Maksutov-Cassegrain

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The Maksutov Cassegrain is a catadioptric design similar to the SCT but with a different optical component that corrects for lower order spherical aberration. In a typical Maksutov Cassegrain the primary mirror is concave with a spherical curvature to the surface, and with a very short focal ratio similar to that in a modern Schmidt Cassegrain. The correction for spherical aberration is achieved by a full aperture thick negative meniscus lens. A negative meniscus lens is a diverging lens with a steeply curved concave spherical outer surface and a steeply curved inner convex spherical surface. These two surfaces are non-concentric. The diameter of the meniscus is the aperture of the telescope, with the primary mirror a little larger than the stated aperture of the telescope. This is because of the diverging property of the meniscus. Light from the periphery of the meniscus would miss the edge of the primary were it the same aperture as the meniscus. With the smaller aperture Maks (the modern abbreviation of Maksutov Cassegrains), produced by the large Asian and American manufacturers, the secondary mirror is an aluminised spot at the centre of the inside surface of the meniscus. This is not ideal for larger aperture Maks for deep-sky imaging, but for apertures up to about 180mm, Maks with this type of secondary produce good quality images as observing instruments. Like the SCT, the Mak has a Cassegrain focus with a moving primary mirror as the focusing optic. In the same way as the light travels through an SCT, the light path in a Mak is refraction through the meniscus, weakly diverging to the primary, a strongly converging cone from the primary with a weaker converging cone from the secondary, through the primary baffle tube and on to focus outside the rear of the telescope. The focal ratios of these types of Maks are quite long, often greater than f/13. This means that they are photographically slow compared to the f/10 SCTs, but nevertheless produce excellent image quality as telephoto lenses. Focal reducer lenses are now available for most of these Maks, which are placed at the rear of the telescope before the eyepiece or camera.

Similar to SCTs the mirrors are aluminised and over-coated, with the meniscus multi-coated for maximum transmission. The secondary mirror diameter is usually a little less than the same aperture SCT, so the central obstruction/aperture ratio is also a little less. Collimation of the Skywatcher and Celestron Maks (if it becomes necessary), is via tilting of the primary mirror cell from the rear of the telescope.

Named after Dimitri Maksutov, the Maksutov telescope originated in Russia during the 1940s and has had a chequered history in the consumer optics market, due perhaps to the culture of its country of origin, and the resulting insecure trade links. The Maksutov Cassegrain was popularised initially in the 1960s and 1970s in the US, by the Questar Corporation with their diminutive Questar 3.5, an expensive tabletop telescope that boasted exquisite engineering, high quality optics, and an attention to cosmetic detail. It was owned by those that could afford one, and coveted by those that couldn't. In the world of amateur astronomy, the Maksutov Cassegrain gained popularity again in the 1990s with Meade's ETX range (still available today). The 90mm ETX became an icon of amateur astronomy in the West, not just because it had more than a passing resemblance to the same aperture Questar, but because it was easily affordable, gave sharp bright images, and fitted well with the concept of "Every home should own one", a rather hopeful message for the inclusion and introduction of non-astronomical families into the general family of astronomers. The ETX90 was cute and inexpensive, what's not to like?

Since then Maksutov Cassegrains have had a mixed fortune, depending on their country of origin. Russian Maksutov Cassegrain telescopes have not been represented well or achieved a status in the West for a number of reasons, and since the revolution in digital imaging, they have struggled to find a prominent place in the evolution of modern observational astronomy and more importantly, modern imaging. The problem is one of usefulness in modern astronomy. For deep-sky imaging, the focal ratios of many Maksutov Cassegrains of 200mm aperture and greater are a little high to suit the needs of the deep-sky imager. Further, the thickness of the meniscus lens in instruments of 200mm aperture and above create problems with differential cooling with the primary mirror and secondary. A thick lens of large aperture, whose role is as an aberration control optic, and made of a different glass than the primary and secondary, will contract or expand at different rates under temperature change. This means that the image can show rather obvious spherical aberration for long periods, until thermal equilibrium is achieved.

If this equilibrium takes a few hours to achieve, then much of the imager's time is spent waiting. The SCT scores well here because the Schmidt plate is thin and cools rapidly. A great many modern astronomers demand an immediacy of use with their telescopes. The easy and rapid backyard set up, plus the culture of "grab and go" astronomy does not sit well with medium and larger aperture telescopes whose optics have marked differential cooling. The other issues with Russian Maksutov Cassegrains were the bulk, weight and price compared to the same aperture Schmidt Cassegrains. Now that the Meade ACF and Celestron Edge HD telescopes are available, there is little to recommend to modern astronomers about medium and larger aperture Maksutov Cassegrains.

Synta, primarily through the brands Skywatcher and Celestron, along with Meade ETX have been successful in offering smaller aperture Maks to the astronomical community. At smaller apertures, the high street prices are lower, weight and bulk is lower, cooling is not that much of an issue, and they are easier to manufacture to a high standard. Apertures from 90mm to 180mm continue to be popular observing telescopes, although imagers still tend to look at alternative designs. With apertures of 90mm to 127mm the Maksutov Cassegrain is an ideal instrument as a small astronomical telescope. Pinpoint star images, no noticeable chromatic aberration and good lunar/planetary images. They are available as optical tube assemblies or as part of a complete telescope, with mount, tripod and accessories. They can also be removed from their mounts, placed on camera tripods and used as high magnification spotting scopes. The small Maksutov Cassegrain is then an excellent multi-purpose telescope and recommended because of this.