The telescope

This is my telescope. It's an 80 mm achromatic refractor, with a focal length of 1200 mm.

A telescope is a device that is capable to project a distant object on a flat surface. This surface can be anything: a piece of paper, photographic film or a CCD chip (an electronic light detector). The picture demonstrates this with a projection of the Sun on a screen.

This image shows the projection of the Sun from a shorter distance. You can even see some sunspots.

The main part of a telescope is the objective (main lens). It focuses incoming light from one direction as a point on the focal plane. The position of this point on the focal plane depends from the direction of the incoming light. In the next picture you can see that the blue and red light rays are focused on different positions on the focal plane, because of their different directions.

Now look at a distant object. All light rays from a single point on the object will fall parallel to each other on the objective, which will focus the rays as a point on the focal plane. All other points of the object will also be projected, but because the camera sees them under a slightly different angle, the positions of the points on the focal plane are also different. This is the reason that an image of the object will form on the focal plane. For more information: Image formation

In this image a screen was hold at the position of the focal plane, where you can see the projection of the Sun. If a photographic film would be hold on the same place as the screen (in a camera), you would get a picture of the Sun.

The same story applies to a camera. A camera is not much more then an objective that projects an object on a photographic film. The difference between a telescope and a camera is the magnification, as we will see later.

The objective has two important properties:

1. The focal length.

   

   x = projection size (mm), f = focal length (mm) and = angle of incoming light.

   You can see that x = f * tan(). This means: the focal length (f) defines the size of the projection (x). The size of the projection is directly proportional with the focal length. If you double the focal length, the projection size will also double.

   f (mm)	x (mm)
   50	0.44
   80	0.70
   100	0.87
   300	2.62
   1200	10.5
   4000	34.9

   This table shows a few examples with = 0.5 degrees. You can also calculate the field of view of a normal camera with 35 mm negatives: x = 35 mm; f = 50 mm gives = 35 degrees

   Take also a look at some photographic examples.

   A normal photographic objective has a focal length of 50-80 mm. This is very small if you compare this with the focal length of 1200 mm of my telescope.

   A focal length of 1200 mm looks great, but is not enough with lunar photography. For more information: Oculair projection

2. The diameter.

   • The size of the surface of the objective defines the light collecting power of the telescope. If you double the size of the surface, then two times as much light will fall trough the telescope on the focal plane, so the light collecting power will also double. If you double the size of the diameter, then the surface will fourfold, and so will the light collecting power. For more information: A measure of the light intensity.
   • The diameter of the objective also defines the resolving power of the telescope. The resolving power means the power of the telescope to resolve details. A telescope with a high resolving power can see smaller details. With twice a bigger diameter you have twice a bigger resolving power. So a big telescope can see smaller details. This is very important with lunar photography. The resolving power of a telescope is measured in arcseconds. Look at angles for more information.
   
   

   Obj. diameter (mm)	res. power (arcsec.)
   60	2.0
   80	1.4
   120	1.0
   200	0.58
   400	0.29
   1000	0.12

   This table shows some examples of the relation between objective diameter and resolving power. These values also depend on wavelength and contrast between objects.

The theoretical resolving power of a telescope can only be achieved if the quality of the optics is good enough. Very cheap telescopes ($100) mostly have very bad optics, which will blur your images. You will never obtain good results with such a telescope.

This can be the difference between a good and a bad telescope.

The focal length is an important factor on image quality. It is more difficult to make good optics with a short focal length than with a long focal length. Good telescopes with short focal length do exist, but are much more expensive. This is the reason that most telescopes used for lunar photography have a large focal length compared to their diameter.

The type of telescope I discussed here is called a refractor. Another very important type of telescope is the reflector (for example: newtonian and cassegrain). In this type of telescopes, the light is not focused by a lens, but by a mirror. The main principles are the same. The story here also applies to reflectors.