The resolution of the telescope in arc seconds is 252,00 times the wavelength of the light divided by the diameter of the telescope. So you can see that the larger the diameter of the scope the smaller the number or the smaller the arc that can be resolved. And the smaller the number the better!



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