The three most common designs of eyepiece you’ll come across as an observer: an orthoscopic (left), a Plössl (middle) and a wide angle (right). Image Credit: The Secret Studio
When starting out in astronomy, it is quite natural to think mainly in terms of the telescope that you want, with hardly a thought given to the eyepieces that go with it.
Indeed, many telescopes come with one or more eyepieces as part of a kit. However, the telescope is only part of the observing equation.
Telescopes work by collecting light and then bending the rays to bring them to focus at a point known as the focal plane, but it is the eyepiece that brings this valuable light to your eye – so its quality and suitability for the purpose are just as important as the scope itself.
Your choice of eyepiece will determine the magnification and the size of the field of view that your telescope will deliver.
Eyepieces and telescopes are denoted by their focal lengths. Together, the two focal lengths produce a magnification, calculated as the telescope’s focal length divided by the eyepiece’s focal length.
The take-home message from this is that the shorter the focal length of the eyepiece, the higher the magnification you will have.
But it’s important to realise that good observing is not all about magnification; in fact, too high a magnification can spoil the view.
Different celestial objects are best seen at different magnifications, which is why most observers have a range of different eyepieces.
Typically, a collection of four – 6mm, 10mm, 15mm and 25mm – will cover most observing requirements.
Another important attribute is the eyepiece’s apparent field of view (AFOV), which will normally be marked on the body along with its focal length.
Values range from 45˚ to over 100˚, but the eyepiece’s true field of view with a given telescope can be worked out with another simple calculation: apparent field of view divided by the magnification.
Also important is the build quality of your eyepiece and particularly its lens elements; quite exotic glass is a prerequisite.
However, as eyepieces commonly have four or more individual lens elements within them, non-reflective coatings are also vital, to cut down reflections between the lens surfaces that would otherwise reduce the contrast and spoil the view.
The quality of these non-reflective coatings is an important factor in the choice of eyepieces.
Sometimes, additional steps are also taken to increase contrast – such as ensuring that the inside of the eyepiece body is a very matt black.
Some manufacturers also blacken the edges of individual lens elements to further reduce reflections.
Three core designs
There are various types of eyepiece available, the three most popular designs being Plössl, orthoscopic and wide angle.
Plössls often ship with new telescopes, though these ‘bundled’ eyepieces can vary greatly in quality.
They commonly have an AFOV of 52˚ and are typically available in focal lengths from 6mm to 32mm.
Plössls of good quality are excellent all-round eyepieces, although the ones with shorter focal lengths do suffer from short eye relief, and this can sometimes make observing a little uncomfortable.
Plössls generally have 1.25-inch barrels, but some 2-inch variants are available.
These have the advantage of a longer eye relief and the ability to accommodate longer focal lengths.
Orthoscopic eyepieces are renowned for their excellent distortion and aberration-free views, although they typically have a narrower AFOV of between 40˚ and 45˚.
They excel at lunar and planetary observations, where a wide field of view is of lesser importance than image purity.
Almost all orthoscopics have 1.25-inch barrels.
Wide-angle eyepieces are available in a range of AFOVs from 68˚ to as wide as 120˚.
These eyepieces help to give the illusion of ‘floating’ in space rather than looking through a porthole, which can be the feeling you get with narrow AFOV eyepieces.
This type of eyepiece is very well suited to scrutinising star clusters and larger deep-sky objects.
Wide-angle eyepieces require 2-inch diameter barrels.
There are, of course other eyepiece designs, and these include Kellners, modified acromats, Huygens and Konigs.
But they are not as popular as they don’t match the quality of the other types.
No discussion about eyepieces would be complete without mentioning Barlow lenses.
Although a Barlow is not an eyepiece in its own right, it is frequently used with in conjunction with an eyepiece to increase your magnification.
A Barlow lens is a tube containing lens elements that diverge the light passing through them.
Barlow lenses are commonly used in conjunction with an eyepiece to increase magnification.
Credit: The Secret Studio
If a Barlow lens is inserted into the light path of a telescope, the effective focal length of the telescope is increased.
As the magnification of an observing setup depends on the focal lengths of both telescope and eyepiece, an increase in the effective focal length of the telescope results in an increase in magnification.
Most Barlows increase magnification by two times.
Light emerging from an eyepiece produces a small circular image known as the exit pupil, the diameter of which is calculated by dividing the eyepiece’s focal length by the telescope’s focal ratio.
Eye relief is the fixed distance from the curved surface of the outermost lens of an eyepiece to the point at which the exit pupil is formed. Longer eye relief gives more comfortable observing.
Power is simply another name for magnification and it indicates how many times bigger the object will appear through the eyepiece in comparison with the naked eye.
Steve Richards is BBC Sky at Night Magazine’s Scope Doctor