INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES  

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FOCAL LENGTH
Another characteristic of the telescope is the focal length (F) or the distance that exists between the objective and the plane in which the image is formed. This is normally measured in millimeters. From this value, the relative luminosity (f) is calculated as is the enlargement factor, both are other characteristic data for a telescope.

FOCAL RATIO The relative luminosity is the relationship between the focal length and the diameter of the objective: with F=1000 mm and D=100 mm.

Therefore the instrument will be 1000/100=f/10. This characteristic, known also as the aperture ratio or focal ratio, is significant also when specific use of an instrument is in needed.

VISUAL OBSERVATION
Enlargement is defined as the apparent increase of the dimensions of an image observed with a telescope. Enlargement or magnification is one of the functions of the focal length of the ocular inserted, in the telescope. The following formula can be used when calculating the enlargement offered by the telescope.

The focal length of most telescopes is fixed by the optic programme of the telescope. As a result, the magnification process varies according to the different eyepieces used. The focal length of a telescope is usually indicated on the telescope itself, or else it is indicated on the instructions manual. In theory, there is no limit to the magnification factor of a telescope. In reality, there is a practical limit to be put on useful magnification. By the same terms, a telescope cannot be used below a certain magnification limit.