Astronomical Optics

Part 6: Evaluating Eyepieces


Main head
Main head

Once I had acquired a basic understanding of telescope optics and eyepiece design, I was ready to make a much closer inspection of the eyepieces in my collection and intelligently evaluate the eyepieces I borrowed from others.

There are many ways to evaluate eyepieces, and many criteria, but I was limited to the evaluations I could make without an optical test or specific testing tools. Some of the tests could be objective (e.g., dimension measurement), but others (contrast, color) would be more or less subjective.

Eyepiece Virtues

If the mere reproduction of the image is accepted as a baseline performance standard, then eyepiece virtues are the ways in which an ocular can qualitatively excel in that task. The four key virtues are sharpness, brightness, contrast; certain comfort or convenience factors are important as well.

Sharpness is simply another word for veridical image reproduction: all elements of the image appear in perfect focus, without any visible distortion or aberration. As all ocular errors increase with field height (distance from the central optical axis), sharpness is typically evaluated as any degradation of the image of a star as it is moved from the field center to the edge.

Brightness is the overall throughput or transmission efficiency of the eyepiece. Ideally, 100% of the luminance of the image will be transmitted to the eye; the eyepiece will absorb or deflect no light. In practice, brightness is reduced by the number of elements and groups (air/glass or glass/glass boundaries) in an eyepiece, and by lenses that are uncoated. These all cause reflections and ghost images, which divert light.

Contrast is the luminance difference between the lightest and darkest areas of an image, and is primarily produced by high transmission oculars that do not scatter light. Light scattering is caused by destructive interference in lens coatings, lens edges or ocular mountings that are not blackened.

The comfort and convenience factors in a good eyepiece will vary with the individual, but typically include (1) comfortable eye relief, (2) eye guards or eye rests, especially in longer focal length eyepieces, (3) size and weight, (4) apparent field of view.

Eyepiece Flaws

Eyepiece flaws occur when the ocular fails the minimum task of accurately reproducing the objective image.

Scatter. Ideally, and separate from the errors of focus and projection, all light entering the eyepiece should be transmitted to the focal plane as a coherent image. When this does not happen, a variety of "stray light" artifacts appear in the image. These are reflections off interior surfaces that are combatted by painting surfaces flat black, installing baffles, etc.

Ghosting is the appearance of secondary images of either the field stop or bright objects in the image. The maximum number of potential ghost images is equal to the pairwise combination formula [N*(N–1)]/2, where N is the number of air/glass and glass/glass boundaries in the optical path that differ in refractive index by more than 0.25. These are normally controlled by antireflection coatings, which are very thin layers of materials with refractive indices that differ from each other and from air or glass by less than 0.25, and therefore provide intermediate steps in the refraction.

Spherical Aberration of the Exit Pupil. If the exit rays of light leaving the eyepiece do not converge in a single focal plane, there is no single, well defined location for the position of the observer's pupil. This typically occurs because the peripheral rays are focused at a point closer to the eyepiece than the central rays, which is a form of spherical aberration: thus, this defect is called spherical aberration of the exit pupil, or SAEP. When this occurs, it produces an off axis "blackout" or blank area in the eyepiece field of view opposite the direction of view, which has a lenticular form that is referred to as a "kidney bean". Generally, observers will not object to an SAEP of 10% or less of the exit pupil diameter.

A. Manufacturing Quality

x. Packaging

x. End caps/cap fit

x. Field stop focus - sharply defined?

x. Field fringe color - overcorrected = green or greenish blue, undercorrected = reddish, will not work well with a fast objective.

x. Field illumination - evenly bright from center to edge: if not, and/or different locations are required to see the field stop and to produce even illumination, then there is spherical aberration of the exit pupil.

x. Distortion - is the entire field stop visible without moving the eye? if not then there is either angular or rectilinear distortion.

x. Scatter - Is the dark boundary of the field stop black, or milky? milky indicates scatter.

x. Mechanical and assembly quality

x. Binoviewer fit

x. Draw tube fit

x. Under cut

x. Weight

x. Scratch/dig ratio - Although optics can be laser tested, a measure of scratch width (in millionths or 10–6 millimeters) and pit diameter (in hundredths or 10–2 millimeters); often as high as 60/40 in consumer optics but as low as 10/5 in military and certain industrial optics. Substitute glare, scatter evaluations. Edmund Optics sells inexpensive, approximate and expensive, precise tools for assessing scratch/dig in finished optics.

x. Coatings - Roland Christen evaluates eyepiece coatings by placing them in lateral daytime shade and viewing them under sky light illumination with the black end lens caps on the field end. The photo (below) shows the results using this method.

He also suggests checking the color of the coatings and glasses by viewing a white surface through the eyepieces, as shown in the next photo (below).

B. Observing Comfort

x. Eye relief - Eye relief is usually shorter in higher power eyepieces. Some astronomers remedy short eye relief in high power oculars by using lower power oculars with a barlow lens.

x. Exit pupil spherical aberration (dark adapted)

x. Critical alignment

x. Eye guard

x. Condensation

x. Focal position. The position at which the eyepiece achieves focus turns out to be an important attribute. Parfocal eyepieces share the same focal position, or nearly; this minimizes refocusing when eyepieces are changed. In commercial SCTs, it minimizes refocusing by moving the mirror, which can be locked and then adjusted only with the crayford focuser. Extreme focal positions may require a focusing tube extender, or may make focusing impossible.

The diagram (right) shows the focusing position of over 60 brand name commercial eyepieces with focal lengths of 3.2 mm to 55 mm. Most of these focus within a relatively small 2 cm interval; most of the exceptions are on the extrafocal side. Eyepiece brands are shown by color: many are parfocal series, excepting only the longest focal lengths, which usually require more focal distance.

C. Image Quality

x. Apparent field of View

x. True Field of View

x. True field accuracy/variation within the line

x. Color - Two excellent tests are (a) moon, (b) WZ Cas

x. Contrast - (a) moon, (b) jupiter, (c) deep sky (illumination level, and pupil size) - faintest visible extended area on the moon.

x. Transmission - faintest visible stars

x. Diffusion - (a) center field, (b) bar, (c) field edge

x. Glare, Scatter - (a) off field sirius, (b) center field sirius

x. Ghosting - how bright? colored? follow, mirror image, or remain in center? in focus or diffuse?

x. Barlow image

x. Spherical aberration

x. Astigmatism - worse with wider field and faster objective

x. Coma - worse with wider field and faster objective

x. Field curvature - (center to edge focus, amount of focuser turn) ...

x. Angular magnification distortion - requires rectilinear pattern

x. Rectilinear distortion - requires rectilinear pattern

x. Lateral color - in almost all eyepieces. increase near edge of field? red toward center — undercorrected, blue toward center — overcorrected.

x. Lateral color

The Eyepieces

These were tested using the above criteria.


These are the most expensive wide field eyepieces that I own.


These are the most expensive wide field eyepieces that I own.


I purchased this simple magnifier or "loupe" lens because the early monocentric triplets were highly favored by late 19th century astronomers.

Hastings Triplet

This is that.

RKE (Reverse Kellner Eyepiece)

This is that.


These eyepieces also have a very strong reputation.


The RKE design, like the Brandon, has a very high reputation among planetary and lunar observers, historical problems with quality control notwithstanding.


On disassembly, the Meade 5000 Plössl eyepieces turn out to be the Erfle II 2-1-2 design.


These are the most expensive wide field eyepieces that I own.


Edmund Optics sells this as a single eyepiece.

The University Optics planetary eyepiece is a modified erfle design.


The Plössl is a straightforward 2-2 design

The Tele Vue plössls


These are the most expensive wide field eyepieces that I own.


The original Abbe ortho design is considered one of the most important eyepiece designs of the last two centuries and is remarkable for its nearly flat field, free of distortion.


The Brandon design eyepieces, now manufactured by VernonScope, have one of the strongest reputations among planetary observers for their brightness, contrast and flat field.


Further Reading

Astronomical Optics, Part 1: Basic Optics - an overview of basic optics.

Astronomical Optics, Part 2: Telescope & Eyepiece Combined - the design parameters of astronomical telescopes and eyepieces, separately and combined as a system.

Astronomical Optics, Part 3: The Astronomical Image - analysis of the image produced by a telescope and the eye that receives it.

Astronomical Optics, Part 4: Optical Aberrations - an in depth review of optical aberrations in astronomical optics.

Astronomical Optics, Part 5: Eyepiece Designs - an illustrated overview of historically important eyepiece designs.

Last revised 1/15/12 — ©2012 Bruce MacEvoy