INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES

page 7

REFLECTORS
With reflector telescopes the light coming from the subject is thrown onto the surface of a mirror (generally parabolic) which is found in the lower part of the draw tube.
The reflection towards the focus causes the light to converge in the upper part of the draw tube, where a mirror (known as secondary or return) redirects it to the focal plane on the outside of the instrument.
In fig. 4, light enters from the left and passes inside along the entire length of the draw tube of the telescope as far as the primary parabolic mirror (1) where it is reflected behind to the secondary mirror (2). This secondary mirror forms an angle of 45 ° with the optic axis of the telescope where the image is put into focus at point F.

This type of telescope is generally free from chromaticism.
Given the ease of the optics a greater aperture may be achieved at relatively low cost. Among the defects to note are that many models are formed of open draw tubes, and the air within can cause problems. Many makes of reflector telescope are available: NEWTON ¯ CASSEGRAIN ¯ SCHMIDT ° SCHMIDT CASSEGRAIN ¯ SCHMIDT NEWTON.

Newton telescope

In a Newton reflector, the rays, once they have been reflected by the primary parabolic mirror are deflected at 90 ° by a flat secondary mirror before converging at the focus. Usually, this system requires that the draw tube of the telescope is a s long as the focus, resulting in a awkward bulkiness in the case of instruments of a certain power. The point of observation is situated on the extreme opposite of the principal mirror. It has a typical focal ratio of f/5 to f/10. It has a simple optic scheme (invented by the physicist Isaac Newton) resulting in a low cost relative to the aperture. It is convenient for observing even objects near the zenith. Its main drawbacks are its bulky size and weight (although these are less notable than in the case of the refractor telescopes) which makes it difficult to carry. Besides, it has less intuitive sighting, for example the ocular is positioned on the side of the draw tube (at 90 ° to the direction of sighting) and not at the base as other models. Sensitive to bumps, the optics need collimating from time to time.

Cassegrain telescope

The Cassegrain system (fig. 6) consists of a principal parabolic mirror with a hole at the centre, and a small converse hyperbolic secondary mirror. The rays reflected by the former converge on the latter and are thrown back again on the former, converging on the focus by means of the hole situated in the primary mirror.
By means of the reflection cast by the secondary mirror, there is a lengthening of the focal from the principal mirror. We are talking here of equivalent focal. This type of telescope is very compact despite the long focal.