A Guide on Choosing Eyepieces for Your Astronomy Telescope

When you purchased your telescope, it most likely came with a few eyepieces. These are typically fine, but they don’t make full use of your telescope. It’s a good idea to have a variety of eyepieces to get varied magnifications from your telescope. So here is a guide on choosing the best eyepieces for your telescope.

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The focal length of an eyepiece determines its classification. This indicates how much magnification it provides. The magnification produced by a certain eyepiece is obtained by dividing the focal length of the telescope by the focal length of the eyepiece—for example, my telescope has a focal length of 350mm, thus a 25mm eyepiece provides 14x magnification. Eyepieces come in a variety of sizes. Most telescopes have the 1.25-inch eyepiece holder, therefore unless otherwise stated, most eyepieces will fit if you have a half-decent telescope.

So I want the shortest focal length possible. Not at all. Remember how I mentioned magnification in the telescope portion, and how it isn’t as crucial as most people believe? Although 14x magnification seems little, it is where I conduct most of my watching. The greater the power of the eyepiece, the lesser the picture resolution (sharpness) and brightness. A smaller picture that is brighter and clearer is always preferable to a bigger one that is fuzzier and darker. The size of the hole you have to look through on an eyepiece is another important to consider; the lower the focal length, the smaller the actual lens is, making it harder to see through. Because the eye relief is reduced, you must approach quite near to the eyepiece to view the picture (especially important for those who wear glasses). This may also restrict the amount of magnification you may utilize. Higher abilities also have a limited field of vision or the amount of sky you can see at one time. A lower magnification will allow you to view more of the sky. Finally, at higher magnifications, things seem to move faster owing to the Earth’s rotation; unless you have a motorized mount, you must constantly move the telescope (at roughly 70x magnification, it will take about 30 seconds to drift across the whole field of vision).

Different types of telescope lenses
Types of Lenses

So I’d suggest getting a nice, high focal length (low magnification) eyepiece with around 15x magnification. This also makes it much simpler to detect objects and is wonderful for dim items like stars and deep-sky objects where you need brightness rather than magnification to obtain a nice view.

When staring at bright objects, such as the moon or planets, it is ideal to have a close-up view. With a short focal length, the high-power eyepiece will provide you with sufficient magnification to observe close-up detail on such items. For close-up viewing, you need to have at least one high-power eyepiece. However, as previously stated, there is a limit to the amount of power you can use- even brilliant things begin to seem dull and the picture becomes fuzzy if you use too much magnification. A basic guideline is that the most magnification you can use is the aperture of your telescope in millimeters times 2 (or in inches times 50)- for instance with a 70mm aperture telescope, 140x magnification is around the limit.

These are the broad guidelines, although as with everything else, certain eyepieces are superior to others. On the same telescope, you may use two eyepieces with different fields of vision for the same focal length. Other eyepiece attributes to check for when purchasing an eyepiece include:

  • Apparent Field of View. This is the field of view you can see through the eyepiece on its own (in degrees). You can get different eyepieces with different fields of view through different lens designs- so wider is better. The true field of view, the actual field you can see when you use it with your telescope is the apparent field of view divided by magnification. Generally, you get about 45° FOV, but you can buy (for a lot more money) and Ultra-Wide eyepiece that has an 86° FOV, meaning you would see an actual field of view nearly twice as wide on the same magnification.
  • Eye relief. As discussed already eye relief is the distance from the lens to your eye that you can see the image at, in millimeters. The longer, the better, as it is easier on your eyes.
  • Aberrations, Distortions and Off-Axis color. Like everything else, you get what you pay for with your eyepieces. Flaws in optical quality can produce aberrations (fuzziness), distortions, and off-axis color (blue or red rims around the edges of objects). These are objectionable, and you want to get an eyepiece with as low amounts of these as possible. Also, note that on cheaper eyepieces the center is sharp, but toward the edge of the field of view it gets very fuzzy. This happens on all eyepieces but is more noticeable on cheaper ones.

Types of lenses

  • Barlow Lens. A Barlow lens is really worth the investment, and I recommend that you buy one first. Effectively, it doubles the focal length of your telescope, so when used in conjunction with an eyepiece, it doubles that eyepiece’s magnification (you can also buy 3x Barlow as well which treble the focal length). The Barlow lens goes in where the eyepiece would, but has an open top so that you can insert another eyepiece on top. By using each eyepiece with or without a Barlow, you effectively double the number of eyepieces you have!
  • Kellner Eyepiece. These types of eyepieces have a couple of lenses together than a third lens. They provide about a 45° FOV, provide quite a long eye relief, and are fairly good.
  • Plossl Eyepiece. Plossls have two pairs of lenses and are very good eyepieces, usually a little more expensive than average eyepieces. They have about a 55° FOV and good eye relief and are excellent eyepieces.
  • Ultra Wide. Ultra Wide eyepieces are newer inventions, having systems of 7-8 lenses. They can offer a staggering 82° FOV, so for obvious reasons are called ultra-wide, but these are much, much, much more expensive than ordinary eyepieces.

I must confess until recently I was rather confused by all the jargon associated with eyepieces.

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