The eyepiece constructional design, dimensions, weight and materials of fabrication govern the degree of comfort, durability, portability and equipment inter-balance. These factors are as important to consider as the image quality through the eyepiece.
The barrel is that part of the eyepiece which is inserted into the focuser of the telescope. The standard barrel sizes are 1.25 inch and 2 inches. In addition, a 0.965 inch diameter exists but is the least common and is generally associated with small, less-expensive scopes.
Modern eyepiece barrels are constructed from polished or anodized aluminum or base metals. Barrels made of chrome or nickel coated brass are typical of older eyepieces. No matter the material used, after many years of constant insertion into the focuser the barrel will wear, revealing the color of the underlying metal. If you are concerned with resale value then it would be better to choose an eyepiece with a solid metal barrel that is uncoated and only polished which will be longer wearing.
Before purchasing an eyepiece, make sure that the barrel is threaded for filters on the field lens end (some oculars are also threaded for filters on the eye lens end). Filter threads are generally standardized for 1.25-inch and 2-inch eyepieces, but there are exceptions. Meade eyepieces and filters, for example, use a thread that is not standard.
Some eyepieces possess a safety feature to prevent the accidental slipping out of the focuser which could happen if the set screw on the focuser or the retaining ring is not adequately tightened. This safety feature can be seen a three different styles, a full undercut barrel, a beveled or tapered undercut barrel, and a tapered barrel. The barrel style is a matter of personal preference and should be tested in the store. It has often been said that the tapered barrel design is preferable as it is more easily inserted and removed from the focuser.
Physical Size and Weight
Physical size becomes an issue when you plan to use an eyepiece on a small telescope with little physical clearance or if you plan to install two identical eyepieces in a binoviewer. For example, the body diameter of many ultra-wide eyepieces is so large that even if you adjust the binoviewer so that the eyepieces touch, you will not be able to look through both of them simultaneously. Alternatively, for those telescopes that have motorized tracking mounts, a heavy eyepiece may require rebalancing of the setup. Also an eyepiece set which is too heavy may have portability issues.
The housing or outer shell of the eyepiece can be made of various metals or plastic polymers such as Delrin. Anodized housings are more wearable than painted housings. If the housing is made of a plastic polymer the wear will show itself as smooth depression. The outer shell of the eyepiece can further be altered to improve secure gripping and comfortable handling in cold weather. The housing coverings could be a diamond etching on the surface of the housing or a rubberized band or sure grip material wrapped around the housing.
The manner by which the eyepiece is labeled is also important. Essentially there are three types of labeling, painted filled engraving, silk screening and surface painting and in order of wear ability.
The Eye Guard
The eye guard is a rubberized attachment which may or may not be included with an eyepiece that aids in the positioning of the eye and acts as a block to external light. Not all eyepieces can be used with eye guards due to eye relief limitations. There are numerous fixed or adjustable designs. Regardless of the design, certain qualities are worth your consideration. The thickness and pliability of the rubber for instance, is an important quality related to longevity. Also for adjustable designs the ability to remain in place once set, resisting pressure is also important.
Check the inside of the barrel for a well-darkened and even appearance. Eyepieces with poor blackening inside the barrel usually result in ghost images around bright objects. The only way to check without disassembling the eyepiece which is not recommended, is to invert the eyepiece and look into the interior. The interior should be completely non reflective.
Good-quality eyepieces are “multicoated”, indicating that at least one lens surface was treated with multiple layers of anti-reflective coatings. Better-quality eyepieces are “fully multicoated” – an indication that all air-to-glass surfaces are multicoated.
Magnesium fluoride (MgF2) is probably the most widely used thin-film material for optical coatings. Coated lenses allow more light to pass through the eyepiece, so giving somewhat brighter images. More importantly, they will reduce the amount of light that is scattered within the eyepiece so that the contrast will be higher.
You can easily check if an eyepiece is adequately coated – if you hold it under a light, you will see blue or purple reflections (and not white) on the lenses.