Choosing a Non-magnifying Finder for Your Telescope
Other than an eyepiece and a good star chart, no other accessory is as important to a stargazer as a finder. That's because most telescopes, even with a low-power eyepiece, show you a very small slice of sky, often less than one degree wide, which makes it very difficult to point the scopequickly and accurately at even a bright target like the Moon or Jupiter. To help you find your way around the sky, many telescopes come with a finder scope, a small telescope with a wide field of view mounted on the main tube and aimed in the same direction as the main scope. But many stargazers rely on another type of finder, a non-magnifying or unit-power finder to quickly and easily find their way around the sky.This article takes you through the pros and cons of non-magnifying finders so you can decide if they are right for you.
2. How Non-Magnifying Finders Work
Unlike finder scopes, which have an objective lens and an eyepiece, non-magnifying or "unit-power" findersare not a small telescope and do not magnify an image.They simply consist of a small ground glass screen onto which is projected a red dot or a circular reticle from a red LED that shows a fixed angular diameter projected onto the sky. The image of the projected red dot or reticle is nearly free of parallax, which means the reticle appears in a fixed position against the distant sky no matter where you place your eye behind the finder.
To use a unit-power finder, simply lean over and look through the finder, move your telescope to aim the finder's reticle at your target, and if the finder is lined up with your telescope, your targetwill appear in the field of view of your eyepiece. Some unit-power finders have a calibrated reticle with a diameter of one half, two, or four degrees which aids in navigating from bright objects to fainter objects with the help of a star chart. Many experienced stargazers learn to love these instruments because they are easy and intuitive to use. Some amateur astronomers only use a unit-power finder, while others supplement these finders with a more traditional finder scope.
Most stargazers use one of three designs of reflex finder: theTelrad, the Rigel QuikFinder, or some version of a 'red dot' finder. Let's have a look at each type.
3. Telrad Finder and Selected Accessories
The Telrad was the first unit-power or reflex finder widely used in astronomy. It takes its name from theacronym for Telescope Reflex Aiming Device. The Telrad finder was developed by stargazer and inventor Steve Kufeld in the early 1970s and it's now perhaps the best known and most widely used reflex finder. Many star charts and planetarium apps incorporate Telrad reticles to aid in star hopping to faint objects.
A modern Telrad is constructed from molded plastic and weighs about 11 oz. It's a large device, about 8.25" long, to accommodate the optical path of the light from the LED and collimating optics and two AA batteries to power the LED. The Telrad includes a base which mounts on a telescope using a strip of secure adhesive tape, so it's not necessary to drill holes in the telescope tube to mount the device. The Telrad can be easily removed from its base when you want to store the device.
To aid in finding your way around the sky, a Telrad has reticles projected on its screen that measure diameters of 0.5 degrees, 2 degrees, and 4 degrees on the sky. You can adjust the brightness of the Telrad's reticle to suit your needs. Three adjustable screws aid in alignment of the optical axis of the Telrad with your telescope.
Because it is so long, a Telrad is usually used with 8-inch or larger Schmidt-Cassegrain telescopes or Dobsonian reflectors. Its large exposed window also renders a Telrad susceptible to dew on humid nights, but you can mitigate dew formation with a passive dew shield, a passive dew shield with mirror to allow right-angle viewing with the Telrad, or with an active heating band that slightly warms the Telrad window above the dew point.Click Here to View All Telrad Accessories
4. Rigel Systems QuikFinder
The Rigel Systems QuikFinder presents a more compact alternative to the Telrad. With a length of just 2.5", this unit-power reflex finder has a much smaller footprint than a Telrad and weighs only 3 oz, so it can mount on essentially any telescope. It's about 4.5" tall which makes it easy to sight without straining your neck. As with the Telrad, three adjustable screws aid in alignment of the optical axis of the Telrad with your telescope.The QuikFinder projects onto its screen reticles of 0.5 degrees and 2.0 degrees in diameter. The reticle illumination can be set to pulsed mode to conserve battery power and decrease the intensity of the reticle so you can locate fainter stars.QuikFinde rare powered with a lithium battery. A separate battery pack that accepts 2 AA batteries mounts on the side of the finder to give even longer battery life.
Like the Telrad, the QuikFinder mounts in a base secured with double-sized tape. It is somewhat less prone to dew than a Telrad.
5. Select Red Dot Finders
Red dot finders are not as versatile as Telrads or QuikFinders, but they are simpler to use and fit on nearly any telescope. They are based on "gun sight" finders and typically have a small window onto which is projected a single red dot, or in some cases, a small cross-hair reticle. You simply aim the red dot where you want to look, or at least in the general direction, and the object should appear in the field of view of your eyepiece. They make an excellent complement to a traditional magnifying finder scope.
Red dot finders are typically mounted on a standard base or bracket that itself mounts into threaded holes on the telescope or telescope focuser. The base can also be secured with double-sided tape if no mounting holes are available.These finders usually include two adjustable screws—up/down and side/side—to help you align the optical axis to that of the telescope.
The drawbacks to a red dot finder? The red dot itself can obscure the object you're aiming at, which can be distracting. These finders, in most cases, do not have circular reticles like a Telrad to help you star hop from bright objects to fainter objects. And like the Telrad and QuikFinder, most red dot finders are susceptible to dew. Red dot finders are available at a cost of $20 to $60. The lower-cost finders are usually made of plastic and have as a reticle a single red dot. Higher-end red dot finders are made of metal and often have a selector switch to choose a from a number of reticle types including red dots, cross hairs, and small circles, for example.
Red dot finders are produced by many manufacturers including Celestron,Baader, Antares, and William Optics. Televue also produces a reflex finder, the Starbeam, a premium device that includes an all-metal housing, a large-aperture window, and a flip window that allows for convenient right-angle viewing through the finder of objects at higher altitudes.
Links for Red-Dot Finders:
6. How to Choose the Right Reflex Finder
Should you get a reflex finder for your telescope? It depends to some extent on your observing interests, skill, and personal preferences. Many amateur observers, even experts, use one type of reflex finder along with a magnifying finder scope. This gives you the best of both types of instrument, and you can switch back and forth between each as you track down objects. All but the smallest telescopes can usually accommodate some type of small reflex finder and a finder scope.
For casual observing of the Moon, planets, and objects with bright stars nearby, or for simple grab-and-go observing with compact telescopes, a red dot reflex finder is all you need. Select one with a larger window, if possible, so you can get a bigger view of the sky as you line things up. Adjustable brightness of the red dot is also a big plus. Red dot finders are usually mounted using a rail on a base that is bolted to the telescope tube. There are many combinations of rail and base, so you must ensure that the a particular red-dot finder will mount to your telescope tube.
If you're comfortable with star hopping with the help of a star atlas, the Telrad is a good choice if your scope is big enough to hold it. Its large, calibrated circles help you find your way from bright stars to fainter stars to even fainter objects. And many star atlases include a Telrad overlay to help you find your way around. With a length of about 8 inches, a Telrad finder will only fit on larger telescope tubes. Before you mount a Telrad, you must make sure your telescope tube has a clear space that will accommodate this footprint, preferable in a location that makes it easy to look through the finder during your observing sessions.
If a Telrad is too large or heavy to fit on your scope, then consider a Rigel Systems QuikFinder. These devices have the compact footprint of a red dot finder, but they have calibrated reticles for star hopping. While most star atlases do not have an overlay for the QuikFinder, you can always make your own.
Both Telrads and Quikfinders are mounted on a base that must be taped to the optical tube. The tape is difficult, but not impossible, to remove so you want to make sure you mount the base in the correct location.