The Confusing World of Eyepieces

   If you have a telescope, you will need eyepieces. A telescope without an eyepiece is like a camera without a lens.
When it comes to buying eyepieces, always buy the best quality.
           a) The difference in quality will drastically improve the view through any telescope.
           b) The difference in cost is not great. The cheapest quality eyepiece will sell for $40.00 whereas the best eyepieces in the world (such as Meade's Super Plossl's & Televue) will sell for about $80.00 to 120.00.

   Most telescopes on the market come with one eyepiece, and most of the time it is a 25mm or 26mm, meaning low to medium magnification. Since the eyepiece allows you to change magnification, you can see why it would be nice to have a few extra eyepieces to choose from when looking at Saturn or the Orion Nebula. The price of the eyepiece is not connected to the power of the ocular, but rather to the design. For example, with four elements, Plossl's are a simpler and therefore less expensive design than an Ultra-wide or Nagler design, which has six elements. There are others that fall between these three in quality and cost. Also remember that eyepiece design does not affect magnification, only the focal length of the telescope in conjunction with the focal length of the eyepiece. In other words, a 9mm eyepiece will give the same magnification for a specific telescope whether it is a Plossl or a Nagler. What will be affected is eye relief, the field of view, and the resolution of the image. In general, the more elements an eyepiece has, the wider its field of view, the better the eye relief, and the crisper the image. Eyepieces come in three barrel sizes: .965", 1.25", and 2".

965" eyepieces come with most department store or catalog telescopes because they are less expensive to manufacture. While you can find better quality .965 eyepieces as accessories, the ones that generally come with a telescope of that caliber are usually plastic and hard to look through, especially in the higher powers. If you have a telescope that takes this size eyepiece, consider upgrading the existing eyepieces that came with your scope for some which are made of glass. You will find viewing through these much more of a pleasure. They are available in a limited number of designs and from a limited number of manufacturers because of the few telescopes that take them.
1.25" eyepieces are the most common size used in telescopes today. They come in a wide variety of styles and prices, from about $50 for the more simple designs to over $300 for the most high-quality, wide angle eyepieces.
2.00" eyepieces are available in medium-to-low power designs, but very few telescopes on the market come standard with a 2"
eyepiece as part of it's package. These beauties are intended for larger aperture telescopes, and give lovely low-power, bright images of the deep-sky. Because of the amount of glass used, they tend to be heavy and on some telescopes, counterweights may need to be used to balance out the system. If you decide to take the 2" plunge, remember that you need a 2" focuser (on a Newtonian) or a 2" diagonal (on a Catadioptric or Refractor)in order to use these.Don't get carried away with buying the highest power eyepieces, especially when considering the second or third addition to your collection. It is important to remember that 50x-60x per inch of aperture is the maximum usable magnification under PERFECT seeing conditionsfor any telescope, but that 10x-15x is the most used power range of seasoned amateur astronomers. That means that if you have a 10" telescope, your most used magnification would be 100x-150x, and the highest power you would be able to use would be 500x-600x. I can assure you, though, that our atmosphere cannot usually support such high magnifications except under rare conditions. Those are the times we stay up all night long, amazed at the luck we had at pulling out our telescope that particular evening!

A term you should know is "parfocal". If a set of eyepieces are parfocal, it means that you will not have to refocus each time you change them as you look at an image under different magnifications. This is nice although not imperative, but if you are building a set of eyepieces from the ground up, I would keep it in mind and aim towards this end. I just wouldn't let it stop you if a great deal came up on a wonderful eyepiece! The second eyepiece I usually recommend to a new telescope owner is one that magnifies the image about 100x. How do you know which focal length eyepiece to buy to achieve that magnification? Here's your answer:

Magnification=focal length of telescope divided by
focal length of eyepiece.

Example: An 8" f10 Schmidt-Cassegrain has a focal length of approximately 2000mm. 2000 divided by a 20mm eyepiece would equal 100x.
Most telescopes have their focal lengths written somewhere on the tube or on the inside rim of the corrector plate. You can also figure out your own focal length by taking the aperture and multiplying it times 25.4. This gives you the aperture in millimeters. Then multiply this number by the focal ratio of the telescope (f10, for example) and this gives you the focal length of your telescope. Example for an 8" f10 Schmidt-Cassegrain: 8 x 25.4 = 203.2mm x 10 = 2032mm of focal length. Eyepieces are referred to by their focal length in millimeters, and so it will be written on the eyepiece itself (not the power!) and on the box it comes in. The power of the eyepiece will change according to which telescope you put them into. A nice set of eyepieces usually numbers between 3 and 6 or so. If you have access to an astronomy club, find others who have the same type telescope you do and ask THEM what their favorites are.

Barlow Lenses
Barlow lenses increase the magnification of an eyepiece by lengthening the effective focal length of the telescope. The Barlow goes into your focuser or diagonal, and then the eyepiece you are using slips into the other end of the Barlow lens, or sometimes the Barlow goes directly into the focusing tube, the diagonal follows, and then the eyepiece goes in the diagonal as usual. A 2X Barlow increases the magnification of the eyepiece two times, a 3X magnifies the image three times, and so forth. The good things about a Barlow are that you can use a lower power eyepiece (which usually has better eye relief) and get the magnification of a much higher power eyepiece. A set of three eyepieces, with the addition of a Barlow, becomes a set of six eyepieces.
If you decide to purchase a Barlow lens, buy the best one you can. A high quality Barlow is an excellent addition to your eyepiece collection, but a cheap Barlow will distort the image and decrease the light coming to your eye to such a degree as to only be a detriment to your observing session.