Maksutov telescope

A 150mm aperture Maksutov-Cassegrain telescope.

The Maksutov is a catadioptric telescope design that employs a full diameter positive meniscus lens (commonly called a "corrector plate") to correct the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication. The chief disadvantage of the design is that it does not scale up well to large apertures (>250 mm/10 inches), since the corrector plate rapidly becomes prohibitively large, heavy and expensive as the aperture increases – most commercial manufacturers usually stop at 180 mm (7 inches). The design is a modification of the Schmidt camera, and was developed by Russian optician Dmitri Dmitrievich Maksutov in the early 1940s. ^[1][2]

Invention

This telescope design uses a spherical primary mirror in conjunction with a positive meniscus spherical surfaced "meniscus corrector shell" at the entrance pupil in a design that takes advantage of all the surfaces being nearly "spherically symmetrical"^[3] to correct spherical aberration, a significant problem in other types of reflecting telescopes^[4]. It was invented in 1941 war torn Europe by Russian optician Dmitri Maksutov. He based his design on an earlier concentric design, Bernhard Schmidt's "Schmidt Camera". ^{[2] [1]} Maksutov claimed to have come up with the idea of replacing the complex Schmidt corrector plate with an all spherical "meniscus corrector plate" while riding in in a train of refugees from Leningrad.^[5]. Maksutov is described as patenting his design in May ^[6], August, or October of 1941^[7] and building a "Maksutov-Gregorian" style proto-type in October 1941^[8]. Maksutov came up with the unique idea using an "achromatic corrector", a corrector made of a single type of glass with a slightly positive meniscus shape that departed from the pure concentric spherical symmetrical shape to correct chromatic aberration.^[9]

Similar designs

In German-occupied Holland, Albert Bouwers submitted patents in February of 1941 (which he built a prototype for in August 1940 ^[10]) for a similar wide field concentric meniscus telescope design (called a "Bouwers meniscus telescope") that slightly predates Maksutov's^[2][11]. Bouwers original design differs from Maksutov's in that it used a purely concentric shaped meniscus corrector that was not achromatic.^[2][11] A later design used a cemented doublet as a corrector shell to correct chromatic aberration^[12]. Because of the similarity between the two designs and their close creation dates, these telescopes are sometimes referred to as "Bouwers-Maksutov" telescopes^[13]. War time secrecy kept both designs under wraps. Maksutov's was published in 1944 in a paper entitled New Catadioptric Meniscus Systems^[14] and Bouwers design was not published until after the war^[15].

Similar independent meniscus telescope designs were also patented in 1941 by K. Penning^[16] and Dennis Gabor (a catadioptric non-monocentric design).^[17]

Derivative designs

The Maksutov-Cassegrain

Light path in a typical "Gregory" or "spot" Maksutov-Cassegrain.

Maksutov's design notes from 1941 explored the possibility of a 'folded' Cassegrain-type construction with a secondary silvered "spot" on the convex side of the meniscus facing the primary mirror.^[14] He thought this would create a sealed and rugged optical system suitable for use in schools^[14]. John Gregory, a designer for Perkin-Elmer, developed a Maksutov-Cassegrain based on the same idea. Gregory later published his design for two f/15 and f/23 telescopes in a 1957 issue of Sky and Telescope. Commercial use of the design was explicitly reserved for Perkin-Elmer. Most Maksutovs manufactured today are this type of 'Cassegrain' design (called either a "Gregory-Maksutov"^[18] or "Spot-Maksutov") that may use all spherical surfaces and has, as secondary, a small aluminized spot on the inner face of the corrector. This has the advantage of simplifying construction. It also has the advantage of fixing the alignment of the secondary and eliminates the need for a 'spider' that would cause diffraction spikes. The disadvantage is that, if all spherical surfaces are used, such systems have to have focal ratios above f/15 to avoid aberrations^[19] . Also a degree of freedom in correcting the optical system by changing the radius of curvature of the secondary is lost since that radius is the same as that of the rear meniscus face. Gregory himself, in a second, faster (f/15) design resorted to aspherization of the front corrector surface (or the primary mirror) in order to reduce aberrations. This has led to other designs with aspheric or additional elements to further reduce off-axis aberration.^[20]

Applications

Commercially manufactured 105 mm aperture Meade ETX "spot" Maksutov-cassegrain.

Astronomical uses

The focal ratio of the Maksutov-Cassegrain design provides high powers and a narrower field of view. This makes them unsuitable for wide-field astrophotography but superb at lunar and planetary imaging. They are also very adept at imaging tightly packed formations such as globular clusters and at splitting double stars. Maksutov-Cassegrain telescopes have been sold on the amateur market since the 1950s. Most early models were small run prestige models that were very expensive. The mid-70s saw the introduction of mass-produced models by some of the major commercial manufacturers. More recently low-cost Russian and, lately, Chinese mass-production have pushed the prices down even farther. Today the design has become a popular choice for the amateur astronomer, if not a 'telescope for the masses', something unthinkable in the 60s when even a small Maksutov-Cassegrains such as the 'Questar 3.5' were quite expensive and within the reach of deep pockets only.

Industrial/Aerospace uses

The Maksutov-Cassegrain design has been used extensively in military, industrial, and aerospace applications. Since all of the optical elements can be permanently fixed in alignment and the tube assembly can be environmentally sealed the design is extremely rugged. That makes them ideal for tracking, remote viewing, and radar calibration/boresighting where instruments are subjected to severe environments and high g-forces.

Names

Names given to this design:

Including other types

• Catadioptric telescope (for the overall design class including Schmidts)
• Meniscus telescope (for the design type including Bouwers, Maksutov, K. Penning, and Dennis Gabor)

Mak specifically

• Maksutov telescope
• Bouwers-Maksutov telescope
• Schmidt-Maksutov telescope ^[21]

For Maksutov-Cassegrain's

• Maksutov-Cassegrain telescope
• Gregory-Maksutov telescope ^[2]
• Spot-Maksutov
• Gregory-Meniscus telescope ^[22]
• Bouwers-Cassegrain

Other types of catadioptric Cassegrains include the Schmidt-Cassegrain telescope, the Argunov-Cassegrain telescope, and the Klevtsov-Cassegrain telescope.

Another type is the Maksutov-Newton ^[23]

References

1. ^ ^{a b} Firefly astronomy dictionary By John Woodruff page 135 Google Books
2. ^ ^{a b c d e} Evolution of the Maksutov design
3. ^ John J. G. Savard, "Miscellaneous Musings"
4. ^ Dmitri Maksutov: The Man and His Telescope
5. ^ Armstrong, E. B., "Geometrical Optics and the Schmidt Camera", Irish Astronomical Journal, vol. 1(2), p. 48
6. ^ Armstrong, E. B., "Geometrical Optics and the Schmidt Camera", Irish Astronomical Journal, vol. 1(2), p. 48
7. ^ Dmitri Maksutov: The Man and His Telescopes By Eduard Trigubov and Yuri Petrunin
8. ^ Dmitri Maksutov: The Man and His Telescopes By Eduard Trigubov and Yuri Petrunin
9. ^ "Astronomical optics" By D. J. Schroeder, page 202
10. ^ The History of the Telescope By Henry C. King, page 360; google books
11. ^ ^{a b} Ian Ridpath, "Bouwers telescope", A Dictionary of Astronomy, 1997 first sentence of article
12. ^ D. J. Schroeder, "Astronomical Optics", page 202
13. ^ Rudolf Kingslake, "A History of the Photographic Lens", page 178
14. ^ ^{a b c} Dmitri Maksutov: The Man and His Telescopes By Eduard Trigubov and Yuri Petrunin
15. ^ Armstrong, E. B., "Geometrical Optics and the Schmidt Camera", Irish Astronomical Journal, vol. 1(2), p. 48
16. ^ Handbook of Optical Systems, Survey of Optical Instruments, by Herbert Gross, Hannfried Zügge, Fritz Blechinger, Bertram Achtner, page 806
17. ^ Lens design fundamentals, by Rudolf Kingslake, page 313
18. ^ James Mullaney, "A Buyer's and User's Guide to Astronomical Telescopes & Binoculars", page 46
19. ^ A Photovisual Maksutov Cassegrain Telescope - by Marc René Baril . "Although convenient, this design is limited to focal ratios above f/15 unless an aspheric correction is applied to some element in the optical system"
20. ^ Rutten, Harrie; Martin van Venrooij (1988). Telescope Optics: Evaluation and Design. Richmond, Va: Willman-Bell. ISBN 0-943396-18-2. 
21. ^ Schmidt-Maksutov telescope. (2009). In Encyclopædia Britannica. Retrieved March 24, 2009, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/527793/Schmidt-Maksutov-telescope
22. ^ The Gregory Meniscus Telescope
23. ^ http://www.company7.com/orion/catadioptric/argo6.html, SkyWatcher http://www.skywatchertelescope.net/swtinc/product.php?id=147&class1=1&class2=104, http://www.cloudynights.com/item.php?item_id=701

See also

(commercially produced models)

• Questar
• Celestron
• Meade
• Orion Telescopes & Binoculars
• Vixen
• Orion Optics
• Explore Scientific

External links

• Evolution of the Maksutov design
• A Photovisual Maksutov Cassegrain Telescope
• Kinds of Telescopes