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F or more than a century, Ritchey-Chrtien (RC) reflectors have beenthe most common type of telescopeinstalled in professional observatories.The design combines a hyperbolic con-cave primary mirror with a hyperbolicconvex secondary mirror to produce

tack-sharp images free of coma, astigma-tism, and spherical aberration. Amateurastronomers, however, have been slow torespond to RCs. Because their mirrorshave curves that are difficult to figure,RCs can be expensive to make.

Recently, Astronomy Technologies(Astro-Tech), based in Norman, Okla-homa, did what many thought wasimpossible: offer RC optics in an afford-able package. The companys 6-inch f/9

AT6RC includes enhanced aluminummirrors coated with quartz, a dual-speedCrayford-style focuser, a Vixen-styledovetail rail for attaching the tube tomany of todays most popular mounts,and a standard finder scope shoe allfor an amazing $795.

The scope does not include a mount, afinder scope, or eyepieces. I supplied myown for this test.

It is important to note that Astro-Techdesigned the AT6RC specifically forastrophotography using digital single-lens reflex (DSLR) cameras and otherimagers with large chips. An observercan use it visually, but the real strengthbehind the RC design is its f lat photo-graphic field. This eliminates the starbloat that plagues photos taken with

many other optical designs.One reason RCs are not as suitable for visual observing as some other designs istheir comparatively large central obstruc-tions created by their secondary mirrors.Central obstructions decrease image con-trast. The larger the obstruction, thegreater the impact.

In the case of the AT6RC, the centralobstruction measures 3 inches (7.6 centi-meters) across, or 50 percent of the aper-tures diameter. That ratio is noticeablylarger than comparably sized Schmidt-

Cassegrain and Maksutov-Cassegraindesigns. The AT6RCs enhanced opticalcoatings offset part of the problem, but visual contrast still will suffer. Astroimag-ers can correct photographic contrasteasily with software during processing,which is why photographers are moreconcerned with sharpness.

High-quality optics and mechanics make this 6-inch Ritchey-Chrtientelescope one astroimagers will appreciate. by Phil Harrington

Equipment review

Astro-Techs AT6RC offers

great imaging on a budget

Contact informationAstronomy TechnologiesP.O. Box 720013Norman, Oklahoma 73070[e] info@astronomytechnologies.com[w] www.astronomytechnologies.com

The two-speed Crayford focuser offers a 10-to-1 reduction between coarse and fine focusadjustments. It accepts 1 1 4" and 2" eyepieces.

The Hercules Cluster (M13, left photo) and Albireo (Beta [ ] Cygni) appear sharp in these images the author took through the Astro-Tech AT6RC. PhilHarrington captured both by attaching his Canon XTi digital SLR to the telescopes focuser and setting the digital speed at ISO 400. He created the Hercu-les Cluster shot by stacking fifteen 30-second exposures. The image of Albireo was a single 10-second exposure.

56 Astronomy March 2010 2010 Kalmbach Publishing Co. This material may not be reproduced in any formwithout permission from the publisher. www.Astronomy.com

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High-quality impressionsThe test scope arrived collimated. Had

adjustment been necessary, I could havereferred to the included instruction sheetto fine-tune the secondary mirrors threescrews. The guidance, however, specifi-cally warns, Do not adjust the sixmirror-alignment screws on back of therear cell as this will void your warranty.

The AT6RC weighs just 13 pounds(5.9 kilograms) and measures only 19.25inches (48.9cm) long, not counting theextension tubes. The fit and finish areexcellent. All interior surfaces, including

the eight well-designed baffles, are flatblack. Proper baffling and blackening arecrucial in RCs to keep stray light frominfiltrating the optical path.

The AT6RCs Crayford focuser acceptsboth 1 1 4" and 2" eyepieces. Both adjust-ments the coarse focus and the 10-to-1reduction fine focus move smoothly.Every photographer will appreciate thefocuser drawtubes numerical scale,which makes returning to a specific set-ting easy. A locking screw ensures thefocuser wont shift.

The focuser drawtube has 1.34 inches(3.4cm) of travel, which alone is not longenough for cameras and eyepieces tofocus properly. Thats why the AT6RCcomes with two 2"-long extension tubes.All of my eyepieces focused perfectlywith one tube in place, but for photogra-phy, my Canon XTi digital SLR needed alittle more out-focus. Together, however,

both tubes proved too long. Once I com-bined my own 1" extension with one of

the supplied 2" tubes, focusing was easy.Accessory suppliers offer a matching 1"tubes for less than $35.

While the focuser worked smoothly, itsagged slightly under the weight of myCanon XTi. You can minimize the sag bychecking that the focusers tension screwis tight. If that screw is loose, the draw-tube also will move too easily for accu-rate focusing. Be certain that you screwthe focuser and extension tubes togetherfirmly. The focuser rotates, which is

handy, but make sure its lock ring is tight.

Sky-testing the AT6RCTurning the test scope skyward quicklyrevealed the first-class quality of itsoptics. Images stayed crisp beyond 300xacross the full field of view. Coma, a

problem inherent with some catadioptricoptical designs, was minimal, as wasspherical aberration.

The AT6RC did a great job resolvingsmall-scale objects, such as globular clus-ters, lunar features, and closely spaceddouble stars. My view of the DoubleDouble (Epsilon [ ] Lyrae) especiallyimpressed me. All four stars remainedclearly split even when I moved them tothe edge of my eyepieces 82 apparentfield. More demanding binaries alsoshowed clean splits.

A bargain astroimaging toolWhether taking photos of the Moon ordeep-sky objects, the AT6RC proves to bea versatile instrument. Its compact sizemakes it easy to transport, while its aper-ture is large enough to capture impressiveimages of a variety of targets. If you arelooking to expand into photography andwant the benefit of a Richey-Chrtiensrazor-sharp images without breaking thebank, I highly recommend the AT6RC.

Phil Harrington is an Astronomy contributingeditor.

The AT6RCs large central obstruction (50 per-cent of the diameter) makes this scope bettersuited for imaging than for visual observing.

Astronomy Technologies AT6RCRitchey-Chrtien telescope targetsimagers who use digital SLRs and other large-chip cameras. Equipment photos: Astronomy : William Zuback

Product specificationsAstro-Tech AT6RC

Type: Ritchey-Chrtien reflectorAperture: 6 inchesFocal length: 1,370 millimetersFocal ratio: f/9Length: 19.25 inches (48.9 cm)Weight: 13 pounds (5.9 kg)Included: Dual-speed 2" Crayford-style

focuser, Vixen-style dovetail rail, two2"-long extension tubes, standardfinder scope shoe

Price: $795

www.Astronomy.com 57

and Janice Fera

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