Hack 53. Install a Unit-Power Finder
Find deep-sky objects quickly and easily.
Installing a unit-power finder, often mistakenly called a zero-power finder, is the best single upgrade any Deep-Sky Object (DSO) observer can make to a scope. A unit-power finder provides no magnification. Instead, it allows you to view the night sky naked eye with a superimposed dim red target-locating pattern. By orienting that pattern relative to the background stars, you can locate most deep-sky objects in a fraction of the time needed if you use an optical finder alone.
A unit-power finder is simplicity itself. No computers, no fancy opticsjust a dim red bulls-eye pattern superimposed on the night sky. Figure 4-18 shows a simulation of using a Telrad unit-power finder to point the scope at the Great Orion Nebula, M42.
Figure 4-18. Using a Telrad unit-power finder to put M42 in the eyepiece
4.11.1. Red-Dot Finders
Rather than a bulls-eye pattern, a red-dot finder superimposes a simple (you guessed it) red dot. Red-dot finders are available in a wide range of prices, from the $21 StellarVue model (http://www.stellarvue.com) to the exquisite $205 Tele Vue Starbeam (http://www.televue.com). Some astronomers even mount the inexpensive red-dot sight made for Daisy BB guns as a telescope finder.
Any red-dot finder allows you to point your scope quickly and accurately. The problem is, that's all they do. Because they don't provide a bulls-eye pattern, they offer no help in locating objects geometrically. We suggest you avoid red-dot finders.
4.11.2. Bulls-Eye Finders
The real competition in unit-power finders is between the $40 Rigel Quik-Finder (http://www.rigelsys.com) and the $40 Telrad (no web site). Both products are extremely popular and have strong advocates in the amateur astronomy community. Any Telrad versus QuikFinder discussion soon degenerates into a religious debate. We have our own strong preference, but we'll try to present both products fairly.
The Rigel QuikFinder has the following advantages versus the Telrad:
The QuikFinder is small, which makes it easier than the Telrad to fit between the focuser and optical finder, particularly on smaller scopes.
The QuikFinder is much lighter than the Telrad, which may be an important consideration if your Dobsonian scope has balance problems. (The QuikFinder appears to be taller than the Telrad, which would be an advantage if true, but is actually an optical illusion caused by the smaller size of the QuikFinder.)
The QuikFinder includes two bases, one intended for the sharply curved tubes of refractors and other small scopes and the other for the lesscurved tubes of larger reflectors and SCTs. Additional bases are available for the Telrad, but you must buy them separately.
The QuikFinder is easier to mount on some scopes than the Telrad because its base plate is shorter. For example, when our friend and colleague Paul Jones bought a Telrad to mount on his 8" SCT, he found that there was nowhere on the tube he could affix the long Telrad baseplate that did not interfere with his mounting rings. Similarly, on some small Dobs and many refractors, there's simply no convenient location to mount a Telrad. The QuikFinder fits in much smaller places than the Telrad, and so it may be your only realistic choice even if you prefer the features of the Telrad.
The QuikFinder includes built-in blinking as a standard feature, which is very useful when you need to orient the bulls-eye against a field of dim stars. You can add blinking to the Telrad with about $5 worth of parts if you're comfortable building electronic projects. You can also buy a $20 third-party Telrad blinker if you want to avoid soldering.
The Telrad also has several advantages versus the QuikFinder:
The Telrad provides a three-circle, bulls-eye pattern0.5°, 2°, and 4°versus only the 0.5° and 2° circles with the QuikFinder. The usefulness of that extra 4° circle is difficult to overstate, particularly when you are trying to locate objects that are far from a bright star.
The Telrad projects its virtual bulls-eye pattern at infinity, which means it has zero parallax. If you center a star or planet in the Telrad, it remains centered regardless of your eye position because the object and the bulls-eye pattern are both at infinity. The QuikFinder has moderately severe parallax because its virtual bulls-eye pattern has an apparent distance of only a few yards. That means moving your eye relative to the QuikFinder also changes the relative position of the nearby bulls-eye pattern relative to the infinitely distant object. Unless you place your eye in exactly the same position relative to the QuikFinder every time, you'll find that the position of the object in your eyepiece is unpredictable. When you use high magnification for planetary observing, the object may be outside your eyepiece field of view. The parallax problem with the QuikFinder is less important for DSO observing, or if you also have a crosshair optical finder installed.
To see the effect of parallax, hold up a finger at arm's length. Close one eye and align the finger with a distant object. Move your eye an inch or two from side to side. The relative positions of your finger and the distant object change, but not by much. Now repeat the experiment with the finger only a couple inches from your eye. The same amount of head movement causes a much larger displacement between your finger and the distant object. If your arm was long enough to put your finger on the distant object, it wouldn't matter where your eye was positioned: the finger would remain exactly aligned with the object. That's how the Telrad works, because its virtual bulls-eye is at infinity, the same as the object. The QuikFinder exhibits parallax because its virtual bulls-eye, like your finger, is at an intermediate distance between your eye and the object.
The Telrad is much more heavily constructed and robust than the Quik-Finder. Both are constructed of plastic, but the Telrad is built like a tank. We leave our Telrads attached when our scopes are broken down for transport, and we've never had a problem. We wouldn't risk that with a QuikFinder, which is much more fragile.
The Telrad dims the view less than the QuikFinder. Both products use ordinary glass rather than coated optical glass, and both dim the view relative to an unobstructed naked-eye view. But the Telrad dims the view noticeably less to our eyes.
You can use either finder with both eyes open to see dimmer stars than are visible looking only through the finder because your brain merges the view from both eyes. The only problem with this method is that it can be physically awkward, depending on the type of scope you use and the location of the object you are trying to find. With a Dobsonian scope, for example, you may find yourself squatting or lying behind the Dob and looking up the full length of the tube.
The Telrad uses standard alkaline batteries rather than the lithium batteries used by the QuikFinder. Both devices seem to live forever on one set of batteries; the dim red LED consumes next to no power. However, if your batteries do fail in the field and you've forgotten to bring spares, it's a lot easier to borrow alkalines for the Telrad than lithiums for the QuikFinder.
The Telrad is more widely supported. More third-party accessories such as dew shields, dew heaters, flip-down mirrors, and so on are available for the Telrad. Also, many field guides, such as Harvard Pennington's excellent The Year-Round Messier Marathon, are based on 4° Telrad circles rather than 2° QuikFinder circles.
We much prefer the Telrad, but we have no argument with those who choose the QuikFinder. But do install one or the other if you have any interest at all in DSO observing. Until you try it, you won't believe how much easier it is to find objects using a bulls-eye finder.
4.11.3. Mounting a Unit-Power Finder
The Telrad and QuikFinder both use two-part assemblies. The finder unit mounts to a separate base, which is affixed to the scope using double-sided foam tape. This method has several advantages. First, it allows you to use the finder on more than one telescope simply by installing additional bases on your other scopes and moving the finder between scopes as needed. Second, it makes it quick and easy to remove the finder when you store or transport your scope, thereby protecting the finder from damage. Third, because the base is permanently affixed to the scope, the finder keeps its alignment quite well. You can usually remove and replace the finder without having to do more than a quick tweak to its alignment. Finally, the double-sided tape, while it provides a reliable mounting, also serves as a break-away failure point. If you do happen to bang your finder on a door-way, the tape usually gives way, preventing damage to the finder itself.
The first step in mounting the bulls-eye finder is deciding where to place it. Figure 4-19 shows what we consider to be the ideal position on a Dobsonian scope, which is between the focuser and the optical finder. Positioning the bulls-eye finder here allows you to use the eyepiece, bulls-eye finder, and optical finder without having to move your head much.
Figure 4-19. A Telrad mounted on a 10" Dobsonian scope
If you're uncertain about the best mounting location, you can use duct tape or heavy rubber bands to mount the finder temporarily. Once you determine which location best fits your preferences, use the mounting tape to affix the base permanently. And we do mean permanently. When we first used it, we thought double-sided foam tape would be a precarious mounting method, but if you affix the foam tape properly it's about as reliable as using bolts. We mounted a Telrad on our 10" Dob with foam tape. After four years of using the scope in everything from intense heat to intense cold, the Telrad base is stuck to the tube as firmly as ever.
Once you have chosen the location for your finder, clean and polish that part of the tube. The finder base attaches with double-sided tape, so you want to make sure the surface is smooth and clean before you press the base into position. Otherwise, the tape may fail to adhere properly and the finder may fall off when you least expect it.
The ideal time to mount the finder is a night with a full moon. (Yes, we know that sounds strange, but bear with us…) To mount the finder, follow these steps:
Make sure all of the alignment screws are set to the middle of their travel. If necessary, count the number of turns from one extreme to the other and then back off half that number of turns. The goal is to leave as much room as possible for adjustment on either side of center.
Connect the finder to the base, and peel off the glossy paper or plastic that protects the sticky surface of the tape. Turn on the finder and adjust the bulls-eye pattern to moderate brightness.
Get the moon centered in the field of view of your telescope, using a medium-to high-power eyepiece. Ideally, use an eyepiece that provides a 0.5° field of view, which corresponds almost exactly to the apparent size of the Lunar disk and to the inner circle of the bulls-eye finder. If your scope tracks, turn on the motors. If you have a Dobsonian or other non-tracking scope, you'll need to work quickly to get the finder attached before Luna drifts very far.
With Luna centered in the telescope field of view, position the bulls-eye finder close to the telescope tube, but not quite touching it. Center Luna in the smallest, inner circle of the bulls-eye finder, which it fits almost exactly. Keeping Luna centered, gently press the bulls-eye finder against the tube. The tape should adhere instantly. Press down sufficiently to make sure the tape is fully adhered.
You can, of course, accomplish the same thing by centering a bright star or planet in the bulls-eye finder, or for that matter a distant church steeple or radio tower. The Telrad and QuikFinder both have more than enough adjustment movement to correct any minor misalignment. But we like the precision of matching the 0.5° finder circle to a 0.5° moon.
Be careful if you mount a Telrad to a refractor, small reflector, or other scope with a small tube diameter. The Telrad mounting bracket is only slightly curved, to fit the curve of larger diameter tubes. As we found out by experience, the curve is insufficient to allow the mounting bracket and foam tape to make firm contact with a 90mm refractor tube. When we mounted it, the bracket appeared to fit and the foam tape seemed to have a solid grip.
Alas, we found out differently as we were setting up for a public observation one night. When we unzipped the refractor carrying case, we found the Telrad lying loose at the bottom of the case. Fortunately, there's an easy way to solve the problem. Simply add a second layer of double-sided foam tape, which you can buy at any hardware store. The additional thickness of the second layer allows the mounting bracket to make firm contact with the tube, as shown in Figure 4-20.
Figure 4-20. Mounting a Telrad on a refractor
If you ever need to remove foam tape, as we did when we replaced the Telrad on our 90mm refractor, you'll find the adhesive is incredibly tenacious. We ended up with part of the tape stuck to the Telrad base and part to the refractor tube, and none of it wanted to come off. We tried peeling it with our fingernails, but it came away only in tiny chunks of foam. The best way we found to remove it was to use dental floss to "saw" under the foam. Once the foam has been peeled away and the adhesive exposed, use WD-40 or commercial adhesive remover to remove the remaining adhesive.