The amateur astronomy society based at the Royal Observatory Greenwich

###### Telescopes Facts and Figures by Martin Male

To get the most out of your telescope here is some information on commonly heard terms.

# Aperture

Simply this is the size of the lens or mirror that gathers the light, usually expressed in inches or millimetres.

# Light Gathering Power

This is not an absolute measurement but rather a method of comparing two optical instruments. The larger the light gathering power, the fainter the objects that can be detected:

light gathering power = square of aperture

Example: an 8 inch telescope gathers 4 times more light than a 4 inch telescope (8×8=64, 4×4=16, 64 / 16 = 4 – note that both measurements must use the same unit, in this case inches). Another example: an 80mm scope gathers about 130 times more light than the naked eye (the maximum aperture of the naked eye is about 7mm so 6400 / 49 = 130.6).

# Focal Ratio

The focal ratio is the ratio of the focal length of the telescope to its aperture. It’s calculated by dividing the focal length by the aperture (both must be in the same units). For example, a telescope with a 2032mm focal length and an aperture of 8″ (203.2mm) has a focal ratio of 10 (2032/203.2 = 10) or f/10.

Smaller f-numbers will give brighter photographic images and the option to use shorter exposures. An f/4 system requires only ¼ the exposure time of an f/8 system. Thus, small focal ratio telescopes are called “fast” and larger f/ numbers are called “slow”. Fast focal ratios of telescopes are f/3.5 to f/6, medium are f/7 to f/11, and slow are f/12 and longer.

Faster f/ number telescopes are more costly to produce and have a shallower depth of focus than slower f/ number telescopes.

Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture – the larger the aperture, the brighter the images. Extended objects will always appear brighter at lower magnifications. The main advantage of having a fast focal ratio with a visual telescope is that it will deliver a wider field of view than slower f/ numbers.

# Magnification

The most commonly used formula in amateur astronomy is used to calculate the magnification of a telescope:

magnification = focal length of telescope / focal length of eyepiece

Example: using a 10mm eyepiece in a telescope with a focal length of 1000mm results in a magnification of 100x (1000 / 10 = 100).

# Maximum Magnification

Since we can simply use different eyepieces to reach different magnification, the temptation is to “pump-up” the power as high as possible. In practice telescopes over 10” even with excellent optics are limited to a magnification of about 300x because of the turbulence in the atmosphere.

# Resolving Power

This is the ability to see detail. Basically the larger the aperture the greater will be the resolving power. However, the optical quality of the telescope and the amount of turbulence in the atmosphere may mean that a smaller telescope can outperform a bigger one. The turbulent atmosphere in the UK is the reason why professional observatories are sited abroad today.

# True Field of View

There are two ways to calculate the true field of view (FOV) in degrees of a telescope and eyepiece combination. The easy way and the method is to divide the apparent field of view (AFOV) of the ocular by the magnification of the system. The AFOV for most eyepieces is provided by the manufacturer or can usually be found by an online search. It is easy to derive the magnification of any telescope/ocular combination. Thus:

AFOV / Magnification = FOV

For example, a 25mm Plossl eyepiece generally has an AFOV of 50-degrees. Used in a telescope with a 1000mm prime focal length, the magnification is 40x. The true field of view is therefore 1.25-degrees (50/40=1.25).