INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES

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TYPES OF TELESCOPES
Telescopes fall into three main categories.
Refractors use an objective made of a compound lens (generally a double achromatic one) to generate the image present on the focal plane.
Reflectors where the same function is carried out by one or more mirrors.
Catadotric telescopes use a combination of lenses and mirrors.
All three types of telescopes have advantages and disadvantages, due to their differences and according to the use to which they are put. As well as cost, a correct choice has to take into account the various needs of the user and the use to which the telescope will be put.

REFRACTORS
In a refractor telescope, the objective is made up two specially constructed lenses. This objective is free from chromatic aberrations and so is called achromatic. Another possible aberration in an objective lens is its sphericity. This too can be corrected by the achromatic doublet.
When good images are required at a great distance, however, other aberrations begin to present themselves. These can include astigmatism, coma or curvature of the field. In order to have good objectives, therefore, free from aberrations, light has to enter from the left and pass across the objective lens. The distance separating the two elements of this lens is very precise (known as air-spacing) for the correction of chromatic aberrations. A ray of light crosses the lens and converges with the focus at point F where the observer is.

Refractor telescopes give well defined and well contrasted images and are available at moderate price, particularly those whose aperture does not exceed 100 mm in diameter. In general, those of greater aperture can be bulky and, as a result, reflector or catadioptric telescopes often interesting alternatives.