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The Astronomy Nexus

f/what?

I throw around the term f/ratio fairly often in my suggestions to new observers. If you're a photographer, you already recognize the concept: it means much the same thing in telescopes as it does in cameras.

For non-photographers: f/ratio is the focal length of the telescope divided by its aperture, the diameter of the main lens or mirror. So, for example, a common telescope design is a 120mm aperture (4.7") refractor with a focal length of 1 meter (1000mm). The f/ratio is thus 1000/120 = 8.3, conventionally written "f/8.3".

What does f/ratio mean to the observer? Several things:

• Telescope Size / Portability: For simple designs (refractors and simple reflectors, like most Dobsonians), the focal length is very nearly equal to the length of the tube. So, for a given aperture, smaller f/ratios mean shorter focal lengths and hence shorter tubes. A 12 inch f/5 Dobsonian is movable by one person, and can be used sitting down or standing up comfortably; a 12 inch f/15 telescope is a massive instrument suitable only for a permanent observatory.
  
  
• Typical Magnification Range:A telescope of a given aperture (say, 6") and small f/ratio will give lower magnification with a given eyepiece than one with a larger f/ratio. For example, a 120mm f/8.3 refractor with a 20mm eyepiece gives a magnification of 50x; a 120mm f/5 refractor would give a magnification of 30x with the same eyepiece. Thus, short f/ratio telescopes are most easily used for wide field viewing; long f/ratio telescopes for high-powered observations (such as planets and double stars).
  
  
• Optical Distortion: Shorter f/ratio telescopes suffer more from certain optical distortions (called "aberrations") than those with longer f/ratios. Short focus refractors suffer from false color ("chromatic aberration") around brighter objects. Short focus reflectors maintain sharp images in the center of the field of view, but have significant distortions at the edges of the field of view. Additionally, many eyepieces do not work as well in telescopes with short f/ratios, and add their own distortions to the edges of the field of view.
  
  

In general, f/ratios under f/6 are considered fairly short, f/6 to f/10 medium, and over f/10 long. The range f/6 to f/10 is a fairly good one for general purpose telescopes; it's possible to get both wide fields (low powers) and high-powered views without unusual eyepieces.

Finally, it's a little easier to make a good-quality short-focus reflector than a refractor. An inexpensive f/6 reflector has only minor image distortion near the edge of the field of view; an inexpensive f/6 refractor will usually have significant false color (often enough to be distracting at high power). Typical f/ratio values for reflectors are f/5 to f/8; for refractors, f/8 to f/12. Refractors shorter than f/8, and reflectors shorter than f/5, are often called "short tubes".

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Last Modification: Dec 5, 2003