Choosing Telescope Eyepieces
You won’t see very far with your telescope without an eyepiece through which to gaze. Eyepieces for telescopes come in many different designs to fit many different budgets. They are made with a variety of specs, which include apparent field of view, eye relief (measured in millimeters) and focal length. Below is a selection of eyepieces in B&H´s inventory, as well as some tips on how to select them.
Long/Short Eye Relief
The distance from the last surface of the eyepiece, from which your eye can see the entire field of view afforded by the optical device, is known as the eye relief. When an eyepiece has minimal eye relief, such as 10mm or less, eyeglass wearers will not be able to see the entire view because their glasses extend more than 10mm beyond the front of their eyes.
Additionally, it can be tiring to peer through short eye relief eyepieces for those who don't wear glasses. It is very common for short focal length eyepieces, such as those of 10mm or less, to have minimal eye relief. Eyepieces with eye relief of 15mm or more are classified as "long eye relief" here, and 15mm is usually sufficient for most users.
Wide Apparent Field
An eyepiece can show a narrow slice of sky while it seems to have a view that surrounds your vision, just as easily as an eyepiece can show a wide chunk of sky while it seems to have a view that's narrow and further away in the eyepiece.
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This is because of "apparent field of view," which is the concept of how wide the view feels within the eyepiece itself. Nearly any observer enjoys a wide apparent view because it makes you feel like you’re “swimming” in space, but this does not come without disadvantages. Generally speaking, those eyepieces that have a wide apparent field will usually have less eye relief than an eyepiece that has a narrower apparent field. Additionally, telescopes which have a “fast” aperture can expose an eyepiece's optical flaws, while the same eyepiece used on a “slower” telescope can appear brilliantly sharp and free of distortions. If you have a telescope with an aperture of around f/5.0 and wish to use an eyepiece with a wide apparent field, you'll need to spend quite a bit of money to get a view which is enjoyable from edge to edge.
Wide Field/Long Relief
These are eyepieces that achieve both goals—longer eye relief and a wider apparent field—in one unit. These eyepieces are often large, heavy and expensive, but they will provide the best viewing experience for eyeglass users or those without eyeglasses.
This designation refers to eyepieces that have an apparent field of view of 59 degrees or less and an eye relief of less than 15mm. The advantage of these eyepieces is that they are often small, less expensive and less of an optical challenge for your telescope.
It is a good idea to space out the focal lengths of your eyepiece collection so that you can observe at different magnifications to match the conditions. Thus, if your telescope came with a 25mm eyepiece, it would make sense to buy another that was, perhaps, 30-40mm, one around 15mm or so, and one below 10mm.
Within one brand and style of eyepiece, it is often true that each eyepiece in that collection will be parfocal with the others. This means that when you switch eyepieces while observing the same object, the telescope will require little or no refocusing with the new eyepiece. This translates into less vibration to the scope and more real time enjoying the view at the eyepiece.
But the best advice of all is this; buy the best quality eyepieces that are within your budget. You'll use these every time you observe and they will critically enhance your viewing experience.