Aperture - focal length
Lenses or mirrors of telescope are characterized by these two values :
The aperture - usually named D - is the diameter of the lens or the mirror. It is often the diameter of the optical element but it can be also the aperture limited by a diaphragm.
- The quantity of collected light varies according to the square of
the aperture. With regard to the eye, having a "standard" aperture
d of 6mm, an objective of opening D = 60mm receives: ( D/d) x ( D/d)
= (10) x (10) = 100 times more light than the eye.
- The resolution is directly proportional to the aperture. With
D in mm, we can quickly estimate it in arc-seconds
by the formula :
a = 120/D
The focal length (F) is the distance
which separates the center of the objective and the point of convergence
of beams resulting from an object very distant - a star is a good one.
Typically, beam of sun light converges
in a point which we named focus or in french "foyer", the place where
is the fire.
The distance focus-objective is the focal length F.
- The dimension of the image is directly proportional to the focal length. In the focal plan, an object of angular diameter "a" will have an image which dimension x is :
x = 2.F.tg(α/2) or F.tg(α) if α is
very small (α in
- The luminosity of the image decreases according to the square of the focal length. (L prop. in 1/F ²)
Focal ratio : F/D
The previous elements thus show us that in theory,
if we keep the same F/D ratio, the luminosity of the image remains unchanged.
Mirrors or lenses with a small
F/D will be said more luminous than optics with high
F/D. This indication also appears on the photographic objectives; we
often speak then about values of diaphragm. The focal being fixed, we
play on a diaphragm to change D and modify so the luminosity of
the image. An objective with F/D = 2 is more luminous than has
Increasing focal length
It is possible to increase the image given by a
telescope by means of a divergent lens, the famous Barlow lens.
It allows to increase the focal of the objective and thus
to operate a magnification of the image. In visual observation, Barlow
lens is constituted by an achromatic doublet.