If you’ve been researching telescope, you may have come across the terms fast and slow telescopes. In this post, I want to go through what these terms actually mean and the advantages and disadvantages of both types of telescope.
What’s the Difference?
All telescopes have an aperture and a focal length. The aperture is the diameter of the lens or mirror that gathers the light, and the focal length is the distance between the lens/mirror and the eyepiece.
If you have two telescopes with the same aperture but different focal lengths, the telescope with the short focal length would be considered fast and the telescope with the long focal length would be considered slow. These terms are borrowed from photography where a lens with a longer focal length will require a slower shutter speed to get the same level of brightness as a lens with a shorter focal length.
The relationship between the aperture and focal length is known as the focal ratio or f/ratio. You calculate the f/ratio by dividing the focal length (mm) by the aperture (mm). So a telescope with an aperture of 130mm and focal length of 650mm will have a f/ratio of f/5. An f/ratio of f/4 or f/5 would be considered fast, above f/8 would be considered slow, and anything in between would be more balanced.
If you’re looking to buy a telescope, you might be wondering whether you should buy a fast or slow telescope. Now we’re going to look at the advantages and disadvantages of both types.
Fast telescopes have a shorter focal length. They have a wider field of view and are generally better suited for viewing deep space objects.
- More portable
- Wider field of view
- Requires more powerful eyepieces for high magnification
- Shorter eye relief assuming same magnification
- Aberrations are more pronounced
Slow telescopes have a longer focal length and are generally better suited for viewing planets in our solar system and the Moon.
- Higher magnification assuming the same eyepiece
- Longer eye relief assuming same magnification
- Aberrations are less pronounced
- Not as portable
- Not capable of wider fields of view
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