INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES
  

page 4

The following formulae are useful in defining the scale of magnification factor in use by most telescopes:
MINIMUM ENLARGEMENT (APPROX)= Diameter x0.15
MAXIMUM ENLARGEMENT (APPROX)= Diameter x 2.4 Example: A telescope of diameter 200 mm can be used in an enlargement interval going from 200x0.15 = 30x at its lower limit 200x2.4= 480x at it upper limit
The formulae listed here are offered only as an indication of the magnification range, usable in practice with most telescopes up to about 12 inches (300 mm) in diameter. These indications are independent of the make and type of telescope (reflector, refractor, Schmidt-Cassegrain etc).
The result of a too high level of magnification is usually a confused image, indistinct and badly defined, offering less detail than that offered by a lower degree of magnification. Using a level of magnification that is too low results in a spot of light from the ocular (see the section on the exit pupil) that is too large for the human eye.

Example: a telescope of 203 mm, used at 200x will show at long range more detail of the surface of the planet Jupiter than will a telescope of 102 mm used at 200x. The telescope of 203 mm collects four times more light than a telescope of 102 mm, and the quality of light collected determines the resolution and the detail of the image.
Effect of the atmosphere:
Much more important than the magnification in visual observation is the stability of the earth's atmosphere.
When the atmosphere is in rapid change, which we can see because the stars "scintillate", it is impossible to obtain sharp, clear images at a high level of magnification. It would be optimistic to expect to use the high level of magnification previously advised. In this case, the observer will have to reduce the magnification in order to obtain well defined images.
The stability of the atmosphere is, in general, used in reference to the quality of seeing in astronomy. Astronomers often speak of conditions of seeing, because the greater the seeing the better the detail will be. Another condition, called transparency refers to the clarity or limpidity of the atmosphere. Ironically, a perfectly dark, limpid sky, which should represent the optimum of desirable conditions for the use of high-level magnification, occurs, just when the seeing, or stability of the atmosphere is not good. Expert observers use magnification which is suitable to the quality of seeing.
In the course of daytime terrestrial observation, lower to mid-level magnification is best, because in such conditions the observer is looking through the lowest part of terrestrial atmosphere. Use of magnification at too high a level will result in an image that seems boiled due to the wave of heat which rises from the earth's surface.
Here are some calculations on one model of telescope.