By Luigi Papagno
As an amateur astronomer getting started with your first telescope, you could easily fall into a trap of collecting more accessories than you really need. Or, worse, you could get telescope accessories that are redundant and leave you feeling frustrated and unsatisfied.
Many people may not know what the essential accessories are and what characteristics these accessories need to have. This article will help clarify what makes quality eyepieces and filters, as well as which types and how many of each you need.
When it comes to purchasing astronomy gear, quality always trumps quantity. You only need a handful of eyepieces and a Barlow lens to be able to have a full viewing experience. Eyepieces are not that complicated to understand, although they have many specifications, such as magnification and focal length, that can often confuse people. Unfortunately, in that confusion, you can easily end up with more eyepieces than you need.
To understand magnification, you need to know what focal length is. A telescope’s focal length is the distance from the primary lens, or mirror, to the point where the light rays come together to focus and form an image. Focal length is measured in millimeters, and it is a determining factor in the magnification power of a telescope. The focal length is usually printed on the telescope and can range from 400 to 3,000 mm.
Eyepieces also have focal length, which lets you know the magnification the eyepiece offers you. Just divide the focal length of your telescope by the focal length of the eyepiece, and the result is the eyepiece’s magnification.
However, don’t be fooled: you do not necessarily want very high magnifications. Some celestial targets, such as the Moon, do well under even high magnifications. However, if magnification is too high you will often end up with very bright, unresolved fuzz. There is also a limit to the amount of magnification the telescope or eyepiece can withstand. To calculate this, take the focal ratio of the telescope (which is the focal length divided by the aperture) and multiply it by seven. This will give you the maximum focal length of an eyepiece that the telescope can accommodate.
Field of View
The apparent field of view (FOV) of an eyepiece is most commonly measured in angular degrees. FOV tells you the width of the sky the eyepiece can “see.” This can range from wider views, such as a whole asterism (a recognizable pattern of stars, which may be part of one or more constellations), to just the full Moon.
However, what is even more important is the true field of view, which is the result that you get by dividing the apparent field of view by the eyepiece’s magnification. This significantly reduces the FOV number but is a far more accurate description of what you will see.
Telescope Eyepiece Designs
There are many designs of eyepieces. Please note that the number of “elements” means the number of lenses contained in each eyepiece.
The Kellner eyepiece is a three-element design that offers good corrections on aberrations and a standard field of view. The downfall is that if you have a telescope with a shorter focal length, this eyepiece will not really stand up to it.
An Orthoscopic eyepiece has four elements, and a Plossl eyepiece has five. These make for excellent eyepieces all around and can be priced quite well. They are well-corrected and have a standard FOV, ranging from 45 to 52 degrees.
Panoptic, Nagler, and Tele Vue are all eyepieces with more than six elements. Their FOV is extremely good (67 to 82 degrees), and they deliver much crisper images. Being at the top of the quality range also means that they are at the top of the price range, but if you can afford them, they are most certainly worth it.
Barlow lenses are not technically eyepieces, but they are almost certainly just as valuable. The Barlow lens fits between the focuser and the eyepiece and doubles, or even triples, the magnification. Just be careful that you don’t end up with extra eyepieces that are easily replaced with another eyepiece paired with a Barlow lens!
Many people may wonder if they need filters at all. The short answer is yes. Filters enhance color, improve visibility, make features pop out, and even cut glare. The way that filters do this is all in the name: they limit certain wavelengths of light from reaching your eyes by blocking them out or reducing them, and allow other wavelengths to pass through.
Living in or close to the city means that you should most certainly invest in Light Pollution Reduction (LPR) filters. Mercury and sodium vapor lamps, often used for street lighting, are responsible for a great deal of light pollution. They emit specific wavelengths, and LPR filters greatly reduce these wavelengths, giving you a significantly improved image.