Finders, Finders Everywhere

Three kinds of finders Not so long ago, finders were little straight-through refractor telescopes and your only choice was the size. Today we have many more choices, and while personal preference is a major factor in picking a finder, not all finders work equally well when faced with light pollution. When dealing with significant light pollution, I've found that a green laser finder combined with a traditional finder-scope works best.

Let me explain why and share my thoughts on how the various types of finders work under washed-out skies....

TELRAD Finders

Red-Dot Finder Red-Dot showing the dot TELRAD finder

TELRAD-type finders are very popular. They work by projecting a “reticle” out at infinity. So when you look through one, you see the sky (unmagnified) with an aiming pattern superimposed (think of a fighter-pilot’s look up display). The aiming pattern can be a single dot (so called “red-dot” finders), a pattern of concentric circles (say 4, 2, and ½ degrees in diameter), or something even fancier. In dark skies you can use these with a good set of charts (some charts are even tailored for use with TELRADs) to star-hop very efficiently to your target. Unfortunately, if there is significant light pollution you usually can’t see enough stars to star-hop with a TELRAD. Also, the glass lens of the TELRAD doesn’t make it any easier to see the stars, and even worse, local stray light often glints on the TELRAD lens.

For these reasons, I have found that TELRADs are not useful in light polluted environments: you can’t see enough to star-hop with them. Under washed-out skies they are only good for aiming your scope at relatively bright stars. This can still be useful as the first step, allowing you to then use a more “traditional” finder-scope to star-hop from that star to your target.

Laser Finders

Green Laser Finder (Pointer mounted in bracket)

Take a green laser, attach it to a telescope, and you have a laser finder. People tend to either love them or hate them, but in my experience they work very well in light polluted skies. Perhaps I’ve been lucky, but even the cheapo green laser I’ve purchased shows up pretty clearly against light polluted skies. In fact, even with a full moon the beam is clearly visible if you simply sight along the laser. I find laser finders much better than a TELRAD or red-dot because you can simply look up at the sky, find that hard to see 4.5 magnitude star (remember, the sky is washed-out), and then you can swing the telescope to point right at it, without ever looking away from the star. For me that works much better than trying to sight that faint star through a TELRAD.

The problem with laser finders is that they don’t work well when it gets cold. Lithium batteries help, but somewhere between 40F and 30F my cheapo green laser pointers fade out. A premium green laser pointer might do better, but even those fade out at some point. Green lasers have two other drawbacks. First, you need to be alert and not accidentally point one towards an aircraft (remember to turn it off once you’ve pointed your telescope). Second, green lasers are banned at many star parties. So as much as I prefer my green laser finder, I also have a simple red-dot finder to use on cold nights or at star parties.


8x50 RACI Finder

Many amateurs (and telescope makers) don’t bother with finder-scopes anymore, using TELRADs to do their star-hopping. But as I mentioned above, TELRADs aren’t very useful in washed-out skies. If there is moderate to significant light pollution, a traditional finder-scope is essential. It’s the only way you can see enough stars to find your way around; in some cases, it’s the only way you reliably see stars fainter than 4th magnitude.

Finder-scopes are available in a variety of sizes. A 6x30 finder-scope is the minimum useful size. I find an 8x50 finder-scope ideal for use in light polluted skies. Finder-scopes are available in straight-through, right-angle, and right-angle, correct image (RACI) versions. Depending on your telescope, a right-angle finder-scope can be more comfortable (e.g., on a Newtonian reflector). I find RACI finder-scopes very convenient because you don’t have to mentally translate between a star atlas and the finder-scope view. But for many years I used a straight-through finder-scope and I never had trouble making the translation. So it really is a matter of what you prefer.

Ultra Wide-Angle Eyepieces

Televue Nagler Type 5 Eyepiece

While technically not a finder-scope, modern ultra wide-angle eyepieces combined with relatively fast (short focal ratio) scopes produce views that are often 2 or even 3 degrees across. It is quite possible to use such a combination as an effective finder-scope. All you need to do is pick a suitably bright star as a “jump off” point, point your telescope at it (perhaps using a red-dot or laser finder), and then star-hop using your main telescope. I personally find that my main telescope shows too many stars, making it hard to match the views to a star chart. But it is definitely workable.



For use in washed-out skies, I find that a green laser finder combined with a RACI finder-scope works best for me. Even with significant light pollution, the green laser lets me quickly point my telescope to the faintest stars I can see, and the RACI finder-scope shows me enough stars to let me star-hop from there. If the night is too cold for the laser finder or if I’m at a star party, I use a simple red-dot finder in place of the laser finder.



Another combo...

I've found that, thanks to quarterly visits to true dark skies, I've learned the sky well enough to look for almost anything I'd reasonably expect to see in the city. For my reasonably fast Dob setup, I've not found any combination that works better than a simple red dot and an ultra wide field eyepiece.

Another combo: Red dot plus ultra-wide field

Thanks for the suggestion. That isn’t my favorite combination in urban skies because I find it hard to line up stars fainter than about magnitude 2.5 in my red dot. That generally leaves a very long star hop, at least with the eyepieces I have (my best FOV is a bit over 2 degrees compared to 5+ degrees with my finder).  As I mentioned in the article, I also find the “extra stars” visible in my main scope make it harder to recognize the star fields. But it is a combo readers should try as they experiment to see what works best for them.

Manual Setting Circles

Ever give any thought to using a digital clinometer on the OTA for altitude and a printed (laminated paper) circle with degree markings on the base for azimuth readings? I'm thinking I'll do that soon... seems others have found they work really well for minimal <$50 investment. Alignment is easy and you use a computer/iPad/whatever to get the alt/az coordinates of your intended target. Then you slew your scope to those readings and the object should be in the viewfinder most of the time.

Manual Setting Circles

I've only used setting circles with my Pal Jr (an equatorial mount).  I used the setting circles on the Pal Jr to observe Venus and Jupiter during the daytime and did it by offsetting from the Sun.  I aligned the mount using a magnetic compass and a protractor-and-plub-bob to set the axis inclination.  It worked surprisingly well, but that was in the 1970s and I haven't tried it since.
I'm quite comfortable star-hopping (and fairly quick at doing it), so I haven't been motivated to try your idea.  However, it sounds pretty cool. 
If you try it, please let me know how it works.  If you are interested in writing up your construction and observing experience using the alt-az setting circles, I would love to post your "guest author" article here.