Types of telescopes

Figure 5.1 shows the optical systems of the most popular kinds of telescopes. Several new types have appeared on the market in recent years.

The diagram doesn't show whether the curved surfaces are spherical or as-pheric, but that makes a lot of difference. Refractors normally use lenses with spherical surfaces; for higher quality, they sometimes use extra elements or extra-low-dispersion (ED) glass.

The Newtonian reflector, invented by Sir Isaac Newton, was the first aspheric optical device; its mirror is a paraboloid. The modern Schmidt-Newtonian is similar, but its mirror is spherical (hence cheaper to manufacture) and it has an additional corrector plate of an unusual shape, making up the difference between a sphere and a paraboloid.

The classical Cassegrain reflector has two aspheric mirrors, a paraboloidal primary and a hyperboloidal secondary. The classical Ritchey-Chretien looks just like it, but both mirrors are hyperboloidal, making it better at forming sharp images over a wide field on a flat sensor.1

1 The name Cassegrain is pronounced, roughly, kahs-GRAN in the original French but is nowadays normally CASS-egg-rain in English. Chretien is pronounced, approximately, kray-TYAN.




Classical Cassegrain, Classical Ritchey-Chretien

Schmidt-Cassegrain, Meade Ritchey-Chrétien

— Maksutov-Cassegrain

Figure 5.1. Optical elements of popular kinds of telescopes. Ritchey-Chretien differs from related types only in use of aspherical surfaces.

Most amateurs use compact Schmidt-Cassegrain telescopes (SCTs). Here the two mirrors are both spherical, and the corrector plate makes up the difference between these and the desired aspheric surfaces. This type of telescope is sharp at the center of the field but suffers appreciable field curvature; that is, the periphery of the picture and the center are not in focus at the same time.

Recently, Meade Instruments introduced a design they call Ritchey-Chretien which is actually a Schmidt-Cassegrain with an aspherized secondary. It overcomes the field curvature problem and is highly recommended for photography. However, the decision to call it Ritchey-Chretien is controversial, and the name may not stick.

The Maksutov-Cassegrain is a classic high-resolution design for planetary work. It also works at least as well as the Schmidt-Cassegrain for deep-sky photography, except that the f-ratio is usually higher and the image is therefore not as bright.

Classical Cassegrain, Classical Ritchey-Chretien

Was this article helpful?

0 0
Learn Photoshop Now

Learn Photoshop Now

This first volume will guide you through the basics of Photoshop. Well start at the beginning and slowly be working our way through to the more advanced stuff but dont worry its all aimed at the total newbie.

Get My Free Ebook

Post a comment