The choice of an optical instrument is always a tricky thing for the novice or beginner. The operation is the more difficult it has no benchmark, no experience ... What instrument do I choose ? The question is very common : Simply browse the many forums for noticing ! In addition, the novice is often dominated by the terrible myth that you need a fastidious knowledge of mathematical formulas and theorems governing optical physics, increasing his apprehension to take the plunge. But all this relative complexity, it is not so ! Only a few basic formulas, which are carried on more modest calculators enough !

An optical instrument is determined by five recurring factors :

  • Objective diameter (in millimeters)
  • Focal distance (in millimeters)
  • Focale / diameter (without unity)
  • Resolving power (in arcseconds)
  • Magnification (in factors)

Three families of optical instruments exist :

  • The binoculars
  • The refractor telescopes and monoculars
  • The reflector telescopes

The binoculars

Easily transportable instrument that can be a great tool for beginners, the binoculars are valuable accessories to study the sky and make trails. Employees in research and comet observation, the binoculars are instruments to very large field and very bright. Their binocular vision allows very long observations with optimum comfort.

There are two prisms editing systems :

  • The reliable and simple system to mount the Italian Ignazio Porro XIX century as my own pair of 7x50 binoculars ; The manufacturing system and very delicate assembly called "roof" that allows a better light and linear transmission.

Two additional parameters come into play line with binoculars :

  • The coverage degrees of arc and the exit pupil
The refractor telescopes and monoculars The reflector telescopes

Instrument oldest optics, the refractor telescope is also the basic instrument for the amateur astronomer. There are many varieties, the most common to the most specialized, depending on their application in astronomy. Monoculars, unique and constant magnification, are integral register part of refractor telescopes. Composed of a more or less complex objective, whether in the material choice or in the number of lenses, the refractor telescope is commonly with a objective in two adjoining lenses : a bi-convex lens crown and flint lens having a concave face and the other very slightly convex. This composition can correct the lens chromatic aberration, which were assigned the refractor telescopes to the the optical era beginning.

Their main advantages are the lack of central obstruction, which predispose them to planetary high resolution imaging and a closed tube making them much less susceptible to turbulence as reflector telescopes.

Their disadvantage is their price :

  • At least three times more expensive than the same diameter reflector telescopes.

Optical instruments invented later than refractor telescopes, reflector telescopes and optical composition take the name of their designer. The objective has been replaced by a concave or parabolic mirror, responsible for collecting the light of the stars. In all reflector telescopes, the primary mirror is assisted by another mirror, responsible for directing the collected light to the eyepiece, which is characterized by a more or less central obstruction. This is the common choice for the amateur to the first economic criterion (a reflector telescope costs on average three times less than the refractor telescope). However, against-part of this economy, its user will have to learn to regularly adjust its instrument (collimation).

The classic Newton telescope 114/900. The concave primary mirror, placed at bottom of the tube, reflects the light towards a secondary mirror plane inclined at 45 ° brings out the light on the intrument side.

I have used for ten years a Schmidt Cassegrain telescope to observe the moon and realize my planetary images. It is a versatile telescope oriented to the global high-resolution. Very compact, most of these telescopes have a F / D ratio of 10 with a focal length of about 2 meters for a tube which does not exceed 50 cm long ! This is a catadioptric telescope. It combines the use of lenses and mirrors. In the telescope case, the light first passes through a thin corrector plate whose faces are aspherical, so as to slightly deflect the light. Then the parabolic primary mirror, the bottom of the tube, in turn focuses the light toward the convex secondary mirror that brings out the light in the back of the tube, through the primary mirror.

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