What fast and slow mean
Telescope “speed” refers to focal ratio, also called the f-number or f/ratio. It is the focal length divided by the aperture. A low number (such as f/4 or f/5) is called fast, and a high number (such as f/10 or higher) is called slow. The terms come from photography, where a lower f-number lets in more light per unit of time.
How to calculate focal ratio
Divide focal length by aperture, using the same units.
- 600 mm focal length, 120 mm aperture: 600 / 120 = f/5.
- 2000 mm focal length, 200 mm aperture: 2000 / 200 = f/10.
You can learn each input in our guides to focal length and aperture.
A rough common grouping is that f/5 or lower is fast, around f/6 to f/8 is medium, and f/8 or higher is slow, though people draw the lines differently.
Fast telescopes
- Wider field of view at a given magnification, since focal length is short for the aperture.
- Shorter exposures for extended objects such as nebulae in astrophotography.
- Often more compact tubes.
- Tend to be more demanding: eyepieces and optics must perform well across the field, and imperfect collimation or coma in fast reflectors can be more noticeable.
Slow telescopes
- Narrower field and higher magnification with a given eyepiece.
- Generally more forgiving of cheaper eyepieces and small alignment errors.
- Longer tubes for the same aperture, unless the design folds the light path.
- Longer exposures needed for imaging faint extended objects.
For visual observing
For looking through an eyepiece, a fast scope does not make the sky look brighter by itself. Brightness of a given object at a given magnification depends on aperture. What focal ratio changes is which eyepiece gives which magnification and field. A slow scope needs longer-focal-length eyepieces to reach the same wide field, and a fast scope reaches high magnification only with very short eyepieces.
For astrophotography
Speed matters more in imaging. For extended targets like nebulae, a lower focal ratio collects light per pixel faster, so you need less exposure time to reach a similar signal. Fast refractors and reflectors are therefore popular for deep-sky imaging, while slower scopes are often chosen for planetary work, where long focal length helps with image scale. Whether a refractor or reflector suits you better depends on budget, mount and target, so treat any single rule as a starting point.
Which should you choose?
If you mostly want wide, rich-field views or deep-sky imaging, a faster scope is attractive. If you favour planets and the Moon, or want something tolerant of simple eyepieces, a slower one fits well. Whichever you pick, check that the mount can handle it. Once you know your scope’s numbers, you can use telescope magnification power to plan eyepieces.