In the decades before the Schmidt-Maksutov’s current popularity, there was essentially only one such instrument available on the open market.  That telescope was the legendary Questar which is still being manufactured.  Even though both the mechanical and optical craftsmanship of Questar can only be described as exquisite,  you should know that it still has the same fundamental optical design as any other Schmidt-Maksutov. Questar’s nearly obsessive attention to detail and quest for perfection aside, I have noticed little if any advantage in optical performance over some lower-priced Schmidt-Maksutovs.  But for those of you who feel that no price is too high to pay for quality of stratospherian heights, this may be the scope for you.

            The Maksutov-Newtonian is the least common of the three hybrid reflectors shown in Figure 4. These telescopes usually have extremely small secondary mirrors.  They have so little diffraction due to the secondary mirror that their image quality is virtually indistinguishable from an apochromatic refractor!  However, such image quality usually comes at a fairly high price: much higher than other catadiotropic telescopes but still far less than an equivalent apochromat.

            Popular for many years among affluent European amateur astronomers and preferred by professional astronomers constructing large new telescopes, the Ritchey-Chrétién optical design was a co-invention of American George Willis Ritchey and Frenchman Henri Chrétién in the 1920s.  The Keck telescopes in Hawaii and the Hubble Space telescope are examples of this type of telescope.

             The classical Ritchey-Chrétién is essentially the classical Cassegrainian, but with both the primary and secondary mirror surfaces having a hyperbolic instead of parabolic curvature.   Its main advantage over other optical systems is the almost complete elimination of coma (an optical aberration described in the part of this section called Understanding Optical Defects).  Furthermore, it exhibits excellent flatness of field when compared to other optical designs.  What do I mean by “flatness of field”?   In telescopes with poor flatness of field, when the image in the center area of the field of view is in focus, then the outer area of the field of view will be out of focus.  Obviously, poor flatness of field also shows itself when the outer portion of the field of view is in focus and the center is not.   Perfect flatness of field results in a sharply focused image across the entire field of view.  These two advantages result in extraordinarily good line edge sharpness no matter which part of the field of view has the observer’s attention when he or she is using a Ritchey-Chrétién.

             Down sides of the classical Ritchey-Chrétién system include: a larger secondary mirror than even the Schmidt-Cassegrainian (resulting in some loss of image contrast due to greater diffraction), higher expense (because of the extra effort needed to grind, polish and precisely correct the more extremely shaped hyperbolic surface), narrower field of view (compared to other systems) and a slight astigmatism (again see Understanding Optical Defects for an overview of this astigmatism).  However, it should be noted that Meade claims that they have eliminated the aforementioned little bit of astigmatism in their new line of Ritchey-Chrétién telescopes by inventing a special corrector plate!

 

copyright 2004 Singularity Scientific

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