Optical Properties


It corresponds to the diameter of the front lens (refractor) or the main mirror (reflecting telescope or catadioptric), that is to say, the inner diameter of the lens. The more it is, the more the instrument captures light and allows to observe faint objects and small in apparent size. In addition, over the amount of light will be important, and we can magnify the image without losing quality. The aperture is usually expressed in millimeters or inches (1 “= 25.4 mm). The larger the aperture is and the greater the instrument is called” powerful. ”

The aperture is the most important factor to consider when purchasing an astronomical instrument, because it is related to the brightness. So premium on the magnification, because without light, an object enlarges have poor resolution.


The focal (or focal length) of a telescope is the length between the center of the main lens or primary mirror focal point of light rays: the home. It is expressed in millimeters or inches.


This is the number of times the instrument magnifies the image observed by the naked eye. Magnification 100x means that the observed object appears with apparent dimensions 100 times greater than with the naked eye. The magnification of a telescope depends on the focal length of the objective and the eyepiece. More the latter is shorter and the magnification. It is the ratio of the focal length of the lens by the ocular (e.g., with a bezel of 800 mm and a 20 mm eyepiece, we obtain a magnification of 40 times):

Magnification = Tube focal length / Eyepiece focal length


Brightness and sharpness decreases with magnification, it is not useful to magnify beyond a certain value. This limit is called the maximum useful magnification. Beyond this limit, nothing is gained more and picture quality. It is 2 times the diameter of the objective (for example, a 60mm bezel opening is 60 x 2 = 120 times).

The magnification équipupillaire, also called minimum magnification is the magnification that produces an exit pupil equal to that of the eye. It allows to exploit the opportunities of the eye. It is equal to the aperture of the instrument divided by the diameter of the pupil of the human eye at night which is close to 6 mm (for a 60 mm, the minimum magnification will be 10 times).

The useful magnification (or solving) is one that allows you to see fine details that the instrument is capable of reproducing. It is equal to three times the équipupillaire magnification, or the tube diameter divided by 2 (for a 60mm telescope, the magnification will be solving of 30 times).


It is the ratio of the focal length and the diameter. A large focal ratio means that the focal length (and thus the tube) is small compared to the aperture and inversely.

Myth about the focal ratio

In most books currently published on the subject and on many websites, it says:
– For global requires a focal ratio of 10 or more.
– For deep sky, you need a focal ratio 6 or less.
– For a versatile observation requires a focal ratio between 6 and 10.

>In fact, you can practice these activities with any focal ratio.


► Below a focal ratio of 6, the aperture is relatively large compared to the focal length, and the instrument will be * naturally * suitable for deep sky.
► Conversely, Above a focal ratio of 10 is relatively small compared to the aperture, the instrument will be * naturally * suited for observation of smaller planets and bright that the deep sky objects.

However, as we have just seen, when the eye is changed, it also changes the magnification (magnification = focal length of the telescope / focal length of the eyepiece)

We can very well increase the magnification of a telescope focal ratio less than 6 (thus a priori intended for deep sky) by providing the eye of a large focal length, and thus adapt to planetary observation; and vice versa.


The resolving power or resolution power is the minimum angle that must separate two contiguous points for them to be properly discerned by the observation instrument. It is thus the instrument’s ability to form distinct images of two close objects (for example, the two stars in a binary star). It is expressed in seconds of arc, which is a subunit of the degree, which is written. “The higher the number the better the resolving power. In addition, the resolving power increases with the aperture.

Power separator = 120 / objective diameter


For example, a 200 mm telescope has a resolving power of 0.6 “(120/200 = 0.6) (angle measuring units. Degrees (°) arc minutes (‘) and seconds arc (”). 1 = 60 = 3600 ” or 1 “= 1/3600 = 0.000 ° 277 °)


The limiting magnitude (or limit value) is the lowest brightness that can be observed with a given instrument. It is calculated from the following formula, where D is the diameter of the objective expressed in centimeters:

M (lim) = 7.1 + 5 × log(10) D


This formula is theoretical result because it takes no account of turbulence or light pollution, no light loss related to the optics of the instrument.


This is the theoretical capacity of the telescope to collect light, so its ability to reveal faint object.
Indeed, it is not because they are smaller than most celestial objects are hard to see, but because they are not bright enough. The most important characteristic of a telescope is not its magnification power, but rather its ability to collect light. The more it is, the more celestial objects will be visible. The light gathering power depends only on the diameter of the lens. We calculated by dividing the objective lens diameter by that of the dilated pupil of a young person who is averaging 7mm and have students all squared:

Power collector light = (objective diameter / pupil diameter)²


This means that with a 70mm opening window, if one has a pupil of 7 mm, receive 100 times more light than the naked eye. So we’ll see 100 times better.

Surface Treatments


► Silvering and aluminide
To increase the reflectivity of the mirrors of glass or ceramic, is deposited over a thin layer of a metal which may be silver or aluminum. This layer is a few hundred nanometers thick. Most mirrors are aluminized amateur instruments.
Mirrors consist of a monolayer of aluminum are very fragile. It is possible to protect with an additional single layer of SiO or SiO2, but this comes at the expense of a loss in reflectivity. Today, almost any telescope mirror in this trade is carried out without this kind of protection. That said, simple aluminide is still used for large mirrors.

► Improved Treatment
It allows 96-99% reflectivity (Enhanced Aluminum, Everbrite).


The lenses glasses present in the objectives of the telescopes have instead need to be as transparent as possible, in order to stop a minimum of light. For this, there are various anti-reflective coatings:

► Single layer MgF2: consists of a monolayer
► Narrow band multilayer: 2 layers
► Broadband multilayer: 3 thin films
► Very broadband multilayer: 5 layers or plusTraitement AntirefletMono-layer or multilayer

The type of treatment applied is usually written on the eye :
► monolayer treaty (Treaty coated)
► multilayer treaty (multiple, multi-treated, MC, multicoated)
► fully-coated (CF) and fully-multi-coated (FMC)> Only his statements ensure that all air / glass surfaces are treated.