Eyepieces: windows on the universe: understanding the basics will help you select from the cornucopia of modern eyepieces.
The heart of your telescope is the objective, the main light-collecting mirror or lens. Its diameter and quality largely determine what your telescope can reveal under ideal conditions. But your choice of eyepieces can greatly enhance your viewing, so let's take a look at eyepieces and their interaction with your telescope and eye.
The essential and familiar eyepiece characteristic is its focal length. Using the same units of measurement, a telescope's focal length divided by an eyepiece's focal length gives the telescope's magnification. For example, a 6-inch f/8 reflector has a focal length of 48 inches (1,220 mm). Coupled with an eyepiece having a 25-mm focal length, the scope magnifies 49 times (1,220/25 = 49).
Another important eyepiece trait is its apparent field of view (AFOV), which has garnered much attention of late. AFOV is the angle our eye sweeps through as we scan from one edge of the field to the other. Simple eyepiece designs generally have 45[degrees] to 50[degrees] apparent fields.
Modern optical glass, the power of computers, and extremely efficient coatings that almost eliminate light loss at lens surfaces have opened new worlds to eyepiece designers. By allowing more optical elements to be added to an eyepiece design with little or no sacrifice in image quality, modern eyepieces can have apparent fields exceeding 100[degrees]. Instead of looking through a porthole, we can now view the universe through a picture window.
A telescope forms a real image at its focal plane. If you point your telescope at the Moon while holding a piece of paper above the empty focuser, you'll see an image of the Moon formed on the paper. Move the paper in and out until you get a sharp lunar image and the paper will then be at the telescope's focal plane.
Knowing what happens at the focal plane helps us understand eyepieces. Let's consider one of my favorite deep-sky objects, the Veil Nebula in Cygnus. The entire Veil complex covers about 2 3/4[degrees] of sky. With a telescope of 600-mm focal length, the real image of the Veil spans almost 29 mm (1.14 inches) in the focal plane. Double the telescope's focal length to 1,200 mm, and the Veil appears twice as large, covering 58 mm (2.28 inches).
When you view a focused image in a telescope eyepiece, the focal planes of the eyepiece and telescope coincide. If you look into the bottom of a simple eyepiece, such as a Plossl, you'll see a sharp-edged ring, or field stop, located below the eyepiece's field lens. This field stop is positioned at the eyepiece's focal plane and sharply defines the edge of your view when you look into the eyepiece. The telescope focal length and diameter of the field stop's opening determine the true field of view (TFOV)--the amount of sky you see in the eyepiece. (The field stop lies between lenses in some complex designs.)
An eyepiece's barrel size limits the field stop's size. The maximum field stop diameter in 1 1/4-inch eyepieces is about 27 mm. With the 600-mm (focal length) telescope mentioned above, a field stop opening of 27 mm would reveal about 2.6[degrees] of sky, not quite enough to see the entire Veil at once. An eyepiece with a 2-inch barrel, on the other hand, allows a field stop up to about 46 mm, which is capable of showing 4.4[degrees] of sky and nicely framing the Veil. I speak from experience when I say that such a view is a lovely sight under dark skies.
In a telescope of 1,200-mm focal length, even a 2-inch eyepiece cannot show the entire Veil in one view. Furthermore, most eyepiece designs don't allow for field stops as large as their barrel diameters, and in many cases the maximum field-stop diameter is but a small fraction of the barrel diameter.
Apparent field is related to true field. Given two eyepieces of the same focal length, the one with a larger apparent field also has a bigger field stop and thus a wider true field. For example, using eyepiece specifications available on the internet, I found an 8-mm eyepiece with a 50[degrees] AFOV that has a field stop with a 6.5-mm opening, and another 8-mm eyepiece with a 100[degrees] AFOV that has a 13.9-mm field stop. The 100[degrees] eyepiece shows more than four and a half times the area of sky in a single view than the 50[degrees] eyepiece.
You can roughly determine the true field of an eyepiece/telescope combination by dividing the eyepiece's apparent field by the magnification. For a more accurate measure, you need the field-stop diameter. The true field, in degrees, is equal to the eyepiece's field-stop diameter divided by the telescope's focal length (both measured in the same units) and multiplied by 57.3.
A high-magnification eyepiece with a wide apparent field can provide an identical or larger true field than a low-power eyepiece with a narrow apparent field. As an example, there are 32- and 20-mm eyepieces that both have 27-mm field stops. The first has an apparent field of 50[degrees] and the second 80[degrees]. In a telescope of 1,200-mm focal length, the first will magnify 38x and the second 60x, but both will show identical true fields spanning 1.3[degrees] on the sky. The higher power is advantageous.
Magnification is often your friend. As magnification increases, the sky background darkens. A star's telescopic image is really a tiny disk, surrounded by faint rings. Up to a magnification where your eye begins to resolve this disk, star images remain tiny pinpoints, and the darker sky background will help reveal fainter stars.
With increasing magnification, extended objects such as nebulae are spread out as much as the sky background, so the image contrast does not change. Your eye, however, more readily perceives faint objects that cover more of your retina, making it seem as though contrast has increased. As such, enlarging a faint object generally makes it easier to see.
On a sunny day, arm your telescope with a low-power eyepiece and point it at the open sky or a brightly lit wall. Hold your eye far back from the eyepiece and note the small disk of light hovering above the eyepiece. This is the exit pupil. It's a tiny image of the telescope's aperture, and it has two important qualities: diameter and distance above the eye lens. When you observe, your eye's pupil should coincide with the eyepiece's exit pupil.
Magnification and telescope aperture determine the diameter of the exit pupil. Simply divide aperture by magnification to calculate the exit pupil's diameter. For example, an 8-inch (200-mm) telescope magnifying 50x has an exit pupil 4 mm in diameter (200/50 = 4). A nice shortcut for calculating the size of the exit pupil is to divide the eyepiece's focal length by the telescope's focal ratio.
Because the exit pupil is an image of the telescope's aperture, stopping it down is the equivalent of stopping down the telescope's aperture. This happens when the exit pupil's diameter is larger than your eye's pupil, a situation that can arise when using very low magnifications that produce large exit pupils. It turns out, however, that the resulting image is still the brightest possible at the given magnification even though you're not using the full aperture of the telescope. There is no lower limit to the useful magnification of a refractor, but almost all reflectors and Cassegrain systems have central obstructions that limit their use with low magnifications.
Have your eyes been dilated during an eye exam? If so, you probably remember that your ability to see details suffered. I once took a pair of 7x42 binoculars, which have a 6-mm exit pupil, to my eye exam. With my pupils dilated, I could not get a sharp focus because a fully dilated eye doesn't work well. Our vision works best with smaller exit pupils (2 to 3 mm), which use only the central part of our eye's lens. This is another reason why higher magnifications (and thus smaller exit pupils) are an advantage.
Another important aspect of an eyepiece is its eye relief--the distance that the exit pupil is located above the eye lens. Longer eye relief is generally better, especially if you wear glasses while observing, since you'll need enough eye relief to place your eye at the exit pupil with your glasses on. If your eye is farther out than the exit pupil, you won't see the entire field of view. How much eye relief you need depends on your facial structure and your glasses. Eyepieces with long eye relief also tend to stay cleaner and are less prone to fogging up in cold conditions because your moist eye stays farther away from the cold eye lens. In the past, short-focal-length eyepieces (the high-power ones) usually also had very short eye relief, but modern designs tend to be much better in this regard.
Try Before You Buy
One thing that many people discover when seeking eyepiece advice from other amateur astronomers is that there are a lot of people who are passionate about their choices. Thus, I recommend trying the eyepieces you're considering purchasing. Amateur astronomers are generally happy to share equipment, and there are many star parties and conventions where you'll find all kinds of telescopes and eyepieces being used.
Trying eyepieces lets you decide which characteristics suit you, and what works best in your telescope. For example, I prefer more than 12 mm of eye relief, while others don't mind less. And today's ultra-wide apparent fields are not for everyone, especially if expense is a concern--those huge fields come at a price.
Most amateurs find that three eyepieces make a basic set. An eyepiece that gives a wide true field helps you find your quarry and enjoy large star clusters and nebulae. Another with a 2-mm exit pupil provides about optimum power for viewing many deep-sky objects. And a third giving a 1-mm exit pupil allows you to see the lunar and planetary detail that your telescope can reveal. But keep in mind that the steadiness of the atmosphere--the astronomical seeing--imposes limits on power. In my area of upstate New York, magnifications above 250x are rarely usable. In places blessed with excellent seeing, however, much higher powers can be wonderful.
Once you discover what you like to observe and become familiar with what your telescope can show, you'll probably want to expand your eyepiece collection. If deep-sky observing catches your interest, consider the widest apparent fields. Enticed by the planets? Consider eyepieces yielding exit pupils of 0.8 to 0.5 mm so that you can eke out the last bit of detail on nights of excellent seeing. Today there are also fine zoom eyepieces that allow you to set the perfect magnification.
Shiny new eyepieces will forever remain a temptation. And if you're searching for the perfect eyepiece, you won't be alone. But always remember that the best eyepieces are those collecting starlight, not dust.
Veteran telescope maker and observer Alan French is a familiar face at astronomical gatherings in the Northeast. He is married to Sue French, author of our monthly Deep Sky Wonders column.
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|Title Annotation:||Telescope Basics|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 2013|
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