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Skywatching Tools

Binocular Types

Refracting Telescope

Reflecting Telescope

Cassegrain Telescope (Catadioptric)

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Telescope Formulas

Magnifying Power

Maximum magnifying power » 20 ´ diameter of objective in inches
because your eye can resolve 100 seconds of arc, so you want the telescope to bring its smallest resolution element to that size: MagnifyingPower×ResolvingPower≈100 – see formula for resolving power below.

Minimum magnifying power should be no less than 3 ´ diameter of objective in inches.
because as the magnifying power gets bigger and bigger, the light from the objective is concentrated into a smaller and smaller spot. If the magnifying power is not big enough, the light is not concentrated down small enough to get through the iris of your eye (you don’t get all the light you should).

Resolving Power

This is known as the “Dawes Limit” and was empirically determined by William R. Dawes. It is about 20% smaller than the theoretical limit of resolution due to the diffraction of light by the edge of the objective (the “aperture” of the telescope). The diffraction formula is 1.22λ/D where λ is the wavelength of light (~550 nm) and D is the diameter of the objective. The Dawes Limit is generally accepted as the best match to a telescope’s actual field performance.

Magnitudes (faintest stars you can see)

Rule of Thumb #1: every time you double the diameter of the objective, add 1.5 to the faintest magnitude you can see. (e.g. 100mm scope can see mag 12.0, 200mm scope can see 13.5)

Rule of Thumb #2: for every increase of one magnitude you can see, you can see 3 times as many stars.
In New Berlin I can see to magnitude 4, at camp we can see to magnitude 6 – that’s two magnitudes increase, so it’s 3×3 = 9 times as many stars you can see.