Match your optical finder to your type of telescope and your observing habits.
An optical finder is a low-power, wide-field refractor telescope with a crosshair eyepiece. The finder has a much wider field of view than the main telescope—typically 4° to 8° versus perhaps 1°—so it is much easier to acquire an object in the finder than with the main scope. If the finder is properly aligned with the main telescope, centering an object in the finder crosshairs also centers the object in the field of view of the main telescope.
Most inexpensive and midrange telescopes come with some sort of optical finder. The type, size, and quality of the finder varies with the size and price of the scope.
Cheap small scopes include a cheap, tiny finder, usually 5 power with a 24mm single-element plastic objective (5X24). These so-called finders are worse than useless and should be replaced immediately with something better.
Mid-size and mid-price scopes usually provide a 6X30 achromatic finder with coated glass lenses. A 6X30 finder may be usable until you can replace it with something better, but it provides insufficient magnification and light gathering. A 6X30 finder shows stars not much below 8th magnitude, which is not deep enough for serious use. Unless you observe Luna and the planets only, make it a high priority to replace the 6X30 finder with a suitable 50mm finder.
Larger and more expensive scopes may include a 7, 8, or 9X50 achromatic finder with multicoated glass lenses. The larger objective and (usually) better coatings of a 50mm finder allow it to show stars down to about magnitude 10, which makes it suitable for all but the largest main scopes. Many bright DSOs, including most of the Messier objects, are at least dimly visible in a 50mm finder from a dark site. (Interestingly, objects are often visible in an optical finder that are invisible in a binocular of identical magnification and aperture. This is because the finder is mounted on the scope and so provides a stable view.) But even if your scope came with a 50mm finder, it may make sense to replace it with another 50mm finder of a different design.
Tip
We consider a 50mm finder the best choice for any scope from the smallest to perhaps 12.5” or 15”. For scopes larger than that, an 80mm finder is usually a better choice.
Finders are made in the following four designs:
- Right-angle, correct-image
A right-angle, correct-image (RACI) finder, shown in Figure 4-21, uses an Amici prism diagonal to provide an uninverted image that is correct left-to-right, as shown in Figure 4-22. A RACI finder has two primary advantages. First, it provides a correct-image image view of the night sky that corresponds to your star charts and to what you see with your eyes, your binocular, and your unit-power finder. You needn’t do any mental or physical gymnastics to match image orientations from one source to another. Second, because the RACI finder positions the eyepiece at 90° to the optical axis of the main scope, you can switch between using the RACI finder, the unit-power finder, and the main scope eyepiece by moving only your head. Depending on the type of scope and where it is pointing, with a straight-through finder you may have to physically contort yourself to look through it.
RACI finders have two disadvantages. First, because the eyepiece of the finder is at 90° relative to the optical axis of the main scope, you cannot use the RACI finder to acquire objects. That means you’ll need a second finder of some sort, such as a Telrad unit-power finder. Second, the Amici prism absorbs some light, so the view through a RACI finder is marginally dimmer than that through a straight-through finder or one that uses a mirror diagonal.
We consider a RACI finder the best choice for Newtonian reflectors, including Dobs, and for any other scope that places the eyepiece at the front of the optical tube assembly (OTA). A RACI finder is also a good choice for a refractor, SCT, or other scope that places the eyepiece at the rear of the OTA, for the same reason that star diagonals are usually used in such scopes: unless the scope is mounted very high and you observe standing up [Hack #60], the eyepiece placement of a RACI finder makes it much more convenient to use than a standard straight-through finder.
- Straight-through
A straight-through finder is simply a standard refractor with the eyepiece in-line with the optical axis of the scope. This is the traditional finder style, and it is the type of finder supplied with most scopes. A straight-through finder produces an image that is inverted but correct left-to-right, as shown in Figure 4-23. That means you can use your star charts simply by inverting them top-for-bottom.
A straight-through finder has several advantages. First, it is the least expensive design to produce, so at a given price point a straight-through finder will generally be of higher optical and mechanical quality than any of the other designs. Second, all other things being equal, a straight-through finder produces the brightest image because it has none of the additional optical elements needed to erect the image or bend it at a 90° angle. Third, because the optical axis of the finder is parallel to the optical axis of the main scope, a straight-through finder can be used directly to point the scope. Finally, again because the optical axes are parallel, you can use the “both eyes open” method to view the night sky directly with one eye while using the other eye to view through the finder. Your brain superimposes the magnified, brighter finder view on the naked-eye view, allowing you to place the finder precisely where it needs to be to put an object in the eyepiece of the main scope.
A straight-through finder has some disadvantages as well. First and foremost, the eyepiece position may make it awkward to use the finder, particularly on a Newtonian reflector that is pointed near zenith. Second, because it inverts the image, you must mentally translate between the finder image and the view of the night sky provided by your eyes, your binocular, and your unit-power finder. Finally, if you use the both-eyes-open method, you are superimposing an inverted finder image against the correct image of the night sky provided by your other eye.
We consider a straight-through finder a usable choice for a refractor, SCT, or other scope that places the eyepiece at the rear of the OTA. In our opinion, a straight-through finder is not the best choice for a Newtonian reflector or other scope that places the eyepiece at the front of the OTA.
- Correct-image, straight-through
A correct-image, straight-through (CIST) finder is, in effect, half of a binocular. Like a binocular, a CIST finder uses a Porro prism to provide an image that is uninverted and correct left-to-right. (In fact, some astronomers literally cut a binocular in half and use each half as a finder scope.) A CIST finder combines the advantages and disadvantages of RACI and straight-through finders. The CIST design has never really caught on for mainstream use, although some inexpensive Chinese short-tube refractors include a 5X24 or 6X30 CIST finder.
- Right-angle
A right-angle finder uses a standard mirror diagonal and produces an image that is right-side-up but mirror-reversed (flipped left-to-right), as shown in Figure 4-24. Although a right-angle finder provides the same easy eyepiece accessibility as a RACI finder, we consider it a poor choice for any type of scope because of its mirror-reversed image. The flipped image means the only way to make the view in a right-angle finder correspond to your charts is to turn the chart face down and shine a light through the chart. Obviously that doesn’t work with charts printed on both sides.
On balance, we believe a 50mm RACI finder is the best choice for nearly any scope. There are many RACI models to choose among. Fortunately, a finder need not be of particularly high optical quality because you are using it only to locate objects, not to observe them. That means finders, even reasonably good ones, are relatively inexpensive accessories.
Considering only the finder itself, we think Antares RACI models are the best choice for a replacement finder. (Antares also makes right-angle noncorrect image finders, which we do not recommend.) Antares 7X50 RACI finders cost $90 ($105 with the optional mount), are available in various colors, and provide a removable eyepiece with diopter correction that allows you to focus the crosshairs independently from the focus of the finder itself. Unfortunately, the Antares finder mount uses the clumsy, old-fashioned, six-screw adjustment method.
Tip
Before you order a replacement finder, either verify that the new finder fits your current finder mount or replace the finder mount as well. If you replace the finder mount, verify either that the new finder mount comes with its own mounting plate or that it fits your current mounting plate.
We also recommend the 9X50 RACI finder sold by Orion for $65 including mount, which is what we use on our own scopes. The Orion RACI finder has the usual mediocre fit and finish of Chinese products, and its coatings are inferior to those of the Antares finders. The eyepiece and prism are not removable, nor does the eyepiece have a diopter adjustment. The crosshairs are rather thick and a bit rough. All of that said, the Orion RACI finder is more than good enough to do the job, and you can’t beat the price. Finally, the finder mount supplied with the Orion RACI finder uses the modern two-screw adjustment method, which allows you to align the finder in seconds and holds alignment well.
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