To determine which magnification a telescope provides, a number of values are required, namely the focal lengths of the objective and the eyepiece. The objective is the largest lens in the telescope and the eyepiece is the small lens through which the observer looks. The magnification of a telescope can then be determined by dividing the focal length of the objective by the focal length of the eyepiece.
The focal length
A property of a lens is that incident light rays are deflected by the curved glass, if these light rays fall in straight lines perpendicular to the plane of the lens then the light rays eventually converge behind this lens in one point, this is called the focal point and is denoted by a large letter F. The distance between this focal point F and the lens is called the focal length, indicated by a small letter f. All lenses have their own specific focal length and this also applies to the objective and eyepiece of a telescope. The focal length is usually expressed in millimeters. Hollow ground mirrors can also function as a lens, the focal point F then comes to lie in front of the lens and not behind it.
The objective and eyepiece of a telescope
On a telescope, the light first enters the lens, this can be a large glass lens on the front of a telescope, but also a hollow cut mirror on the inside of the telescope. The first type is called a refractor telescope, the second type a reflector telescope. With a large lens, more light enters the telescope and the telescope is better suited to achieve high magnifications. So the larger the objective, the better the performance of the telescope can be, with emphasis on the word ‘can’, because even with small telescopes, bright objects such as the moon and planets can be perfectly observed.
Eyepiece
The observations are made by looking through the eyepiece with the eye, this is the small lens that is located in a small adjustable tube at the beginning of the telescope (refractor) or on the side of the telescope (reflector). The eyepiece can be exchanged with another eyepiece with its own specific focal length, for example 10 mm or 25 mm. By exchanging the eyepiece, the magnification of a telescope can thus be reduced or increased. The focal length is always indicated on the eyepiece.
Determine the magnification of the telescope
The lens of a telescope always has a fixed focal length, this is often indicated on the telescope just like the diameter of the objective. A ‘750/150’ telescope has a focal length of 750 mm and an objective of 15 cm. Suppose a 10 mm eyepiece is placed in this telescope, the magnification of the telescope at that moment is 750 mm / 10 mm = 75 X. With a 25 mm eyepiece, the magnification is 30 X.
The specifications of a telescope often indicate the maximum meaningful magnification, usually twice the focal length of the telescope divided by ten. This means that an eyepiece of 5 mm must be used, 750 mm / 5 mm is 150 times. In practice, however, this does not often produce very good observations and it is best to work with an eyepiece of at least 10 mm. In fact, it may even be that with an eyepiece of 25 mm more and better is observed than with an eyepiece with a smaller focal length.
What is the F value?
The F value is a number that says something about the brightness of the telescope or a photo camera. The F value of a telescope is the same as that we know from photography, it is partly determined in photography by the diameter of the aperture. Now a telescope actually only has one type of aperture and that concerns the diameter of the lens. The F-value is calculated by dividing the focal length by the diameter of the lens, or by the diameter of the aperture in photography. A ‘750/150’ telescope then has a value of F5. With a photography lens of 50 mm and an F-value of 5.6, the aperture is 8.93 mm open.
Brightness
A telescope with a low F-value is said to be bright, but has the disadvantage that the depth of field is limited. We know this phenomenon mainly from photography and it produces atmospheric pictures with a sharp object placed in the foreground against a blurred background. The limited depth of field ensures that the area in which we can focus a telescope is more limited than with a telescope with a high F-value of F10. Of course, factors such as the quality of the lenses and the components also play a role in properly focusing a telescope. In general, a telescope with a high F-value is suitable for observing planets and a telescope with a low F-value is suitable for observing galaxies and nebulae. By the way, keep in mind that a 70 mm lens already gives 36% more light than a 60 mm lens, so that quickly adds up.
