To a beginner, looking at the night sky through a telescope feels like reading a newspaper through a straw. Telescopes, even “wide-field” models, have relatively narrow fields of view. (To get an idea of how much sky an “average” telescope shows, hold up a quarter at arm’s length. Not much, is it?) This narrow window on the sky can make it very difficult to locate objects.
To minimize the problem, you need a “finder” eyepiece to provide the widest possible true field of view in your telescope. How wide that field can be is determined by the focal length of your scope and the focuser size. For example, a typical 8” SCT with a focal length of 2,032mm and a 1.25” focuser has a maximum possible field of just under 0.9°. Conversely, a short-tube refractor with a 400mm focal length and a 2” focuser has a maximum possible field of about 7° (which means the scope can serve as its own finder).
To determine approximately the widest possible field of view your telescope can provide, make the following calculation:
If your telescope has a 1.25” focuser, divide 1,750 by the focal length of the scope in millimeters. For example, if your scope has a focal length of 1,200mm, the widest possible true field of view in degrees is about 1,750/1,200=1.46°.
If your telescope has a 2” focuser, divide 2,800 by the focal length of the scope in millimeters. For example, if your scope has a focal length of 1,200mm, the widest possible true field of view in degrees is about 2,800/1,200=2.33°.
Although the difference between 1.46° and 2.33° doesn’t seem great, the amount of sky visible is proportional to the square of the field of view. That means the 2.33° eyepiece shows more than 2.5 times as much sky as the 1.46° eyepiece, a very significant difference. For example, Figure 4-8 shows a simulation of the star field around the Crab Nebula (M1) with a 1,200mm scope using a 2” 40mm eyepiece with a 70° apparent field of view (outer circle), a 32mm 52° Plössl (middle circle), and a 1.25” 25mm 50° Plössl (inner circle).
Obviously, it’s much easier to locate the object with the 2” wide-field eyepiece, but even the 1.25” 32mm eyepiece shows significantly more sky than the 25mm Plössl eyepiece typically bundled with inexpensive scopes. When you’re first getting started, you can get by with the bundled 25mm or 26mm Plössl eyepiece, if your telescope came with one. If you observe mostly Luna and the planets, your first priority will probably be to buy additional highpower eyepieces. But, if you have any interest in observing DSOs [Hack #22], you should probably make it a high priority to buy a better finder eyepiece.
When you are evaluating finder eyepieces, a quick way to determine their suitability as a finder eyepiece is to multiply the focal length of the eyepiece in millimeters by the apparent field of view in degrees and compare that result to the numbers given previously. For example, a 1.25” 35mm Celestron Ultima Plössl with a 49° apparent field of view yields 35 x 49=1,715, very close to the 1,750 figure for the maximum possible true field of view in a 1.25” focuser. Similarly, a 2” 40mm Pentax XW eyepiece with a 70° apparent field of view yields 40 x 70=2,800, which is right at the limit for the widest possible true field of view in a 2” focuser, as is a 41mm 68° Tele Vue Panoptic at 41 x 68=2,788.
For a premium finder eyepiece, the $295 24mm Tele Vue Panoptic is hard to beat. Its true field is nearly the widest possible in a 1.25” focuser, and its relatively high power darkens the sky background relative to a longer focal length eyepiece.
For a finder eyepiece in the $100 range, choose the 30mm or 35mm Orion Ultrascopic (also sold under the Celestron Ultima brand name) or the Tele Vue 32mm Plössl. All of these eyepieces are superb optically and have excellent fit and finish. They’re less expensive than the Panoptic because their apparent fields are in the 49° to 52° range versus the 68° of the Panoptic.
For a finder eyepiece in the $50 range, choose a 32mm Chinese Plössl, such as those sold by Orion under their Sirius brand name. These eyepieces are a step behind the $100 eyepieces. Their coatings and lens polish aren’t as good, which means they show lower contrast and some scatter. Mechanically, they’re obviously not up to the Japanese standard of the $100 eyepieces, but they’re not terrible either.
Avoid 40mm 1.25” eyepieces. The 1.25” barrel diameter limits their apparent field of view to about 42°, which is like looking through a drinking straw. A 32mm Plössl provides almost exactly the same true field of view at much higher magnification, which darkens the background sky, making it easier to locate objects. A 40mm 1.25” eyepiece is entirely worthless. We don’t understand why anyone makes them, let alone why anyone would buy one.
If you have a 2” focuser, choose a finder eyepiece according to your budget, as follows:
For a premium finder eyepiece, the $500 Pentax 40mm XW and the $495 Tele Vue 41mm Panoptic are superb choices, as they should be at that price.
For a finder eyepiece in the $225 to $300 range, buy a Celestron 40mm Axiom or a University Optics 40mm MK-70 (http://www.universityoptics.com). At $226, the MK-70 is about $50 cheaper than the Axiom, has less eye relief, and is less suitable for scopes with focal ratios of f/8 [Hack #9] or faster. If you are comfortable buying a used eyepiece, a 40mm Pentax XL is better optically than either of these eyepieces, and it sells for about $225 on Astromart (http://www.astromart.com). In 2004, Pentax replaced their superb XL series eyepieces with the even more superb XW series, but XL eyepieces are still widely available on the used market.
In the $150 range, the best bet is the Orion 40mm Optiluxe, which is Japanese-made and has excellent fit and finish. Optically, it’s a mixed bag. With an f/8 or slower scope, such as an SCT, the Optiluxe provides excellent image quality. At the f/4.5 to f/6 focal ratios common on Dobs, the Optiluxe is extremely soft at the edges of the field. With a fast scope, you probably won’t want to use the Optiluxe to actually observe large objects, but some edge softness is acceptable if you are using the eyepiece primarily for locating objects rather than viewing them.
In the $75 range, several companies resell 3-element Chinese 2” eyepieces with varying focal lengths and apparent fields of view. Orion, for example, resells these eyepieces as their DeepView series. The 42mm DeepView has a 52° apparent field. Although this is not even close to the maximum possible field in a 2” eyepiece, it is still much larger than the true field available in any 1.25” eyepiece. The downside is edge performance in fast scopes, which is simply hideous. Stars near the edge of the field appear as elongated blobs rather than as sharp points.
Figure 4-9 shows two top-notch finder eyepieces, a 2” 40mm Pentax XL (left) and a 1.25” Orion 30mm Ultrascopic.
There are several myths and misconceptions about finder eyepieces, so it’s worth taking a moment to set the record straight. Here are some things to be aware of:
Many astronomers refuse to use very wide finder eyepieces because they believe that such eyepieces “waste light” as a result of their very large exit pupils. For example, a 40mm eyepiece used in an f/5 scope provides an 8mm exit pupil, much larger than the maximum entrance pupil of your eye [Hack #7]. If your eye can dilate only to 5mm, that 8mm exit pupil reduces the effective aperture of your scope by the same factor. Although that is true, it matters little because you are seeing the brightest possible image at that image scale regardless of the exit pupil size. If you substitute an eyepiece that provides the same true field of view with a shorter focal length and a wider apparent field, objects appear larger but dimmer as a result of the higher magnification.
The myth persists that finder eyepieces produce low contrast. As with many myths, there’s a kernel of truth here. Finder eyepieces are of long focal length, which yields low magnification. Like any extended object, the background sky is dimmed by higher magnification. If you observe from a light-polluted site, using low magnification (and an accordingly large exit pupil) means the background sky appears bright and washed out. This has nothing to do with the inherent contrast of the eyepiece, but only with the observing conditions and low magnification. From a dark site, that same eyepiece provides a pitch-black sky background and all of the contrast it’s inherently capable of providing.
Figure 4-9. Two excellent “finder” eyepieces: 2” Pentax 40mm XL (left) and 1.25” Orion 30mm Ultrascopic
If you’ve ever wondered why anyone would pay $295 for a 24mm Panoptic that provides almost the same true field as a $100 32mm Plössl, this is why. The higher magnification of the 24mm eyepiece provides a darker sky background, which is prettier and makes it easier to locate and see objects. Similarly, some well-heeled observers pay $620 for a Tele Vue 31mm “TermiNagler.” The true field of view of the 31mm Nagler is actually slightly narrower than that of the Pentax 40mm XW or the 41mm Panoptic, but the 31mm Nagler provides much higher magnification and a more pleasing view.
Many astronomers believe that wide-field eyepieces produce coma, the aberration that makes stars near the edge of the field appear as short streaks instead of pinpoints. In fact, this aberration in the form of off-axis coma is inherent in any parabolic mirror, which means that every Newtonian primary mirror suffers from coma in proportion to its focal ratio. Even an imaginary “perfect” eyepiece would show coma in a Newtonian reflector near the edge of its true field. Lenses, including eyepieces, can also suffer from coma as well as other edge aberrations, and it is these eyepiece aberrations that most astronomers are referring to when they speak of coma. In fact, a well-corrected wide-field eyepiece like the Pentax 40mm XW or the Tele Vue 41mm Panoptic produces almost no coma of its own, and will show off-axis coma from the primary mirror only at focal ratios below f/5. Very fast scopes, such as f/4.2 and f/4.5 Dobsonians require a coma corrector such as the Tele Vue Paracorr when used with wide-field finder eyepieces.