Thursday, January 6, 2011

Eyepieces for Telescope

What is the Eyepiece

The eyepiece is the lens assembly at the focal point of the telescope that forms and magnifies the image. Most telescopes, even department store types, come with at least one for initial use. Sadly today many of the major manufacturers deliver their telescopes with just that one eyepiece. Today’s Celestar 8 and Nexstar 8 telescopes,the current editions of the legendary Celestron C8, come with only a single 25-mm Plossl eyepiece (though Celestron does offer a nice observing kit that adds five more eyepieces and a filter set for under $100).

How to choose your telescope eyepiece

When you purchase eyepieces, remember to get the proper size for your telescope. Eyepieces are sold in three industry-standard sizes (0.965 inch, 1.25 inch, and 2 inches) encompassing several different designs. The major things that distinguish eyepieces from one another are eye relief and field of view. Eye relief is the maximum distance from the telescope that you can see the entire field of view. The farther away you can keep your eyes, the more comfortable you will be. Field of view is the angular measure of how far you can see from the left edge to the right edge of the field.If you own an economy or department store type telescope it likely came with an eyepiece of the Huygens or Ramsden optical design.

These are two-element eyepieces designed to be low cost and produce satisfactory images to the undemanding eye. Eye relief is minimally adequate and the field of view is usually less than 30 degrees. The perception that I always had with these eyepieces is that I was looking through a pinhole. If my head wandered slightly, my eye would wander out of the field and if my eye were to tear at the eyepiece, I would lose sight of the target. These eyepieces are almost universally sized at 0.965 inch and while they serve department store telescopes adequately, they are not suited for use in high performance instruments.

Telescopes intended for high-quality astronomical work, are equipped with 1.25-inch eyepieces. The cast iron cell that seals the end of a Celestron or Meade telescope has an opening in it with a universal thread to allow attachment of 2-inch accessories. This opening is usually fitted with an optional adapter called a visual back that stops that opening down to 1.25 inches and has a thumbscrew to secure accessories with a non-threaded draw tube. This arrangement allows the telescope to accept 1.25-inch or two-inch accessories. The preferred size is 1.25 inch since there are a much wider variety of accessories at more modest cost and they are much lighter. Using 2-inch accessories usually requires that the telescope be carefully counter balanced.

For these telescopes, the lowest cost eyepiece one should consider is the Kellner design. These eyepieces generally cost less than $50 per unit and provide a field of view of about 40 degrees. Kellners provide bright images and good eye relief as long as you do not use too much power. At higher powers, eye relief becomes uncomfortably short. The Kellner eyepiece uses three lenses in its design. Light first strikes a convex element that then focuses light on a second convex lens. That lens is directly mated to a concave third element. If you need to add more magnification flexibility to your telescope at a modest cost, orthoscopic eyepieces might be the way to go.

Orthoscopic eyepieces utilize four elements in their design. Light first contacts a three-stage lens stack of convex, concave and convex lenses. These focus light onto a concave lens at the viewing end. Eye relief is better than in cheaper designs and, combined with their field of view, the Orthoscopic eyepiece was for many years considered the best all-around telescope eyepiece. It has over the years lost its top-of-the-line stature to more modern designs. Orthoscopics may also suffer from field curvature near the edges of the field of view. This may cause star images to appear slightly streaked rather than perfect pinpoints near the edge of the field of view. I added two Orthoscopic eyepieces to my equipment box after buying the Celestron, an 18-mm (111×) and a 12.5-mm (160×). These additions allowed me more magnification in conditions where my 7-mm would be useless.

For those with a little more money to spend after the big purchase, consider adding more Plossl eyepieces to your collection. This design has replaced the Orthoscopic as the most popular among discriminating observers. The Plossl also utilizes four optical elements. The first and second elements are mated and are concave and convex. This stack sends light to a second stack that is convex and concave bringing the image to focus. The design has the advantage of providing an image that is uniformly sharp from center to edge. Each one can cost some where between $60 and $100 depending upon size and manufacturer. Plossl designs feature excellent eye relief since the exit end of the eyepiece is considerable wider than less costly designs. The field of view is about 50 degrees on a typical model. Unlike with the Kellner design, the Plossl provides excellent views at both high and low powers.

Since the Plossl design is usually the featured eyepiece inmost manufacturers product lines, you may have choices of equipment with special over coatings to cut glare and improve light transmission. Eyepieces such as Celestron’s Ultima line provide improved image quality and sharpness at a considerably increased cost. Celestron now offers Plossls as part of a discount equipment kit. This allowed me to add 32-mm, 20-mm, 15-mm, 9-mm, 6-mm and 4-mm eyepieces to my collection (along with a Barlow and a filter set) for minimal cost of less than $100. You can now buy six of them for what a single Plossl cost twenty years ago.

Two other highly popular high-end eyepieces are the Erfle and Nagler designs. These are super-wide field eyepieces that more resemble looking through a window than a telescope.

This is accomplished using what is usually a six-element array providing a field of view as great as 70 degrees. With such extreme width in the field of view, these eyepieces are generally best used for low-power views. Some loss of image sharpness may be experienced when using these eyepieces with very short focal lengths.

For those desiring the most field of view and maximum eye relief for maximum comfort, consider adding 2-inch accessories for your telescope. A special visual back allows 2-inch accessories to be added easily. Newtonian users might have to install a new focuser.

Some elements might be directly attached to the rear cell of the telescope. Two-inch accessories are much larger and therefore more expensive to purchase.

A single eyepiece can run to well over $200. They are also much heavier than 1.25-inch equipment and require counterbalancing. This in turn will cost more money.

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