A look at seven unit-power finders.
The 1x or unit-power finder is supposed to eliminate visual gymnastics by letting you point the telescope while scanning the real sky with both eyes. Finding objects is simple, according to the instructions of the seven models I tested. To use unit-power to full advantage, your sky should be dark enough to show plenty of naked-eye landmarks. You just position yourself so the finder's display is visible and move the telescope until the bright image is centered on your chosen spot in the sky.
A plywood testing platform was constructed and bolted to the tube rings of an equatorial 6-inch f/5 Newtonian reflector. All seven finders were then mounted on it for simultaneous testing. A 13-millimeter eyepiece giving 58x was used to align the finders and check their pointing accuracy. I chose the four-day-old Moon, Venus, and the galaxy pair Messier 81 and 82 as targets. These range from around magnitude -9 to +8.
Telrad. While unrefined in appearance, the Telrad is durable and well thought out. In fact, the current model is virtually unchanged from the 1970s original. This reflex sight uses a red LED to backlight a target reticle of three concentric circles. The pattern is reflected by a first-surface mirror, through a plano-convex lens, to a plate-glass viewing window mounted at 45 [degrees] to the line of sight. This diagonal window reflects the reticle's image to the viewer's eye. The reticle's target circles have apparent diameters of [1/2.sup.[circle]], [2.sup.[circle]], and [4.sup.[circle]]. With the largest base plate of all the finders tested, Telrad is stable. Installation was simple, and it was easy to attach and remove the finder from its base even while wearing gloves. Optional additional base plates allow a single unit to be transferred between different telescopes.
Turning three small nuts on the back panel tilts the mirror to align the unit. I did find it necessary to realign Telrad nearly every time I reattached it, but this took less than a minute. The optimal viewing distance is about a foot behind the window. Less of the pattern is seen at longer distances, until it completely disappears at about 6 feet.
Telrad has a nice, big lever on the right side, a combination on-off switch and brightness control. It works smoothly and proved helpful when switching from bright targets to the dark sky. However, it is easy to accidentally knock the freely swinging lever into an invisibly dim setting, risking battery drain. Advantage: Instead of lithium button cells, which you might have trouble finding, Telrad uses two ordinary AA batteries.
Rigel Systems QuickFinder. Nearly five inches tall, this finder has a higher profile than any of the smaller finders tested. With its angled viewing window, the design is reminiscent of Telrad. But a shorter-focus lens and mirrorless vertical design, plus the use of a small, 3-volt lithium button battery, make it half the size and weight. Although QuickFinder has been available for some time, its construction is almost prototype-style. The housing is made of two pieces of square-section PVC pipe cemented together. Cosmetics didn't affect performance, however, and it proved rugged and simple to use.
QuickFinder projects a red-illuminated reticle with variable brightness. The optimum viewing distance for the [1/2.sup.[circle]] circle with stubby crosslines and a clear center is from about a foot away. It is the only unit tested that has a blinking reticle. A tiny knob next to the on-off/brightness control varies the rate from about one pulse per second to an attention-getting flicker. At first I felt this feature might be a gimmick, but it proved its worth in the field. I could keep the reticle nice and bright and still see dim stars that disappeared under a constant setting.
Three thumbscrews act on the base of the clear viewing window to align the axis of the finder with the telescope. QuickFinder has an acrylic window, so care should be taken not to scratch it when cleaning. Interchangeable bases are available.
Star Site. This device consists of an elegant aluminum tube on a mounting stalk, both finished in flat black. The tube has a phosphorescent ring inside the sky end, and a field stop at the eye end. To charge the phosphorescent ring, you shine a flashlight (red worked fine) into the eye end for a few seconds. The soft green glow is apparent for about five minutes, long enough to find most objects.
Permanent installation is simple, using two screws or bolts through holes in the unit's mounting base. The top of the base also has a flat groove on either side, for semipermanent attachment to large binoculars or small telescopes using the supplied nylon cable ties.
The alignment process for Star Site is similar to that used with any finder. You center a target, such as a star, in the main telescope. Then you adjust the finder until the target is centered in it as well. Unlike the other unit-power finders tested, Star Site has no optics and relies totally on your eye's detection of parallax for its function. You align it by adjusting the tube until the target is in the field of view, while at the same time the phosphorescent ring appears symmetrically round to your eye. Tightening two small screws then fixes the alignment of Star Site on its mounting stalk.
To use Star Site, place your eye about 14 inches behind it and move the telescope to your chosen spot in the sky, while keeping the glowing ring in view and symmetrical. It is immediately apparent when your eye wanders off-axis because the ring is deformed by the encroaching edge of the field stop.
Star Site doesn't project an image, so the user can't focus on the sky and the ring at the same time. It took a little practice not to instinctively focus on the ring and thereby throw the sky out of focus. After a time, however, viewing became intuitive - as if the green ring were a ghostly mental image.
There is no provision for easy, temporary removal, and the base plate is rather small for a unit this size. Advantages: there's no battery to worry about, and no window to attract dew and frost. My overall impression? A nifty and very affordable device.
Tele Vue Starbeam. This product accents precision in glass and machined aluminum, with sleek lines and a flawless matte-anodized finish. Starbeam is by far the fanciest illuminated sight reviewed. You could buy most of the other six finders here for what one Starbeam costs. After 10 minutes of viewing it became clear that every feature of the unit is first rate.
The heart of Starbeam is a glass window that acts as both a mirror and a viewing lens (see the diagrams on page 49). The interior surface of this mirror-lens is ground to a curve that focuses the image of a red LED dot at infinity. The outside surface of the window is figured to an opposite curve. This figuring cancels out the magnification of the interior surface, giving a true 1x view of the sky.
The crisp, red dot projected by Starbeam marks your target. Its brightness is smoothly adjusted by turning the knurled knob on a chromed housing that holds two small 1.5-volt batteries. The mirror-lens, with its exterior antireflection coating, virtually disappeared while observing. With the LED at full brightness I could easily see the red dot when viewing the Moon. At dimmer settings, the dot appeared as a tiny, translucent disk; bright stars remained visible when I set the dot on top of them.
I assumed the Starbeam's large mirror-lens would be a magnet for dew, but the tube shields it well. The unit came supplied with bolts (and matching Allen wrenches) to attach it to Tele Vue refractors, along with some hardware for other scopes - the directions cover installation in excellent detail. Rigid alignment is achieved by adjusting thumbscrews on the mounting plate and then locking the unit down with a large central screw. Starbeam was the only finder reviewed that came with dust covers, a nice finishing touch.
Orion EZ Finder. EZ Finder has a molded plastic housing, tiny on-off slide switch, and a button battery exposed to the elements. A two-position switch gives a choice of low and high settings. A glass window at the eye end seals the optical system. The 20-mm diameter mirror-lens has a red dichroic coating that reflects most of the light from the LED back to the viewer. The bright setting is for daytime, and the low brightness setting can be varied by removing the tube assembly from the base and adjusting a resistor. I found this operation wasn't very practical in the field.
The mounting brackets seem designed for the rear cells of Schmidt-Cassegrains, though EZ Finder can be attached to any telescope with the supplied double-sided foam tape. Unfortunately, there is no provision for an interchangeable base. Alignment by lateral and vertical adjustments similar to Starbeam's held well. The view through the EZ Finder has a slightly green cast, which makes the sky seem darker. The target-pointing dot is not quite round, but it is effective. I achieved the most accurate aim with a view from a few feet back.
Tele Vue Qwik-Point and Celestron Star Pointer. These two are so nearly the same in appearance and operation that they invite direct comparison. In fact, their housings are almost identical black plastic. Modified from BB-gun sights (the Tele Vue model says "Daisy" on the base) they are small and light enough to be at home on modest instruments, including mounted binoculars.
Both units have a window about 7 mm in diameter with a semi-reflective interior coating. As with EZ Finder, alignment is achieved with lateral and vertical adjustments. Qwik-Point requires a screwdriver for its lateral adjustment, while Star Pointer has an oversize nut that can be turned with your fingertips. A miniature thumbscrew jack at the back end of both units makes the vertical adjustments. Once set, the alignment of both finders was very solid, and homing in on the test objects was simple and efficient.
Both finders can be quickly removed from their dovetail tracks, and extra bases are available. The Star Pointer has a nicely machined aluminum base with bolt holes. Double-sided foam tape is also provided as an alternative means of mounting. Qwik-Point has a rectangular mounting block that bolts directly to Tele Vue optical tubes. but hardware is provided for other scopes. Both finders have tiny on-off switches. but they were easy to operate even with gloves on.
Like EZ Finder, both finders have exposed batteries, and Qwik-Point also has a small, exposed circuit board with a dimmer control and brightness switch. I was initially concerned that dampness would affect performance, but it didn't cause any problem over the short testing period. Star Pointer's brightness is adjustable over a wide range by turning a tiny screw on the unit's left side. It was easy enough to adjust while observing, but I had to keep a tiny screwdriver around, the kind of thing observers tend to misplace in the dark.
Both surfaces of these units' mirror-lenses have the same curvature, and the outside surface is uncoated. Thus, both surfaces produce reflected dots, focused at infinity. When viewing slightly off-axis, the reflections show up as two closely spaced red points. I found that this actually helped in centering, since the two dots blended back into one when my eye was right on axis. The viewing windows are rather small, and pointing accuracy was slightly increased by sighting from a few feet away.
All six LED-illuminated finders demonstrated that an image projected at infinity works very well as an accurate sky-target locator. All the units tested easily centered the Moon and Venus in the eyepiece. Centering on the spot where M81 and M82 lurk was about equally difficult in each unit, using a chart and naked-eye stars as references. There was some inaccuracy due to parallax - the reticle or dot in all the finders can wander as much as a degree on the sky with changes in the observer's head position. With careful centering of the eye, all units allowed star-hopping with about [1/2.sup.[circle]] accuracy.
Manufacturers would do well to replace the small, nearly useless finder-scopes on their starter telescopes with 1x finders like these. Customers would be far more likely to find their first sky targets.
RELATED ARTICLE: How 1x Finders Work
Six of the units tested project an illuminated reference target: either a reticle pattern or a point of red light. In both types a bright image is created at the focus of an optical element and is reflected into the line of sight. Telrad and QuickFinder are reflex sights that use a lens to focus the image of a bright reticle. A clear window then reflects the image to the eye. The others use the inside surface of a lens-shaped viewing window as a focusing mirror. This mirror lens projects the image of an LED-illuminated pinhole into the line of sight.
Both optical configurations produce the virtual image of an illuminated pattern or point, focused at infinity. Aiming is intuitive. The brain interprets the illuminated image as an object "floating" at an infinite distance, superimposed on whatever field the eye is viewing. (Of course, nearsighted users whose eyes don't focus at infinity should use their glasses or contact lenses to take full advantage of the effect.) Projecting the image through a tube or aperture adds accuracy by providing a reference for the eye to center on and by eclipsing the image when the eye moves too far off-axis.
This is a necessary part of the design, since there is no other fixed frame of reference for the user's eyepoint. Such projection systems are effectively small, reversed telescopes, with the light source at the position normally occupied by an eyepiece. The projected-pinhole type functions like a reversed Newtonian without a diagonal - a Herschelian reflector. The projected-reticle finder can be considered as a reversed refractor.
RELATED ARTICLE: The Unit-Power Finder's Military Roots
Steve Kufeld (Telrad) and Al Nagler (Tele Vue) deserve independent credit for bringing the reflex-type unit-power finder to amateur astronomy. In 1979 Kufeld was perusing a military surplus catalog when he noticed a reflex sighting device that projected an illuminated target pattern onto a glass window. He ordered one, confident that he could adapt it to his telescope. It proved too cumbersome, so Kufeld designed and created the smaller, lighter Telrad - the first popular 1x finder. According to Kufeld, 40,000 are currently in use.
Nagler worked with the U.S. Navy on head-up displays (HUDs) to assist aviators. These devices use a fast lens to relay instrument data displayed on a cathode-ray tube to a partially silvered glass window in the pilot's forward line of sight. Nagler applied similar optical principles to his 1988 design of the Starbeam unit-power finder.
David Regen is a freelance photographer and writer who reviewed beginner's telescopes in the December 1995 issue. He observes the night sky at 1x from New York City and Long Island.
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|Title Annotation:||includes related information about testing and history of the devices; astronomy equipment|
|Author:||Regen, David N.|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 1996|
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