Everything You Wanted To Know About Telescope Eyepieces

Telescope Eyepieces are a vital part of any telescope and key to providing magnification. We look at the different types and how they work.

The image that is acquired through the lens of a telescope becomes visible through an eyepiece. Each eyepiece provides a given magnification which is determined by the ratio between the focal length of the objective and the 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 Telescope Eyepieces Designs

There are many different types of eyepiece designs, 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.

Kellner

Carl Kellner designed this first modern achromatic eyepiece in 1849 intending 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.

Plössl

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 a relatively large FOV. This makes this eyepiece ideal for a variety of observational purposes including deep-sky and planetary viewing.

Orthoscopic

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°

Super Plössl

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.

Telescope Eyepieces 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.

Telescope Eyepieces 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.

Telescope Eyepieces Magnification

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.

Telescope Eyepieces Coatings

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.

Telescope Eyepieces Sizes

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.

Telescope Eyepieces Filters

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.

Telescope Eyepieces Vignetting

Vignetting is caused when a lens of an eyepiece is not able to field all the light 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.

Telescope Eyepieces 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.

Telescope Eyepieces 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 observation.

A 1mm exit pupil gives you maximum planetary detail and is excellent for splitting binary stars.

Barlow Lens and Powermates

Both Barlow lenses and Powermates act as image amplifiers by increasing the effective focal length of your telescope, thereby increasing the magnification achieved with any eyepiece.

Typical Barlow Lens have a magnification of 2x and are composed of two or sometimes three elements in a single group, forming a negative lens. This lens produces a diverging ray of light which as well as magnifies the view, moves the eyepiece exit pupil farther outwards thereby increasing the eye relief. This effect is not too noticeable with short to medium-focal length eyepieces, but it is more significant in longer focal lengths as it can lead to vignetting if the eyepiece lenses aren't wide enough to let the full width of the altered light cone through. Eyepiece to Barlow lens spacing has a significant effect on the magnification achieved.

A Powermate is a telecentric design with four lenses in two doublet groups - a smaller negative lens and a larger positive lens arranged so that light exits the Powermate in parallel rays. This design ensures that the original eye relief of the eyepiece remains unchanged, making it more suitable for longer-length eyepieces and avoiding the risks of vignetting. A Powermate's magnification remains fairly constant irrespective of where the eyepiece is positioned.

From a lunar and planetary imaging point of view, Powermates are the better choice as they can be stacked to produce high levels of magnification.

What are 45 Degree Erecting Prism and Diagonals?

The 45° Erecting Prism is a clever little device which serves three main purposes.

1. It allows a more comfortable viewing angle for visual observations
2. It corrects the orientation of the sky.
3. Allows refractor telescopes to be used as a spotting scope during the day for birds.

As a novice, I was always having problems with the reversed view you get through the eyepiece, and often getting lost when star hopping by moving the telescope in the wrong direction!

This is because astronomy-refracting telescopes show upside-down reversed images. This is normally fine for astronomy but can be a little confusing for the novice. Erecting Prisms correctly orientate the image and the 45° prism makes the viewing angle a lot more comfortable.

There are however a few disadvantages of using erecting prisms. They will cause the view to be slightly dimmed as some of the light gathered is lost inside the prism. You may also notice a reduction in the field of view. Cheaper Erecting Prisms can introduce Chromatic Aberrations which are caused when the different wavelengths of light come into focus at slightly different points. This results in lower image quality and is often observed as a purple fringe around stars.

What is a Diagonal?

A Diagonal is very similar to an Erecting Prism however it uses fewer elements inside and only flips the image horizontally. They are often cheaper than erecting prisms, suffer less in the way of chromatic aberrations, have a greater field of view and have less loss of light. The disadvantage is that images will still appear upside down.

What's the Difference Between an Erecting Prism and a Diagonal?

The key difference between an erecting prism and a diagonal is that with an erecting prism, images appear vertically and horizontally correct. For a diagonal, on the other hand, the image appears vertically upright but horizontally flipped. The difference is caused by how light travels through them. Erecting prisms reflect the light twice, while diagonals only reflect it once.

Erecting Prism and Terrestrial Observation

Erecting Prisms are also good for using your astronomical telescope for more terrestrial viewing. This is because they will correctly orientate the image of the birds in the eyepiece.