The Maksutov revolution

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1 2009 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. EQUIPMENT ROUNDUP If you re looking for optical quality in a small package, consider one of these fine telescopes. /// BY STEVE EDBERG The Maksutov revolution Among the telescopes available today, the Maksutov- Cassegrain (or Maksutov, for short) ranks fourth in popularity. Russian astronomer Dmitri Maksutov ( ) invented the optical design in Dutch astronomer A. Bouwers, who may have preceded Maksutov, independently developed the design. What we now call the Maksutov telescope is a hybrid optical system using both mirrors and a lens to produce images at the focus. Such systems are called catadioptric. The more common Schmidt-Cassegrain telescope is also a catadioptric system. The primary difference between the two is the corrector lens found on the sky end of the telescopes. Whereas a Schmidt- Cassegrain uses a thin lens shaped with a complex curve, the Maksutov lens is thick and has matching spherical curves ground into it. The lens is a thick shell of glass, which conveniently minimizes spherical aberration and provides sharp images. The corrector s curvature is oriented concave out. This means the lens would hold rain if the telescope were left outdoors pointed at the sky. The secondary mirror often is coated directly on the inner surface of the corrector lens. Schmidt-Cassegrain telescopes employ a cell to house the secondary mirror. The cell usually protrudes through a predrilled hole in the corrector. With its thicker corrector, a Maksutov is a bit heavier than a Schmidt-Cassegrain of the same aperture. Optically, Maksutovs are easier to make, requiring only three spherical surfaces to be ground and polished. The secondary mirrors of Maksutovs are usually smaller than those on Schmidt-Cassegrains Steve Edberg has a variety of telescopes so he can choose the right one for every occasion. His wife is not amused. because their slower (larger f number) optics don t require as large a secondary mirror. All other qualities being equal, a smaller secondary mirror means that the viewed images will have higher contrast than through telescopes with larger secondary mirrors. Contrast in the viewed image depends heavily on scattered light, and a smaller secondary scatters less light. One concern with Maksutovs can be their optical alignment, or collimation. The corrector lens and the secondary mirror attached to it must be aligned perfectly with the primary mirror or the final image will be degraded. Collimation is not easily adjusted so make sure the out-of-focus star images in your Mak show nicely centered donuts at the center of the field of view. Astronomy evaluated Maksutov telescopes from Celestron, LOMO, Meade, Orion, and Questar. These telescopes were tested with any eyepieces supplied with them and also with some of my favorite eyepieces. A few of the telescopes came with mounts (which were not evaluated) while others were supplied as only optical tube assemblies. Mounts for this group can be obtained from the telescope source or other vendors. All knobs and flip controls on these telescopes are easy to manipulate, even with light gloves on and even on the darkest night. X

2 Astele $1, LOMO America, Inc. 15 East Palatine Road, Unit 104 Prospect Heights, IL [t] [w] LOMO America imports the Russian-made Astele (the aperture in millimeters, equal to 5.3 inches) Maksutov telescope, as well as other sizes. This telescope operates at a focal ratio of 10.13, close to that of many Schmidt-Cassegrains. The faster f-ratio requires it to have a larger secondary mirror than other brands of Maksutovs with similar apertures. Its focal length is 1352mm. A 25mm Plössl eyepiece (54x) is included, along with a prism star diagonal, a T-adapter for a camera (you ll have to provide your own T-ring), and a dew cap, making for a complete package. The finder is a generous 8x40 right-angle system. I liked the tripodmounting bracket on the base of the telescope. Although it had only one hole for a tripod, it has a dovetail cross-section that allows easy balancing on a mount specifically set up to receive the bracket. The telescope is boxed well but it does not come with a case. ETX-125 $995 Meade Instruments Corporation 6001 Oak Canyon Irvine, CA [t] [w] Meade Instruments offers Maksutov telescopes in several sizes. I previously reviewed Meade s ETX-90, so for this roundup I report on the 5-inch f/15 ETX-125EC. This telescope comes on a dedicated two-tined fork mount. The package includes a 26mm Plössl eyepiece, a small, rightangle finder, a tripod, and a go-to hand controller. The telescope has two exit ports for its images. The one at right angles to the optical axis is normally used with eyepieces for visual observing. The other exit port, at the back of the scope, is used with a camera for a straightthrough configuration (with accessories required). Switching between optical configurations is easy just turn a knob and refocus. Note that the eyepiece can end up in awkward viewing positions when the telescope and mount are used in an equatorial configuration and when you are viewing objects with high declinations.

3 Questar 3.5 $4,050 Questar Corporation 6204 Ingham Road New Hope, PA [t] [w] NexStar 4 $479 Celestron 2835 Columbia Street Torrance, CA [t] [w] Celestron s NexStar 4 is the successor to its C90 Maksutov, which was available in a variety of forms. This new telescope comes mounted on a singletined fork with computer control. The optical tube assembly has an aperture of 4 and is f/13. The package comes with 25mm and 10mm eyepieces and a red-dot rifle-type pointer/finder. The telescope has two exit ports. The one at right angles to the optical axis is normally used with eyepieces, while a camera is used in the straightthrough configuration (with accessories required). Turning a large, rubber coated knob slides a mirror back and forth to switch between configurations. I didn t have the photographic accessories to check, but I expect refocusing is also necessary when the switch is made between camera and eyepiece. You might want to get or make a tube to parfocalize the photographic and visual foci. It should be noted that the eyepiece can end up in awkward viewing positions when the telescope and mount are used in an equatorial configuration with viewing towards the high declinations of your hemisphere. The Questar 3.5" is a gem of fine machining and optical perfection. It comes as a complete package on a two-tined fork mount and with short legs that permit tabletop use as an equatorial telescope. (It can also be tripod-mounted, alt-az, or polar/equatorial.) The package includes screw-in 16mm (81x) and 24mm (54x) eyepieces, a finder, and solar filters for the main telescope (sub-aperture) and finder all packaged in a leather carrying case. The telescope has two optical exit ports. The one at right angles to the optical axis is used with eyepieces while a camera and other visual accessories can be used straight-through. Switching configurations is easy just turn a knob. But more switching is available on the rear of this telescope. While eyepieces can be exchanged in the usual manner, you can also turn a knob to bring an internal 1.5x Barlow lens into position. The finder works the same way. Turning a knob moves a prism in the main telescope out of the way. You now have a right-angle finder view through the eyepiece. The Questar 3.5" uses a separate, small objective lens and mirror to bring the finder s view of the sky into the light path. 84 astronomy /// october 04

4 Apex 127 $399 Orion Telescopes and Binoculars P.O. Box 1815 Santa Cruz, CA [t] [w] Orion s Apex 127mm Maksutov- Cassegrain optical tube assembly appears to be the same as its StarMax 127 that comes with a German equatorial mount (and at a lower price). Orion also markets the Apex line in 90mm (3.6-inch) and 102mm (4- inch) apertures. The Apex 127mm has a 5-inch aperture, a focal length of 1,540mm, and a photographic speed of f/12.1. It comes with a 6x26 straightthrough, erect-image, achromatic finder scope, a 45 correct-image prism diagonal, and a 1 1 4", 25mm Plössl eyepiece, which provides a magnification of 62x. All this is packaged in a nice, roomy, padded, nylon-polyester case with pockets for accessories. The Apex 127 is connected easily to any telescope mount or sturdy camera tripod thanks to a built-in 1/4"-20 adapter plate. T-threads on the eyepiece adapter allow you to attach a 35mm camera body (with optional T-ring) to the telescope for prime focus astrophotography.

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