FINDERS FOR ASTRONOMICAL TELESCOPES

 The use of some sort of sighting instrument, or "finder" is required in order to put celestial objects in the main telescope's relatively narrow field of view. There are two basic types of finders: finderscopes, and unity-power "sight" finders.

FINDERSCOPES are small low-power wide-field secondary telescopes mounted alongside the main instrument in adjustable mounting rings for aligment with the main telescope. They usually have an eyepiece with crosshairs for object centering, and a wide field of view which is several times that of the lowest power field on the main telescope. Once the finderscope properly aligned with the main telescope, the user can put an object in the telescope's field by merely moving the main scope around until the object of interest appears centered in the finderscope's crosshairs. It should then be in the field of the main telescope. Some finderscopes have a source for low-level illumination of the crosshairs, which can be useful on a dark sky, as long as the illumination does not wash out the target stars. Many finderscopes use a star diagonal to put the viewing eyepiece in a more convenient position, although it does reverse the field, making reference to star atlases more difficult. Some amateurs solve this problem by making their own finderscopes with two star diagonals, but most just put up with the minor inconvenience of having to stoop a bit to look straight through the finder.

 The minimum aperture for a finderscope is a subject of some debate. Most observers have difficulty seeing much through finderscopes with objectives smaller than 30mm, and for a "standard" finder, a 50mm aperture is recommended. As for magnification, the finder should have at least 7x of power and at least a five degree field of view, so that objects may be located easily. Some seasoned observers who go for very faint targets with large telescopes will even recommend using 60mm to 80mm finders in the 7-15x power range, although with the larger sizes, the number of faint stars visible with these large finders may end up confusing the inexperienced amateur.

 1x FINDERS: These are just what the name implies: finders which do not magnify or enhance the view. They range from simple sighting tubes or gunsight-like devices, to illuminated reticle-type systems. The simple tube sights will get you in the ballpark, but in dark skies they can be a little hard to use. The "reflex" sights with illumination put an illuminated dot on a tilted glass plate, allowing the user to look through the plate at the sky, see the dot and place objects on it for finding. However, the object should be visible to the unaided eye for this to work well. Many amateur astronomers have built their own inexpensive dot-reflex finders from simple ones found on some gunsight systems.

One of the best known commercial 1x finders is the TELRAD, a reflex type finder which puts an unique three-ring reticle of adjustable brightness in the viewers field. The rings are 0.5 degrees, 2 degrees, and 4 degrees in diameter on the sky, and are most useful not as a simple bulls-eye, but as a "pattern maker", for star hopping. By using overlays which resemble the Telrad's reticle pattern on star atlases, and centering the target object's chart location in the overlay, star patterns and alignments with the Telrad rings in the area around deep-sky objects can be worked out which can then be duplicated by looking through the Telrad at the night sky. This can make finding even faint non-naked eye objects much easier than with the simple "dot" finders. The Telrad is a bit bulkier than some of the simpler "dot" sights, but it is very popular, especially among Dobsonian telescope users. As for which is better (Telrad Vs. Finderscopes), it really depends on the individual's preference. Many amateur astronomers use both a finderscope and a Telrad on their instruments for locating objects in the night sky.

David Knisely, Prairie Astronomy Club, Inc. sky.