Choosing the right eyepiece is just as important as choosing the right telescope. The eyepiece can make all the difference between seeing an object and not.
Never select an eyepiece based on the magnification that it will deliver through a given telescope. This advice might be a bit against conventional wisdom, but magnification plays second fiddle to exit pupil when observing under less than ideal conditions. The exit pupil is the diameter of the beam of light leaving the eyepiece. You can see the exit pupil of a telescope or binocular by aiming the instrument at a bright surface, such as a wall or the daytime sky (not at the Sun!). Back away and look at the little disk of light that appears to float just inside the eye lens. That's the exit pupil.
The exit pupil's diameter will change as magnification goes up and down. To find out its diameter, use one of the following two formulas:
|Exit pupil ||= diameter of the telescope's objective lens or primary mirror in millimeters / magnification |
| ||= focal length of the eyepiece in millimeters / telescope's focal ratio (its "f/ number")|
Here's an example. Imagine we have a 25mm eyepiece in a 6-inch (152mm) f/8 telescope. This combination produces a magnification of 48x.
Therefore, the corresponding exit pupil would be:
Exit pupil = (152mm / 48) = (25mm / 8) = 3.1mm
Knowing the diameter of the exit pupil is critical, because if it is too large or too small, the resulting image may prove unsatisfactory. This is especially true under light pollution. Light pollution greatly lowers the contrast between a sky object and the background. If an eyepiece produces too large an exit pupil, contrast will suffer greatly. On the other hand, if the exit pupil is too small, images may be so dim that objects are nearly impossible to see and focus.There is no single best exit pupil for viewing all objects.
The table below offers suggestions for urban conditions.
|Target||Exit pupil (mm)|
| Large star clusters, complete lunar disk ||3 to 5|
|Small deep sky objects (especially planetary nebulae and smaller galaxies), double stars, lunar detail, and planets on nights of poor seeing ||2 to 4|
| Double stars, lunar detail, and planets on exceptional nights ||0.5 to 2|
As you are making your selections, make sure your eyepieces come with eyecups.
Eyecups are designed to guard against localized light entering the corner of the observer's eye. They can make a big difference! Personally, I find full eyecups to be more effective than the so-called wing style.
Eyepieces must also be kept clean, because even a slight amount of oil or grease on the lens will smear and degrade the view. The outermost eye lens is especially susceptible to grease from eye lashes. To clean a lens, use lens tissue dampened with lens-cleaning solution. Lightly wipe the lens surface with the tissue to remove the grease. Lens pens are also good for this purpose, but to prevent scratching the lens's soft optical coatings, never rub with any pressure.
Many people are under the misconception that light-pollution reduction (LPR) filters are the salvation of urban astronomers everywhere. On the surface, it makes sense. Many outdoor lights do not shine evenly across the entire visible spectrum. Instead, they emit their light at only a few discrete wavelengths. For instance, the common high-pressure sodium streetlight used throughout North America shines principally in the yellow wavelengths. LPR filters are designed to suppress the broad portion of the visible spectrum that includes those wavelengths, while passing others through. They do little, however, to lessen the impact of most car headlights, lights on buildings and houses, and other sources that use incandescent bulbs. Incandescent lights shine at all visible wavelengths.
Let's put another urban legend to bed once and for all: LPR filters do not make deep-sky objects look brighter. They block all light at their specified wavelengths without regard to where it came from. That means these filters actually dim everything in view. In the process, however, the background sky is usually darkened more than the target, effectively boosting contrast. So, LPR filters should make an object easier to spot but will not make it appear brighter.
Finally, one of the most useful accessories for viewing the sky, regardless of where you set up, is binoculars. While they make wonderful tools for touring the universe on their own, binoculars also help bring urban stargazers into the country.
If the naked-eye limiting magnitude from an observing site is only 3rd magnitude, 50mm binoculars will show stars as faint as 6th, possibly even 7th magnitude, which is the naked-eye limit from the best dark sites. Binoculars will also help you find reference stars that are invisible by eye alone, whether they are used for aligning a go-to telescope mount or for star-hopping. Don't leave home without them!