The light-gathering power of a telescope is directly proportional to its surface area. This means that for faint objects or faint detail, you will see more with a larger-diameter mirror.
Resolving power determines how clearly you see something and is the most important power of a telescope. If you have a small blurry image and magnify it, you just get a big blurry image. It doesn't become any clearer by making it bigger.
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The theoretical resolving power of a telescope is directly proportional to the diameter of the telescope, but also depends on the wavelength of the signal. Radio telescopes do not resolve as well as optical telescopes because radio waves are longer wavelength than optical light. This is why radio telescopes are so big.
- Resolution also depends on the quality of the optical components, but also to a large extent on the steadiness of the air. If Earth’s atmosphere is turbulent, it causes “bad seeing” and even the biggest telescopes see blurry images. This is being overcome to some extent by adaptive optics.
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For optical telescopes, the resolving power is given approximately by the formula:
Angular resolution = 138 / D
where the angular resolution is measured in seconds of arc and the diameter of the telescope D is measured in millimetres. The dependence on diameter again means that bigger telescopes are better under good seeing conditions.
Magnifying power is a relationship between the focal length F of the primary (main mirror or lens) and the focal length f of the eyepiece. Where both focal lengths are measured in the same units, usually millimetres, magnification is given by the formula:
Magnification = F/f
- Beginners are advised to start off with low magnification, around 50x or less. If you have a choice of eyepieces, this means choosing the eyepiece with the biggest number written on it. Then objects don't move out of the field of view too rapidly, and it's much easier to find targets at low magnification than at high magnification.
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