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Off-Axis Vision As Light Reflects Off a Mirror, Some Light Is Want to Trigger a Debate at Your Next Lost

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Off-Axis Vision As Light Reflects Off a Mirror, Some Light Is Want to Trigger a Debate at Your Next Lost EQUIPMENT REVIEW The DGM Optics OA-3.6ATS off-axis reflector brings amateur astronomers the contrast of a refractor coupled with the color clarity of a reflector. /// BY PHIL HARRINGTON Off-axis vision As light reflects off a mirror, some light is Want to trigger a debate at your next lost. Special coatings, called “enhanced,” club meeting or star party? Drop this seemingly innocent increase reflectivity from the standard question on your favorite star gazers: “Which is better: a 89% to 96%. But nothing in life is free. Coated mirrors generally have a shorter refractor, reflector, or catadioptric telescope?” Then, as fans life expectancy than uncoated ones. This of each arm themselves with statistics, preferences, tradition, coating also may be harder to strip off and their own personal idiosyncrasies, disarm them with when realuminizing time comes. Balancing the pros and cons of special this: I know a reasonably priced telescope coatings, DGM enhances its secondary that combines the high contrast of refrac- mirrors but not its primary mirrors. tors with the true-color views of reflectors. As they settle down, remind them of the The test subject pros and cons of each design (see “Name For this review, DGM supplied their small- that telescope,” page 84). Refractor lovers est instrument, the OA-3.6ATS, a 3.6-inch must cope with chromatic aberration in all f/11.1 reflector. The optical tube assembly but the most expensive apochromatic (OTA) measures 40 inches from tip to tip instruments, and for reflectors and cata- and is built around a white PVC tube. dioptrics, contrast is their weakness. Paying a bit more gets you an aluminum Now that your friends are moonstruck, tube, and on some of the larger DGM tell them about Dan McShane, an amateur instruments, a lightweight carbon-fiber astronomer and telescope maker from tube. The OA-3.6ATS features a 2" RCF-1 Westminster, Massachusetts, who founded Crayford focuser from Jim’s Mobile, Inc. DGM Optics in 1996 and began offering (JMI), but owners must supply their own clear-aperture reflecting telescopes. THE FRONT VIEW of the DGM OA-3.6ATS shows finder scopes and eyepieces. the off-axis nature of the secondary mirror, Each DGM instrument may be pur- Hidden mirrors seen on the left edge of the tube. DANIEL G. MCSHANE chased as an OTA alone or on a furniture- Outwardly, McShane’s design looks just quality Dobsonian mount crafted from like a conventional Newtonian reflector. ture? Imagine taking a standard primary 5-ply Atlantic birch plywood triple-coated But a peek into the tube reveals a differ- mirror and cutting out three or four small, with clear polyurethane. Mount bearings ence. Where’s the secondary mirror? Look round sections near the rim. As you can are Ultra-High Molecular Weight closely. DGM mounts its flat secondary guess, one edge of each small mirror (UHMW) Polyethylene and ride smoothly mirror not centrally, but off-axis. would be higher than the opposite edge. on Teflon pads. Realizing that a small The secret to this strange geometry is All DGM’s off-axis mirrors, manufac- Dobsonian mount can rock, McShane the primary mirror. It looks conventional, tured by Dodgen Optical of Flagstaff, wisely places three short diagonal legs, also but it’s not. If you slice a traditional pri- Arizona, begin as large, on-axis concave made of UHMW Polyethylene, under the mary in half, you find the low point of its paraboloids. Dodgen produces large “par- ground board. Together, the telescope and curve exactly in the center. The DGM off- ent” mirrors that DGM sections to pro- mount assembly weighs 25 pounds; the axis instruments, however, are “off-center,” duce its off-axis paraboloids. As McShane assembly can be carried by gripping the or more correctly, off-axis. Hard to pic- explains, “The parent must be high quality handle on the front of the mount’s rocker and free of irregularities because it will box with one hand while placing the other Phil Harrington is always on the lookout for new have a greater negative impact on image hand under the mount’s base. Construc- and better telescopes. You can purchase his new quality than with a conventional mirror.” tion quality of the OA-3.6ATS’s tube observing guidebook, Star Watch, at bookstores The performance of a reflecting tele- assembly and wooden Dobsonian mount everywhere or from www.philharrington.net. scope also depends on mirror coatings. ranks with the best on the market. 82 astronomy /// october© 2009 Kalmbach 03 Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com DGM OPTICS OA-3.6ATS off-axis reflector is lightweight and simple to set up. An easy- open, hinged bracket holds the telescope tube in place, allowing quick adjustments to the telescope’s balance and rotation. ASTRONOMY: WILLIAM ZUBACK www.astronomy.com 83 Construction time the tube fore and aft for perfect balance as collimate, and the manufacturer’s instruc- The OA-3.6ATS arrived in two boxes; one well as rotate the tube to position the tions guided me through the process. contained the tube and optics, and the focuser at a convenient angle. The first step, adjusting the secondary other the pre-assembled Dobsonian rocker Finally, a Teflon strap clamps over an mirror, proved unnecessary as the mirror box and ground board. Good packing kept altitude bearing to secure the telescope to arrived in perfect alignment. If necessary, the secondary in alignment. Installing the the mount. Tightening the strap controls the mirror can be moved back and forth primary mirror and cell was simple thanks how freely the telescope moves in altitude. and left and right. Its holder does not have to a well written instruction book. conventional adjustment screws but moves A hinged, birch plywood cradle with an Optical alignment as a unit instead. adjustable latch joins the individual pieces Many multiple-mirror off-axis reflectors Primary mirror collimation for the OA- of the DGM optical tube assembly. The are difficult to collimate. But the simpler 3.6ATS is similar to a standard Newtonian hinged cradle lets the observer both slide design of the OA-3.6ATS made it easy to telescope. Aim the telescope toward a /// NAME THAT TELESCOPE REFRACTING TELESCOPES focus light through two or more lenses. This straight-through view refracting telescope of the sky offers excellent contrast. Lenses, objective however, bend some colors more than others, eyepiece creating chromatic aberration — a misalignment of the colors of an object. Apochromatic refrac- tors use combinations of lenses to focus all colors as closely together as possible. This added clarity, however, adds to the price. REFLECTING TELESCOPES, such as Newtonians reflecting telescope (the design found on virtually all Dobsonian eyepiece mounts), use mirrors to gather and focus light. A “primary” mirror found at the telescope’s base collects the light, which is then bent out of the tube and into an eyepiece by a “secondary” mirror. secondary primary mirror This mirror is suspended on a structure called a mirror “spider.” The secondary mirror and spider obstruct part of the tube, decreasing the amount of light hitting the mirror — decreasing image contrast. CATADIOPTRIC TELESCOPES, sometimes called catadioptric telescope compound telescopes, use both lenses and mir- rors to focus light. Two popular catadioptric corrector eyepiece telescopes are the Schmidt-Cassegrain and plate Maksutov-Cassegrain designs. These instru- secondary mirror ments collect light through a corrector lens and primary mirror onto a primary mirror. The light reflects to a sec- ondary mirror and into a detector (eye, camera, or CCD). Like a reflector, these systems often have reduced contrast. eyepiece DGM Optics off-axis telescope THE DGM OPTICS OFF-AXIS DESIGN differs from a Newtonian reflector by the shape of the pri- mary mirror and the location of the secondary mirror. DGM’s off-axis telescope uses an asym- primary metric mirror to send light to a secondary that mirror hides on the side of the telescope’s tube. From secondary this location, the secondary blocks no light, mirror giving the telescope the contrast of a refractor, while possessing the clarity of a reflector. 84 astronomy /// october 03 white background and look through the In addition to Dobsonian-mounted focuser without an eyepiece. Move your instruments, DGM offers OTAs without eye toward the front edge of the focuser — mounts, giving customers a chance to toward the front of the telescope tube. The mate telescopes with mountings of their barest sliver of the secondary should choice. The OA-3.6ATS would go perfectly reflect onto the edge of the primary mir- on a Vixen Great Polaris, Orion SkyView ror. Adjust the primary’s aim by turning Pro, or Celestron CG-5 German equatorial three spring-loaded screws on the back of mount. Each of these would have the the mirror cell one at a time. Initial colli- added advantages of slow-motion controls mation is complete when the reflected as well as optional clock drives. sliver of the secondary lies along an imagi- DGM off-axis reflectors are great for nary line running the length of the tube planet watching and double star observ- from the secondary mirror to the primary. ing, but the coarseness of the RCF-1 As a final check of collimation, center a focuser made attaining a sharp focus diffi- star in a medium-power eyepiece and THE MIRROR CELL of the DGM OA-3.6ATS can cult. DGM will substitute any focuser, slightly de-focus the image. If the unfo- be adjusted the same way as any Newtonian charging the price difference between the cused ball of starlight looks oval, move the telescope.
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