Eyepieces

People buying a telescope for the first time, tend to focus mainly on the telescope and somewhat on the mount. This is wrong! I have run in to people who have a great telescope on a good solid mount that complain about there bad views they keep getting. I remember one amateur who kept sending his telescope back for repair and having it returned with a letter from the manufacture stating that they could not find anything wrong. Once I placed one of my eyepieces in to his telescope, everything became clear.

The telescope makes up only one third of the total picture. The telescope, mount and eyepiece all have equal weight during the buying phase. But unlike the telescope and mount, you will probably wish to buy more than one type of eyepiece. In addition, eyepieces can be used on more than one telescope or may be moved to any future updated telescope purchase you might make.

Knowing the language of the eyepiece

The following is a list of some, but not all, of the more important terms you need the learn and know. When selecting the right eyepiece, you will need to understand and consider several factors about eyepieces. Review the list of terms below and become familiar with them. At first it will be a little overwhelming but read the list through once then take a short break. Think about what you have read and then re-read the list of terms again. It is important that you do become comfortable with them since you will be using them for as long as you remain active in amateur astronomy. 

Make sure you know the size of eyepiece your telescope can except. Most of today's quality amateur telescopes are designed to use a standard 1.25 inch diameter eyepiece while the less expensive department-store telescopes tend to use a .965 inch eyepiece to reduce the cost. Some of the high end telescopes are now sporting large 2 inch eyepiece.

An Element is simply any single glass lens within a eyepiece.

The Eye lens is simply the lens element that is closest to your eye.

The Field lens is the lens element farthest from your eye or the lens element that is facing inside the telescope.

Magnification (or in some cases lack of magnification) is equal to the telescopes focal length divided by the eyepiece focal length. For example: My telescope has a 1220mm focal length and I plan to use my 32mm Plössl eyepiece. The total magnification with this setup would be 1220mm / 32mm = 38x. Remember that higher magnification is not always required. May times people are surprised to find out that the view they are admiring is only at 30x. Stealer
objects like nebulae and star clusters are best viewed using low powers.

Exit pupil is defined as the diameter of the light beam leaving any eyepiece and traveling to your own eye's pupil. Knowing the exit pupil is important. If the exit pupil is too large or too small, the resulting image you see might prove to be unsatisfactory. Let me explain. The average human eye can dilate to about 7mm while in a dark condition. If the exit pupil is larger than 7mm, some of the light coming from the eyepiece can not enter your eye and is wasted.
This reduces the brightness for the viewed object. Exit pupil can be calculate by using ether of the two formulas below: 
Diameter of the telescope's primary objective ÷ magnification OR eyepiece focal length ÷ telescope's focal ration (f-number).

Lets look as an example: My telescope has a 203mm (8 inch) primary mirror. Using the Magnification above of 38x, the Exit pupil is 203mm ÷ 38x = 5.3mm.

Apparent field of view is the most common method use by manufacturers of eyepieces to proudly tout their eyepieces monster fields of view. Unfortunately, this number can be a little confusing. The apparent field of view refers to the eyepieces edge-to-edge angular diameter when seen by the observer's eye. This number does not directly tell you how much sky you will see. To find out how much sky will be viewed, you need to find the real field of view covered
next.

The Real field of view is defined as the total area of sky that can be viewed through the eyepiece and is given in degrees. If you know the Apparent field of view and the total magnification, you can calculate the real field of view using the formula: Real Field = Apparent Field ÷ Magnification

For example, the Plössl used above to calculate magnification has a Apparent Field of 45°. Using the Magnification calculated above of 38x, the Real Field of View for this eyepiece would be 45° ÷ 38x = 1.2°

Eye relief is the distance from your eye to the top of the eyepiece when you can see the entire (or largest) field of view. A good eye relief is important for comfortable viewing. If an eyepiece's eye relief is too small, your eye in very close to the eyepiece. This will become a big problem if you use glasses during you viewing.

Contrast is how sharp the overall image is thought the hole Field of View.

A Parfocal eyepiece is one designed to allow a change of eyepieces without refocusing the telescope. A note to the wise, I have found that eyepieces only tend to be parfocal only within one manufacturers line of eyepieces and not between manufacturers.

The different types of Eyepieces

Eyepieces come in a variety of different designs and sizes. Which design is right for you depends on several factors. What do you plan to observe, how finicky you are about the image, how much sky you wish to view through the eyepiece, and of course how much money you wish to spend. The five most common types of eyepieces in use today are shown below. Review each type to help decide
which eyepiece or eyepieces are right for you.

In 1849, Carl Kellner designed the first three element, achromatic eyepiece known today as the Kellner eyepiece. The Kellner gives sharp, bright images in the low to medium power range. They have good eye relief and an apparent field of view between 40° and 50°. Best used on small to medium sized telescopes, the Kellner is a good low-cost performer for the budget-conscious observer.

Once considered the best all-around eyepiece, the four element Orthoscopic (or Ortho for short) has lost some of its luster to the newer wide field designs. Introduced in 1880 by Ernst Abbe, the Ortho offers excellent sharpness, good color correction and contrast. Like the Kellner, Ortho's also sport a 40° to 50° apparent field of view but with a longer eye relief. The Ortho is especially good for planetary and lunar observing.

In 1860, G. S. Plössl developed a four element eyepiece while living in Vienna, Austria. The Plössl is today's most popular eyepiece design. Providing excellent image quality, good eye relief and an apparent field of view of around 50°. High end Plössls exhibit high contrast and pinpoint sharpness out to the edge of the eyepiece. The Plössl is the
ideal all around eyepiece for planets, lunar and deep-sky viewing.

Developed for the military in 1917, the Erfle eyepiece is optimized for a big "picture-window" sized apparent fields between 60° and 75°. Containing five or six elements, this eyepiece will get you impressive deep-sky views at low powers. This is not the eyepiece for lunar or planetary viewing since image sharpness suffers at the edges at higher powers.

Today we have Super and Ultra wide eyepieces that incorporate six to eight elements to produce apparent fields of view up to 85°. Best used at low-to-medium powers, these eyepieces have so wide a field of view, you need to move your eye around to take in the whole panorama. The ideal eyepiece for viewing galaxies, star fields and other
deep sky objects. Image quality is very high, but the number of elements slightly reduce light transmission. You will also pay for the ultra wide field of view!