Tips for Purchasing Affordable Eyepieces
With the great variety of eyepieces on the market no wonder most new amateurs are confused when it comes to either upgrading or adding to their current collections. I am going to try to outline a few of the basic principles regarding both eyepiece design and quality. I am also going to put together a few examples of good eyepiece systems for under $100.00
What Does the Eyepiece Do
The eyepiece is really just a small magnifying glass for the light coming through your telescope. Amazingly a telescope does not make an image look big the eyepiece actually does. When the lights is either reflected or refracted onto a mirror the image formed is incredibly tiny, it takes an eyepiece to make this image appear much larger than it is. So not only does an eyepiece show what the telescope is pointed at, but it plays a part in determining the power at which the telescope is operating (if you have any questions about magnification or power read my guide to magnification right now).
There are a few things you should first know about eyepieces before I get into specific designs or features. The most important are eye relief, apparent field of view, and focal length.
The eye relief of a telescope is quite simply how close your eye has to be to the lens to see the entire “picture”. An eyepiece with a very good eye relief (usually measured in mm) will allow people who wear glasses a much more comfortable view. If you do wear glasses eye relief is a major issue when purchasing an eyepiece. Luckily many of the newer affordable eyepieces offer a good amount of eye relief. Without glasses a good eye relief translates into a much more comfortable view.
The apparent field of view directly determines how much space will be seen through a particular eyepiece. Think of two windows, one small and one big. You will see more of the outside through the larger window without actually being closer to it. Although this is not the best example just think of it as the size of the view. The field of view is generally measured in degrees.
The focal length of an eyepiece is the distance it takes that particular eyepiece to focus incoming light. This is measured in mm and is directly related to the magnification that the telescope is operating at. As a rule the higher the focal length number the lower the power. Most eyepieces range from about 4mm (high power) to 40mm (low power). The focal length usually also influences the eye relief.
Types of Affordable Eyepieces
Ramsden and Huygien ($10.00 - $20.00)
These are the oldest and simplest of all of the eyepiece designs. These are most commonly found with lower priced telescopes of all brands and designs. They are two element eyepieces meaning that they only use two lenses to focus and correct the incoming light. These eyepieces typically suffer from a very small apparent field of view, usually in the 20 to 25 degree range and they also have extremely short eye relief. Do not expect to use these while wearing your glasses. These are usually to blame for the poor dim views associated with many of the lower priced telescope.
Kellner ($15.00 - $ 40.00)
Quite a leap up from the Ramsden and Huygien eyepieces the Kellners are a much more advanced design. They feature a well corrected three element lens system that adjusts for many common flaws found in the simpler designs. Most notably they have a much wider apparent field of view around 40 to 45 degrees and much better eye relief except at short focal lengths. This means that in most Kellner’s with a focal length above about 10mm you will be able to comfortably wear glasses. They also correct some distortion at the edge of the field where simpler designs suffer. The Kellner is a much better eyepiece than the Ramsden and Huygien designs, for this reason it is advised that you should at least buy Kellner’s.
The Orthoscopic (Ortho) is once again quite a leap above the Kellner design. An Ortho is a four element highly corrected eyepiece that is well known for its ability to show bright and sharp images. This was at one time the most popular eyepiece but due to its comparatively narrow apparent field of view it has been surpassed by newer designs. Both eye relief and apparent field of view are average in an Ortho. Usually the apparent field of view on an Ortho is between 40 and 50 degree. Eye relief also gets very short on the shorter focal length eyepieces. However, there are very few eyepieces that are as sharp or well corrected as a good Ortho, any lunar or planetary observer should really consider giving them a chance.
The Plossl has quickly become the new standard for high quality, low cost eyepieces. The Plossl design has taken the astronomy world by storm over the past few decades. These are probably the most common of all mid priced eyepieces on the market today. The Plossl features both well corrected optics and very good eye relief along with a larger apparent field of view. The Plossl is a four element design that offers between a 45 and 60 degree apparent field of view with very good edge of field correction. Almost every serious amateur will have quite a few Plossl’s lying around due to their low cost and very good performance.
Now that we have covered the both the basics and designs we can get into more specigifc information. Optical coating are one of the more important but often overlooked features of any eyepiece. These coatings reduce the amount of incoming light reflected off of the glass thereby allowing more light (information) to reach the eye. Good coatings can make or break the difference between finding a very dim object or not. The coating come in a few very basic levels.
Coated: This is the most basic level of optical coating. Usually only one glass to air surface has a layer of coatings. While this is an improvement over non-coated glass it does not offer very much extra. Most eyepieces that ship with low cost telescopes offer simple coatings or none at all.
Fully Coated: At this level each glass to air lens element is given a separate layer of coating. This is probably the most common level of coatings available on sub $50.00 eyepieces. Looking into the top lens you will likely see a light blue glare, this is a good indication of fully coated lenses.
Multi Coated or Fully Multi Coated: Each and every glass to air lens is treated with multiple layers of anti reflection coatings. These coatings are usually the standard on more expensive premium eyepieces but can be found on some of the lower priced models as well. Multi coated lenses will show a very dark and distinct red, blue, or green hue when looked at from certain angles.
Some certain brands also use proprietary names for coatings such as Ultra Multi Coated or Super Multi Coated. In essence these are the same as Multi Coated of Fully Multi Coated lens systems. However more expensive eyepieces typically have more generous amounts of optical coatings applied to them.
Eyepieces also come in three different barrel sizes. They are .965 inches, 1.25 inches, and 2 inches. As you may have thought the general rule is the larger the eyepiece the more expensive it is. The size that you can use is determined by the size of your focuser (reflector) or the size of your diagonal (refractor). The standard size is considered to be the 1.25 inch format while the .965 inch format is usually limited to Asia and some of the lower priced models. The 2 inch eyepieces have recently become much more popular even though they have extremely high price tags.
Usually the diameter of the eyepiece also determines how large a portion of the sky you will actually see. The .965 inch variety will show the smallest slice of sky for a given focal length and apparent field while the 2 inch eyepiece shows the largest. Also as a general rule 2 inch and 1.25 inch eyepieces are higher quality than their .965 inch cousins, however this is not always the case and there are plenty of good .965 inch eyepieces on the market today.
If you do happen to own a telescope that only takes .965 inch eyepieces you still have many options. Along with there being many good .965 inch eyepieces there are converters that will allow your scope to use 1.25 inch eyepieces also. In most cases they cost less than $20.00 and will greatly benefit your telescope.
Sample Eyepiece Collections for Under $150.00
Deep Sky Observer
Lunar and Planetary Observer
Submitted by Curt Irwin - firstname.lastname@example.org - Grand Rapids, MI