Eyepieces - A Brief Introduction
Good quality eyepieces are an essential part of a successful astronomical telescope, in fact without an eyepiece a telescope is not a telescope at all, but a lens (from an optical designers point of view). Similarly, a telephoto lens with an eyepiece held at the back focus, ceases to be a lens and becomes a telescope. Information that has a touch of geekiness about it maybe, but it leads neatly to the next statement. Without the necessary high quality eyepiece in place (i.e. with the emphasis on high quality eyepiece, rather thanan eyepiece), the complete image information and detail the telescope objective is providing, may not be visible. If we took our camera sensor chip and roughed up the surface, bend it a little, distort the pixels toward the edge of the chip, and generally damage its ability to faithfully record the image information incident upon it, we would not be very happy with the result. If chips were like that anyway, our camera stores would likely be well used to receiving visits from unhappy customers.
When we look at the focused image in the eyepiece, we can often see field distortion, field curvature and chromatic aberration toward the edge of the field. Occasionally we can even see an astigmatic image on-axis. We need to be aware that many of the obvious aberrations we see in the image are created by the eyepiece and not the objective of the telescope. There are many astronomers that spend a great deal of money on a high quality telescope, but are happy to use poor to average quality eyepieces with it. This is equivalent to a high quality camera lens forming an image on a poor quality chip. The important message here is that whether we have a high quality or standard quality telescope, anything less than a good quality eyepiece will deteriorate the image quality further.
This is beginning to sound expensive. However, we are fortunate in that the major Asian manufacturers now produce eyepieces that are of perfectly good astronomical quality and exceptional value for money. Brands such as Celestron, Meade, Skywatcher, Baader and Vixen offer many eyepiece designs of good quality, such as Plossls, Orthoscopics, Wide-angle and Super wide-angle eyepieces, for lunar and planetary observing as well as wide-field deep-sky observing.
The important aspects of a successful astronomical eyepiece are full multi-coatings on all lenses, lens edge blackening (to prevent light scatter), as free as possible from off-axis coma, low distortion, low field curvature, good correction from lateral colour and at the very least, no on-axis astigmatism. We also need to have good eye relief with our eyepieces, as there is little point having wide-angle eyepieces if we can't see the edge of the field unless we push the eyepieces into our eye sockets! We like to have soft rubber eyecups that can be folded back, or those that can be pushed up or down, and we would like our eyepieces to be aesthetically attractive with attention to cosmetic detail. All of this is a difficult task for optical designers to manage, let alone the manufacturing of them within a strict budget. Recently though, over the past five years or so, the quality of Chinese and Taiwanese eyepieces have seen a marked improvement in quality and consistency as higher standards are requested, and manufacturing and testing facilities improve. It has been possible for over a decade to obtain high quality eyepieces from Taiwan, after all, many of the very expensive esoteric eyepiece designs from Televue in New York, have all been manufactured in Taiwan for many years. Its just that now, Asian manufacturers are offering quality eyepieces at much lower prices.
Eyepieces are commonly available in two barrel diameters - 1.25" and 2". With most complete telescopes, one or sometimes two 1.25" eyepieces are included with the package. This is the continuation of a long-held tradition, and to enable the telescope to be sold as a complete telescope package. It is worth bearing in mind that although the eyepieces included with many budget and mid-priced telescope packages are adequate for getting "up and running", the quality of the image will almost certainly be improved by adding one or two better (and still inexpensive) eyepieces to your budget. 2" eyepieces are mainly useful for lower magnification views when a wider field of good illumination is required. A refractor with a long focal length (>25mm) 2" eyepiece will give superb wide-field (sometimes described as rich field) views of star fields and large deep-sky objects. Similarly, a Newtonian with an aperture of 200mm or more, plus a long focal length 2" eyepiece, can give images so bright and full of stars, it provides the observer with one of the defining experiences of visual astronomy.
Eyepieces are threaded at the bottom of the inside of their barrels. This is so filters can be threaded in. Fortunately, the thread pitch is the same on nearly all eyepieces of the same barrel size, so we can fit filters from one manufacturer onto eyepieces from another.
The field of view of an eyepiece is termed the apparent field, and is an expression in degrees of arc of the measurement of the diameter of the field stop, a metal ring at the focal plane of the eyepiece. As it is on the eyepiece focal plane, this means it is always in focus as a sharp circular edge to the field. The diameter of the field stop also places a restriction on the amount of edge aberrations that can be seen, that the eyepiece creates. It would be nice to have no visible edge to the field of view, but the edge of the field would increasingly become more distorted and aberrated. A finite window defining an image with minimal aberrations is much preferred. Eyepiece designs have traditional apparent fields associated with them. The classic orthoscopic has been and still is a popular choice for planetary observers. Its apparent field is often between 40o and 50o, not exactly wide-field, but as planets are small, even when highly magnified, a wide-field is not required because we ideally want to keep the planet at the centre of the field, where the image is at its sharpest. Plossls and Super Plossls have fields of between 40o and 60o. Wide-field eyepieces can be thought of as having apparent fields greater than 60o. Some eyepiece designs reach extreme field widths of >80o. A wide-angle eyepiece fulfils its usefulness when the edge of the field stop is visible by the observer, and the eyepiece has good aberration correction toward the field edge.
It is always a sensible idea to choose eyepieces that are of good quality, not only because of the improved image quality they can provide over stock eyepieces, but also as they can be retained if you decide to upgrade your telescope. Wide-angle eyepieces are best applied to lower and medium power deep-sky observing, (e.g. magnifications of 30X up to 180X depending on telescope aperture), and for wide-angle views of the moon at medium magnifications. Narrow-field eyepieces such as orthoscopics and Monocentrics have attributes that lend themselves to high resolution views of bright extended detail objects, such as the moon and planets, or maybe close double stars, at higher magnifications (e.g. >200X depending on telescope aperture). A recommendation for a choice of eyepieces for any telescope would be three different focal lengths. Two wide-angle eyepieces, one of long focal length for low magnification deep-sky viewing, one of medium focal length for more detailed deep-sky images plus lower power lunar and planetary images, and one narrow-field eyepiece with a shorter focal length for higher power lunar and planetary images. Add a Barlow lens into the mix, and we have six possible magnifications that will cover every aspect of visual observing.