Beginner's Questions

Q: Explain the importance of focal length, and how it relates to different types of telescopes.


Telescopes have two main attributes: aperture and focal length.

Aperture describes the diameter of the telescope's primary objective. This is the size of the primary mirror in a reflecting telescope, and the size of the primary lens in a refracting telescope. The larger the aperture, the more light the telescope collects and the greater the resolving power.

Focal length is a measure of the distance from the primary objective to the telescope's focal point.

Today if someone says that they have an 8 inch telescope, or a 127 millimeter telescope, they're referring to its aperture, but this wasn't always the case. If in the 18th century you had asked Charles Messier what size his telescope was, he'd probably say, "6 foot" or whatever the specifics were for his instrument. He was referring to his scope's focal length, not its aperture. That's because in the 18th century nearly all telescopes were simple refracting telescopes. These telescopes suffered from chromatic aberration. Longer focal lengths reduce the effects of chromatic aberrations. At the time, using a longer focal length telescope implied that the image quality was better.

Today, most professional telescopes, including the Hubble Space Telescope, use reflecting designs. Reflecting telescopes generally don't suffer from chromatic aberration. In today's refracting telescopes, chromatic aberration is reduced, or eliminated, by using a combination of lens coatings, exotic glass and multiple lens elements. Focal length is no longer as important as it once was in determining the quality of an instrument's images.

If you have two telescopes of the same aperture, regardless of their design, but differing focal lengths, the one with the longer focal length will magnify the image more than the shorter one, but the longer one will have a narrower field of view. A wider field of view can be more desirable than high magnification. If for instance you're viewing with a large aperture Dobsonian that you have to position manually, and it doesn't track with the sky, then the wider field of view will make it easier to find your target, and view it for longer before the object drifts out of the eyepiece's view, and the telescope has to be repositioned. But to an observer that wants to see the detail on the planet Mars, the higher magnification provided by the longer focal length instrument may be more important than the size of the field of view. Keep in mind that the ability to see that detail is dependent on the instrument's aperture, rather than its focal length all other things being equal.

How a telescope will perform then is really a combination of the scope's aperture and focal length. Dividing the focal length by the aperture (in the same measuring units) gives the scope's focal ratio. A scope with a focal length of 2000mm and an aperture of 200mm has a focal ratio of f/10, commonly referred to as its "f-number". Visually, a lower f-number telescope will have wider, lower-power views than a telescope with a greater f-number. When doing astrophotography using a lower f-number telescope exposures can be shorter than when using higher f-number telescopes. Debatably telescopes of f/8 or less are considered fast, and those scopes that are f/8 or higher are considered slow. Fast optics are demanding. The faster a telescope the more critical the collimation becomes. Slower telescopes are more forgiving.

When using Schmidt-Cassegrain telescopes adjusting the focus moves the primary mirror. This means that changing the focus also changes the focal length by a small amount. But with any telescope the focal length can be adjusted to a degree with the use of barlow lenses (increasing the focal length) and reducers (decreasing the focal length). Schmidt-Cassegrains tend to have slow optics and astrophotographers typically use focal reducers to make them perform faster (decreasing exposure times).

In today's market telescopes for beginners tend to have focal ratios between f/4.5 and f/8.5. For those scopes aimed at intermediate and advanced amateur astronomers, large aperture Dobsonians have focal ratios ranging from f/3 to f/6. Other Newtonian reflectors and refractors typically have focal ratios between f/4 and f/8. These scopes are often used by astrophotographers that prefer fast optics. However, with some digging, you can find refractors designed for solar system viewing that are longer. Most Schmidt-Cassegrain telescopes are f/10, but f/8 to f/15 catadioptric designs are available. Some of these scopes, such as models from Celestron that are Faststar capable, allow a camera to be placed at the Cassegrain focus resulting in fast focal ratios such as f/2. Since the telescope market is always evolving you're experience may vary.

Whether to buy a telescope with a short focal length or a long focal length depends at least in part on what type of objects you enjoy viewing the most. Observers of open clusters and nebulae tend to prefer the wider fields of view that a short focal length telescope provides. While solar system objects, double stars, planetary nebula, globular clusters and most galaxies benefit from the higher magnification that longer focal length telescope can provide, assuming the telescope has the necessary aperture. My recommendation is to buy (at least) one of both. :)

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