The universe is as mysterious as it is beautiful. For those of us intrigued by the wonders of celestial objects, the telescope is the ultimate tool through which we can get a sneak peek. But to be able to see more than just a blurry planet, it’s crucial to understand the power of eyepieces. These incredible inventions can radically change the whole experience of telescope stargazing, transforming a blurry celestial smudge into a crisp, detailed view of cosmic wonders. How do they work and what type suits your needs as an amateur astronomer?
How Do Telescope Eyepieces Work?
While telescopes gather light and create the primary image, an eyepiece is responsible for magnifying and focusing that image for your eyes. This key link in the chain is a multi-lens component that allows you to achieve different magnifications with your telescope and enables your eye to focus more closely than you normally can. The quality of the eyepiece can significantly enhance the performance of your instrument. But before we go into the mind-boggling range of eyepiece telescope options, we’ll get familiar with the optical terms used to describe and define them.
Magnification
The first thing you need to know is your telescope’s focal ratio – its focal length divided by the aperture. For example, if you have an f/10 telescope you can use almost any decently made eyepiece and you will get quality, sharp views. But, if you use an f/4 telescope, you have to combine it with the best optical systems to achieve image clarity. Simply put, the smaller the focal ratio, the better eyepiece you need to use.
Eyepieces are referred to by their focal length in millimetres. The length of the eyepiece coupled with the telescope's focal length determines the magnification you will achieve. To figure out magnification, you will divide the telescope's focal length with that of the eyepiece. For example, with a 1000mm telescope and 25mm eyepiece, you will get 40X magnification (1000 ÷ 25 = 40).
However, magnification is not everything. In fact, excessive magnification can actually degrade the quality of an image. As a rule of thumb, most astronomers agree that the maximum useful magnification is around 50x per inch of aperture (or 2x per millimetre). Beyond this limit, you will get a dim and fuzzy image. This explains why a small telescope over-magnifying Jupiter does not reveal more detail than a larger telescope providing moderate magnification.
Field of View
Apart from the eyepiece’s focal length, another important feature is the apparent field of view (FOV). Measured in degrees (from 30° to 90° or more), this tells you the width of the sky, in angular terms, that is presented to your eye. Eyepieces with larger apparent FOV take in greater gulps of sky compared to smaller ones. A wider field can be particularly useful when observing extended objects, such as nebulae, or when simply trying to keep oriented during prolonged observation of the night sky. However, wider fields, in general, come with a higher price tag and are more liable to exhibit edge aberrations in the less costly designs. Users should also consider the true field of view, which is obtained by dividing the apparent field by the magnification the eyepiece delivers.
Eye Relief
Another very important aspect, especially for the eyeglass wearer, is eye relief - the distance from the eyepiece at which the observer can still see the whole field. Longer eye relief (15 mm or more) will make for a more comfortable view, which is important if you intend to observe with your glasses. In general, longer focal length eyepieces tend to offer more eye relief, while those of shorter focal lengths can be more challenging to use, especially in designs such as Plössl.
Eyepiece Designs
Eyepiece designs have evolved remarkably in recent years, with each type offering its own set of advantages. The simplest design, the Kellner eyepiece, offers decent views at a reasonable price, thus making it quite popular with beginning observers. The Plössl, on the other hand, has a four-element design and hence developing more detailed images, has become the standard for many amateur astronomers. More advanced designs like Orthoscopics, perform excellently with planetary observation, while eyepieces such as Nagler offer immersive views of larger deep-sky objects.
Building a Collection
When putting together an eyepiece telescope collection, start with three basic focal lengths. Low-power eyepieces in the 20-32mm range provide a wide field for locating objects and viewing the larger deep-sky objects. Medium-power eyepieces ranging from 12 to 15mm, serve as versatile general-purpose options suitable for most viewing conditions. High-power eyepieces (6-9mm), work splendidly for planetary and lunar observation when atmospheric conditions allow.
Quality Matters
Construction quality matters significantly in eyepiece performance. Well-made eyepieces for telescopes feature fully multi-coated optics to reduce internal reflections and increase light transmission. They should feel solid and slide smoothly in and out of the focuser. They should have cleanly blackened internal parts and a smooth barrel surface. While premium eyepieces might not be more costly, they maintain their value and, when properly taken care of, may last long enough to be handed down several generations through the family tree.
Maintenance
Proper maintenance of astronomy eyepieces is necessary for optimal performance. Keep them stored in their protective case when they are not in use and take special care not to scratch them or allow fingerprints to get on the optics. Clean them only if they are dirty and do this according to optical cleaning procedures. A well-maintained eyepiece can serve for decades.
To Wrap Up
For the beginner, it may be wise to purchase one or two quality eyepieces instead of many obscure ones. As you gain more experience and your preferences develop, you can gradually add to your collection. Before making large investments, try borrowing or testing different eyepieces when you are at the astronomy club meetings. Remember, the best eyepiece is the one fitted to your observing goals and your telescope's capabilities.
