All About Telescope Eyepieces
The image that is acquired through the lens of a telescope becomes visible through an eyepiece. Each eyepiece provides a given magnification This is determined by the ratio between the focal length of the objective and eyepiece.
Eyepieces are so called as they are the part you look into the telescope using your eye. Different eyepieces have different properties such as magnification and eye relief, and different designs affect the quality of the image you see.
Type of Eyepiece Designs
There are many different types of eyepiece design, each having a different arrangement of elements (glass lenses inside the eyepiece). Some lens designs offer a cheap design with some imperfections, while other designs feature more elements to correct imperfections and distortions, together with a higher price tag. Here are a few common eyepiece designs.
Carl Kellner designed this first modern achromatic eyepiece in 1849 with the intention to correct the chromatic aberrations in the simpler eyepiece designs of the time. The biggest problem of Kellner eyepieces was internal reflections, however, today's anti-reflection coatings make these usable, economical choices for small to medium aperture telescopes with focal ratio f/6 or longer.
The Plössl is an eyepiece usually consisting of two sets of doublets, designed by Georg Simon Plössl in 1860. The compound Plössl lens provides a large 50° or more apparent field of view, along with relatively large FOV. This makes this eyepiece ideal for a variety of observational purposes including deep-sky and planetary viewing.
The 4-element orthographic eyepiece consists of a plano-convex singlet eye lens and a cemented convex-convex triplet field lens achromatic field lens. This gives the eyepiece a nearly perfect image quality and good eye relief, but a narrow apparent field of view - about 40°-45°
Officially there is no design called a super-plossl, however several manufacturers have taken the original Plossl design and added a fifth element similar to the Erfle design of rifle sights. The Erfle design is seen as a logical extension to Plössls.
Eyepiece Focal Length
The focal length of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converge to a single point. When in use, the focal length of an eyepiece, combined with the focal length of the telescope or microscope objective, to which it is attached, determines the magnification. It is usually expressed in millimetres when referring to the eyepiece alone.
High magnification eyepieces have lower focal lengths, so a 20mm eyepiece has a higher magnification than a 40mm eyepiece.
Eyepiece Field of View
Each eyepiece has an apparent field of view, measured in degrees (°). This tells you the apparent width of the sky, in angular terms, that is presented to your eye. Eyepieces with larger apparent fields take in a greater area of sky than smaller ones.
Simpler eyepiece designs tend to have apparent fields of about 45° and widefield designs may be 60° or more.
Every telescope has a stated focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. Typically depending on the aperture and type of telescope focal lengths tend to be between 400 and 3000 mm. Eyepieces have focal lengths too, and to calculate the magnification, simply divide the focal length of the telescope by that of the eyepiece. So, for example, a 2000mm focal length scope used with a 25mm eyepiece will, therefore, deliver 2000/25 = 80x. Note that the same eyepiece used with a different focal length scope will, therefore, give different powers.
Elements in the eyepiece construction typically have several coatings, from anti-reflection and glare to chromatic aberration reduction and protective coatings. You may see these as purple or green tinted reflections when looking at the glass.
Most modern telescopes accept eyepieces with a diameter of 1¼ inches (31.7 millimetres) which slide into their push-fit focusers. In addition, there are designs intended to show you wide views with eyepiece barrels 2 inches (50.8 mm) in diameter. Telescopes with 2-inch focusers usually include an adapter that allows them to accept 1.25 eyepieces.
Most, if not all eyepieces, accept a threaded filter which can be stacked. Filters allow specific frequencies of light to pass, or block, for example, a light pollution filter will block the wavelengths of light emitted by mercury-vapour street lights.
Vignetting is caused when a lens of an eyepiece is not able to field all the lights rays coming through the previous lens. Vignetting presents itself as a noticeable darkening of the field of view towards the edges.
Vignetting is usually a problem in cheaper eyepiece construction and can only be corrected with higher quality optics and design.
Eyepiece Eye Relief
Eye relief is the distance from the eyepiece to the observer's eye. The shorter this distance, the more difficult it can be to observe. Also, if you wear glasses short eye relief eyepieces can be very difficult or impossible to use. Long focal length eyepieces (usually low power) tend to have long eye relief, so they do not need to be specially designed to increase eye relief. Short focal length eyepieces (usually high power), on the other hand, do not inherently have long eye relief. To counter this some of the more expensive eyepieces are specially designed with multiple elements to make them easier to use.
Eyepiece Exit Pupil
The exit pupil is the diameter of the beam of light coming out of the eyepiece. To find the eyepiece exit pupil simply divide the eyepiece focal length by the telescope focal ratio. The brightness of extended objects (galaxies and nebulas) is proportional to the square of the exit pupil.
From dark sky sites, a 5mm to 7mm exit pupil is best for observing Milky Way star clouds, open clusters and large nebulas. From light-polluted suburban sites, a 3mm to 4mm exit pupil improves the contrast by darkening the light-polluted skies without overly dimming the objects themselves.
A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing.
A 1mm exit pupil gives you maximum planetary detail and is excellent for splitting binary stars.
Last updated on: Monday 23rd July 2018