Spectrum Scientifics Blog

A telescope’s eyepieces are often one of the most underrated parts of the optical system. Many folks get caught up with aperture fever or drool over high-end apochromatic refractors while neglecting the fact that the eyepiece is almost 1/2 the optical system!

Most telescopes include 1 or 2 eyepieces. Depending on the model and the manufacturer these might be junky, perfunctory eyepieces or decent lines like Plossls or Kellners. We’ll go over the various features of some of these “common named” lines as well as more modern designs. But first a bit about the eyepiece’s size and features:

Size

Eyepieces come in three sizes, .965″ Diameter, 1.25″ Diameter, and 2″ Diameter. Of these, the .965″ size is not really made much anymore, but some older models may pop from time to time. This size was an old standard used on small Japanese imports but it has been phased out for the most part.

1.25″ Diameter eyepieces are by far the most common size. All the telescopes Spectrum Scientifics sells use this size, and an eyepiece from one telescope will almost certainly fit another one these days.

2″ Diameter eyepieces usually only work in larger telescopes. These eyepieces provide very comfortable viewing as the eyepiece has a nice, large piece of glass to look into. The disadvantage of 2″ eyepieces is that they are low powered. So you can’t take advantage of the comfortable viewing and crank up the magnification.

All telescope eyepieces should have threads in their base to accept filters. There are some exceptions, but they should be rare.

For looking at various optical designs, we will look at the “Named” eyepiece designs first. These have been around for a long time and are named after the scientist who developed them.

Ramsden and Huygenian . These are very old eyepiece designs that date back as far as the 17th century. They use a minimum number of glass elements and these days are considered very cheap designs. If you eyepiece has a latter ‘H’ or ‘R’ before the number it is one of these old designs.

Kellner eyepieces have at least three glass elements and are a vast improvement over the older designs. These make good ‘economy’ eyepieces and work well with small, beginner telescopes. A good example of Kellner designs would be the Orion Explorer II line

Plossl Eyepieces are a step up from the Kellner line, having four glass elements: two achromats (two pieces of glass cemented together to improve optics) are placed facing each other and give excellent images. Most intermediate or advanced telescopes include Plossl eyepieces. A good example of Plossl would be the Orion HighLight Line:

New Optical Eyepiece Designs

These are more modern optical designs, without common names but rather brand names. These eyepieces are so numerous these days that we will describe them by their major features rather than by any names. New eyepiece designs are usually more expensive than the traditional designs as they involve more design and more glass elements.

Long Eye Relief Eypieces are great for eyeglass wearers or for folks who don’t want to look through a pinhole for high magnifications. Eye relief is the point past the glass where the image forms. Short eye relief means that your eye must be very, very close to the eyepiece glass, which can be frustrating for eyeglass wearers. These designs of eyepieces reduce those problems by using multiple elements and often special glass in one or more of those elements. In addition to the long eye relief the viewer can also benefit from a wide open end to the eyepiece even at high powers (traditional designs mean one is looking through a pinhole at high magnification). The trade off is that there is a lot of glass in the optical systems of these eyepieces, so there is more than a little bit of light loss, even with decent optical coatings. Also, using a barlow with these multi-element eyepieces is not good for the light path.

A good example of Long Eye Relief eyepieces would be the Orion Epic series

Ultrawide or Wide Field-Of-View Eyepieces are designed to provide the wider field of view at the same magnifications as traditional eyepieces. This means you see more of the sky in the eyepiece than you would with a Plossl or Kellner without sacrificing magnification. A good example of these would be the Orion Expanse Eyepieces.

These are just a couple of examples of many other designs out there.

The more modern designs will invariably be more costly than the traditional eyepieces. As to which eyepiece to use, that depends on what you want to do. If you are on a budget, stick to Kellner & Plossl designs. If you must wear eyeglasses when you view, consider the long-eye relief designs, if you want to set up your telescope for public viewing consider an easy-to-look-through 2″ diameter eyepiece (if your telescope can take them). If you are looking at wide deep-sky objects, consider the wide-field eyepieces. People’s needs will vary from person to person, so there is no one ‘perfect’ eyepiece.

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Filed under: Guides, Optics, Telescopes