Colour filters, which usually thread into the eyepiece barrel, are almost a necessity for viewing planetary detail. By using an appropriate colour, you can highlight a specific planetary feature. This often allows you see two to three times as much detail as in an unfiltered view.

A motor drive is necessary for many types of astrophotography, but it is more than just a convenience for visual observation as well. At 200x magnification, the Earth's rotation will move an object out of your field of view in about two seconds. A Right Ascension motor drive will keep an object in the centre of the field where the image is the best without producing the objectionable vibrations experienced with manual tracking. Adding a Declination motor drive and a hand controller allows you to guide for astrophotography.

This often asked question is quite irrelevant, as different design's performance varies with different telescopes. Different eyepiece designs have various characteristics. For example, an expensive wide field design is not required for planetary viewing, where the only important thing is maximum contrast. A Plossl or Orthoscopic would probably be best, but almost all designs are good performers on-axis for any f/ratio. Telescopes with F/ratios>10 are quite tolerant of simple low element eyepieces up to 55 deg. A.F., but telescopes <6 inch are a different matter. Off-axis performance requires powerful correction to properly image the highly convergent beam. Each eyepiece and telescope performs as a system, and their image can only be evaluated as much.

Only for some objects, although undermagnification is often a problem, even for experienced observers. The penalty for increased magnification is reduced field of view and brightness; faint objects grow fainter as the magnification is increased This is why larger aperture telescopes are so effective on faint objects; they provide enough light to stimulate the eye at high magnifications. For example, a 4-inch telescope will only view a globular cluster effectively at 80X, and it will appear as a blob. A 6-inch will resolve the outer stars at 130X, an 8-inch will resolve further in at 200X. 10 and 12.5-inch telescopes will make them glitter to the core at 300 and 400X.

If brightness is not a factor, choose the eyepiece that will encompass the object, then allow for a suitable backdrop. If you want to know the actual field on view the eyepiece will give (True Field), this can be calculated as: True field of view = Apparent field of view/ Magnification

When you pay more for an eyepiece you are usually paying for: Field of view: Eyepieces that have many lenses to correct for the five major aberration (these aberrations give increasingly worse, the lower the focal ratio of the telescope) have obviously higher costs in lenses and coatings. Eye relief: Using larger, more expensive elements in eyepieces allows for a greater distance between the eyes and eyepiece. Coatings: 2-layer multicoatings on both faces of all lenses will typically add 25% to the cost of an eyepiece, but this is absolutely necessary to preserve the contrast of the image when the light has to go through 7-9 lenses. Advertising: Those ads aren't free.

The only time the eyepiece alone may perform as well, is on-axis, in a high-contrast application, as the extra optics of the barlow may cause a slight depreciation. Optically, for all other uses, the eyepice+barlow outperforms the eyepiece working alone. The reason? Most of the aberrations caused by positive spherical lenses (Coma, Astigmatism, Curvature of Field and Spherical Aberration) can be reduced and sometimes almost eliminated by introducing a negative system (barlow) which has the same aberrations in negative quantities! Spherical aberration of the system is reduced as the positive spherical aberration of the eyepiece is cancelled by the negative spherical aberration of the barlow. The other aberrations cancel in a similar way! This is one of the eyepiece designer's most powerful weapons, and it is used in most of the shorter focal length ultra-wide designs. Another great benefit of this idea is that the longer eye relief of the longer f.l. eyepiece used with the barlow is retained.

Parfocal eyepiece sets reduce the amount of refocusing when changing powers, but it is rare when no refocusing is required. Parfocallizing of eyepiece sets is a non-performance factor when choosing oculars.

Eyepieces are the most critical factor concerning the performance of your telescope, excepting a dark sky. Eyepieces create the image your eye will see, and the right ones will give you the experience that makes amateur astronomy so rewarding. Even the best instrument will never perform to it's potential visually with poor oculars. Since most manufacturers sell their telescopes with inexpensive ones, and since most people selling a telescope keep their good eyepieces, the aftermarket is your best source. Borrow as many as you can and try them out; for every object there will be an eyepiece that works best with your particular telescope. You'll probably be satisfied with 5-8 good eyepieces; and you'll use your telescope much more often with good ones.