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Latest Revision: September 19, 2002
DISCLAIMER: All this information is accurate to the best of my knowledge; if there are any omissions or errors, please let me know. This document is intended to be an overview, not the end-all-and-be-all of a given topic. If you want to find out more about a specific gadget, accessory, or thingy, consult the references listed in each section.
Most people have seen conventional finders before -- they're just little telescopes that have cousins in areas well distant from amateur astronomy (monoculars, rifle scopes, etc.). Like all telescopes their main purpose is to gather light and magnify the image -- but not too much, since the main goal is to have a wide field for finding.
Newcomers often hear about this thing called a "Telrad" and wonder what it is. Telrads are well known to experienced amateurs, but are often less familiar to newcomers to amateur stargazing. In a nutshell: A Telrad isn't a telescopic finder. It's a heads-up display. You look through it and what you see is a bullseye pattern about 4 degrees across, which appears to be focused at infinity. By centering the bullseye on a star that's also in the field of view of the telescope, you align the Telrad and are ready to go. Using one on naked eye objects is easy -- you simply push your scope until the bullseye is directly centered over the object, and presto, there it sits in the scope (at least with a low power eyepiece).
The diameters of the circles in the Telrad bullseye are 4 degrees, 2 degrees, and 0.5 degree -- the last is small enough to ensure that centering the object will put it in the field of view of a low power eyepiece (assuming the Telrad is aligned properly).
It is worth noting that numerous new Telrad clones have popped up. Probably the most popular of these is the Quickfinder, by Rigel Systems, which is very similar. It has a 2-ring instead of a 3-ring reticle, but otherwise works much the same way as a Telrad. The Quickfinder is much smaller (about the size of a deck of cards) and lighter, so it's popular on very small telescopes, or telescopes where balance is a major concern. A number of other "Telrad-inspired" finders have done away with the reticle, and simply project a small red dot onto the "heads-up" display rather than a reticle pattern. I personally think the bullseye reticle is a better pattern, but feel free to experiment.
Also, there's an LED-based BB gun sight made by Daisy that works along the same principles as the "dot" finders; this may be something cheap, homebrew experimenters may want to look at. The Daisy LED sight is a little too bright for astro use, but some folks on sci.astro.amateur have thrown a resistor or pot into the circuit to adjust the brightness. A few of the "dot"-based unit power finders work on essentially the same principle. From a recent Sky & Telescope review (June, 1996):
|Star Site||*||Reticle (single ring)|
|EZ Finder||Telescope and Binocular Center
**modified from the Daisy LED gun sight described earlier
Which is better -- telescopic finders or unit-power finders? This is one of the perennial religious wars on the sci.astro groups. I decline to take sides, on the grounds that there are few, if any, decisive advantages either way for a large group of people. In practice, it boils down to an individual observer's tastes and abilities.
I personally like Telrads on Dobsonians and conventional finders on equatorially mounted scopes. If you have enough naked eye stars around to make "star hopping" feasible, and a good set of charts, using a Dobsonian or other alt-az scope with a Telrad becomes very fast and natural with only a little practice. On the other hand, on equatorial scopes, a conventional finder of known field of view simplifies finding, since you can "step off" in known increments of right ascension and declination towards your target -- handy if you know the coordinates, or are navigating with the help of a star atlas. Try both types of finders (on scopes as similar to yours as possible) and decide for yourself.
Even if you prefer to use a telescopic finder scope, there is one case where having a Telrad-style finder is very handy. If you have a finder scope with a right-angle diagonal for convenient viewing near the zenith, the image you see in the finder will normally be mirror-reversed (not just upside down). The image won't match a star chart unless you mentally "mirror reverse" the chart as well, which many people find difficult. With a right-angle finder, a Telrad or other non-magnifying finder is often very handy to get you into the correct general area of sky. Your desired target will probably appear in the finder without any extra star-hopping, so you won't have to struggle with mentally reversing your charts while searching for the object.
One Telrad is pretty much the same as another (ditto for all other "heads-up" style finders), but with conventional finders, there are significant differences between models. You'll probably have to experiment until you find a brightness (mostly aperture related) and field of view (mostly magnification and eyepiece design related) that's comfortable for you.
Factors such as aperture, field of view, and magnification depend on your choice of main telescope. As with your main scope, a larger aperture gives you brighter images and generally higher magnification, but may (depending somewhat on design) give you a smaller field of view. For medium size scopes, say about 4" - 8", a 50mm finder is a fairly common choice. Owners of large aperture scopes often put a very large finder on, such as an 80mm or even a small conventional refractor, to make finding dim targets easier. For example, the 12" refractor at the University of Illinois's old observatory has a 3" f/15 refractor -- an eminently capable instrument in its own right -- as its finder. If you have a small telescope such as a 60mm refractor, you probably don't need more than a 30mm finder or so.
Try not to go below 30mm, for reasons of construction quality, if nothing else. If you already have a finder smaller than 30mm, e.g., 5x24, odds are it's poorly made. Save up the bucks and buy a good 30mm or larger finder from one of the major outfits, e.g. a Celestron or Meade dealer.
Some things to look for in telescopic finders:
The right angle versions have the advantage of being more comfortable from more positions, but have the serious disadvantage of introducing an extra reflection in the image, so that the sky appears mirror-reversed when compared to charts. The ability to mentally un-reverse the image varies from person to person, so, again, some experimentation is in order here. Sometimes, you can get finders with Amici (erecting) prisms, which don't suffer from this reflection.
Conventional finders: Same places that make or sell telescopes. Telescope and Bincoular Center (aka Orion), Lumicon, Astronomics, 'most any Celestron or Meade dealer...
Telrads: Originally made by Steve Kufeld, in California. Now, they're available through all major vendors.
Non-Telrad "heads-up" pointing devices: most are made by major mfrs. such as Celestron, Orion, and TeleVue, so the usual addresses apply.