INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES  

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REAL FIELD

REAL FIELD
The field of view (in degrees) that the observer sees by looking through a telescope using an ocular, called real or effective field. There are two parameters: the apparent filed of the ocular is the magnification given according to the following formula.

Example: a wide-angle ocular of 14 mm has an apparent field of 84 degrees. When it is used with a Schmidt-Cassegrain 200 mm, this ocular provides 2000/14 mm=143x. The Real field with this ocular is, therefore 84/143, or 59 degrees.

EXIT PUPIL
The diameter of a beam of light coming from the ocular of a telescope is defined as the exit pupil. Determined by the diameter and the focal length of the telescope as well as by the focal length of the ocular. It is calculated as follows:

The pupil of the human eye has a maximum diameter of about 7 mm, and, as a result, cannot accept an exit pupil larger than this diameter. Oculars of low level magnification offer bigger exit pupils. It is for this reason that telescopes have a minimum limit of magnification, or cannot be used below a certain level.
Example: A telescope with a diameter of 203 mm and a focal length of 2000 mm, used with an ocular with a focal of 25 mm will result in a magnification of 2000/25=80x. The diameter of the exit pupil when an ocular of 25 mm is used, is therefore 203 mm/80=2.5 mm. The concept of the exit pupil is important because it is directly related to the luminosity of the telescope image.

FOCAL MULTIPLIER
The Barlow lens uses a negative (concave) lens or group of lenses, to increase the focal length of a telescope. The draw-tube in which the lens is situated fits into the ocular holder of the telescope in such a way that the lens is positioned before the focal point of the telescope. A 2x Barlow lens doubles the focal length of the telescope. For example, it can bring it from 2000 to 4000 mm, and it also doubles the working focal ratio. The result is that an ocular that magnifies the sun by 100x gives magnification of 200x when used in combination with a Barlow lens. These lenses have two major advantages: doubling the magnification possibilities of the oculars and making a higher level of magnification possible with longer focal oculars while maintaining comfortable eye relief. A disadvantage is that luminosity goes to be diminished.

DIAGONAL PRISMS
Observing with a Schmidt-Cassegrain or refractor telescope, where the ocular is positioned on the lower part of the telescope's tube, the diagonal prism, a device for right angle viewing, allows a more comfortable viewing position. Most telescopes present inverted images, with the right in place of the left. In astronomy this is not a problem. When observing objects on Earth, however, the image needs to be rectified. By using the diagonal prism, the image is rectified downward while remaining inverted sideways. This is common in all telescopes and most observers readily adapt to the image presented in in this way.