![]() 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.Ī straight-through finder has some disadvantages as well. ![]() Finally, again because the optical axes are parallel, you can use the âboth eyes openâ method to view the night sky directly with one eye while using the other eye to view through the finder. 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. 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. 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. That means you can use your star charts simply by inverting them top-for-bottom.įigure 4-23. The inverted image provided by a straight-through finderĪ straight-through finder has several advantages. A straight-through finder produces an image that is inverted but correct left-to-right, as shown in Figure 4-23. This is the traditional finder style, and it is the type of finder supplied with most scopes. Straight-throughĪ straight-through finder is simply a standard refractor with the eyepiece in-line with the optical axis of the scope. 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, the eyepiece placement of a RACI finder makes it much more convenient to use than a standard straight-through finder. 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). 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. That means youâll need a second finder of some sort, such as a Telrad unit-power finder. 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. 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.įigure 4-22. The correct image provided by a RACI finder (Interestingly, objects are often visible in an optical finder that are invisible in a binocular of identical magnification and aperture. Many bright DSOs, including most of the Messier objects, are at least dimly visible in a 50mm finder from a dark site. 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. ![]() Larger and more expensive scopes may include a 7, 8, or 9X50 achromatic finder with multicoated glass lenses. Unless you observe Luna and the planets only, make it a high priority to replace the 6X30 finder with a suitable 50mm finder. A 6X30 finder shows stars not much below 8th magnitude, which is not deep enough for serious use. A 6X30 finder may be usable until you can replace it with something better, but it provides insufficient magnification and light gathering. Mid-size and mid-price scopes usually provide a 6X30 achromatic finder with coated glass lenses. These so-called finders are worse than useless and should be replaced immediately with something better. Cheap small scopes include a cheap, tiny finder, usually 5 power with a 24mm single-element plastic objective (5X24).
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