Telescope de Cassegrain

Télescope de Schmidt-Cassegrain
Introdution
From James Gregory to John Gregory
The 300 Year Evolution of the Maksutov-Cassegrain Telescope
In 1663 a Scottish mathematician named James Gregory designed the first reflecting telescope, envisioning a small secondary mirror
that reflected light through a hole drilled in the main mirror. The design also called for two concave mirrors, with the secondary mirror
placed beyond (outside) the focal point of the main mirror. Unfortunately, the technology of the mid-1600's could not manufacture
precise enough mirrors to turn Gregory's design into a working instrument.
In 1672 a Frenchman named Cassegrain wrote to the Paris Academy of Sciences on the topic of the megaphone. In a cover letter to
Cassegrain's paper, Henri de Berce of Chartres also described a reflecting telescope designed by Cassegrain. The main differences
between Cassegrain's design and that of Gregory was that Cassegrain used a convex secondary mirror and moved it inside the focal
point of the main mirror.
Nearly 260 years passed before the next major development took place. This was in 1930 when an Estonian astronomer, lens and
mirror maker, Bernard Schmidt, developed a lens called a 'corrector plate' that would compensate for the Cassegrain's optical
distortion - specifically it corrected for spherical aberration. This arrangement was the first lens and mirror - or catadioptric - telescope
and was used for astronomical photography. In the Schmidt Camera, a curved photographic plate holder was positioned at the focal
plane of the main mirror for maximum sharpness. Later the Schmidt Corrector Plate was applied to the Cassegrain design and the
Schmidt-Cassegrain Telescope was born.
Eleven years later, two men, A. Bouwers of Amsterdam, Holland, in February of 1941 and Dmitry Maksutov of Moscow, Russia, in
October of 1941 independently conceived the idea of replacing Schmidt's complexly shaped corrector plate with a curved lens, called
the Meniscus Corrector Shell. This shell also corrected for the spherical aberration and was easier to manufacture which opened the
way for the telescope we have come to know as the Maksutov-Cassegrain.
Then, in 1957, an optical engineer, coincidentally named John Gregory, developed further refinements in the Maksutov-Cassegrain
design. Most notably he did away with the separate secondary mirror and replaced it with a mirrored spot on the corrector shell itself.
This brings us to the modern instrument which could be called the Gregory-Maksutov-Bouwers-Schmidt-Cassegrain-Gregory
telescope, but which most people simply call the 'Mak.'
Le Télescope de Cassegrain
Rôle du secondaire
Le secondaire grandit l’image . Comme une Barlow
Le coefficent est determiné par
Coefficient de déformation (Variante du Cassegrain):
Type
Cassegrain
Gregorian
Dall-Kirkham
Pressmann-Camichel
Ritchey-Chretien
Primaire
Parabolique
Parabolique
Ellipse
Sphérique
Parabole/Hyperbole
Secondaire
Hyperbolique
Ellipse
Sphérique
Hyperbolique
Hyperbolique
Calcul des baffles internes :
A ne pas négliger car le tube pointe vers le ciel
Performance :
Réalisé avec JODAS : http://www.myoptics.at/jodas/twomirror.html
Le RC est sans Coma : Dénomination chez Meade : AFC ; Celestron : Edge HD.
Pourquoi Schmidt ?
La chambre de Schmidt est une chambre photographique de grande ouverture conçue pour l'astrophotographie. Elle est
basée sur un miroir primaire sphérique et une lame déformée spécialement réalisée pour compenser l'aberration
sphérique. La luminosité des prises est exceptionnelle grâce à un rapport f/D très faible (environ 2).
Chambre de Schmidt :
Schmidt a réussi à contourner la difficulté de réaliser un primaire parabolique très ouvert.
En plaçant une lame de fermeture (asphérique) en amont, qui transforme le primaire sphérique en un miroir
parabolique.
Avantage du Schmidt Cassegrain :
-
Formule très compact
Adapté pour de grande focale. (Planétaire)
Le foyer est accessible permettant d’installer beaucoup d’accessoire (Binoculaire, roue à filtre, Caméra, etc.)
Inconvénient :
- Un peu de chromatisme (Lame)
- Miroir très ouvert imposant une collimation rigoureuse
- Mise en température (tube fermé)
- Risque de buée sur lame
- Obstruction centrale assez élevée. (Design du commerce)
- Système de MaP du commerce (shifting)