Eyepiece

Today, engineers assisted by computer-aided drafting software have designed eyepieces with seven or eight elements that deliver exceptionally large, sharp views.

For many years, simple eyepiece designs with a minimum number of internal air-to-glass surfaces were preferred to avoid this problem.

These thin coatings are only one or two wavelengths deep, and work to reduce reflections and scattering by changing the refraction of the light passing through the element.

The effect can create a ring of false colour around point sources of light and results in a general blurriness to the image.

Achromats are lens groups that bring two different wavelengths of light to the same focus and exhibit greatly reduced false colour.

Longitudinal chromatic aberration is a pronounced effect of optical telescope objectives, because the focal lengths are so long.

This definition of lens power relies upon an arbitrary decision to split the angular magnification of the instrument into separate factors for the eyepiece and the objective.

It represents the distance at which the mirror or objective lens will cause light from a star to converge onto a single point (aberrations excepted).

Eyepieces for telescopes and microscopes are usually interchanged to increase or decrease the magnification, and to enable the user to select a type with certain performance characteristics.

A larger eye relief means that the optimum position is farther from the eyepiece, making it easier to view an image.

Good design guidelines suggest a minimum of 5–6 mm to accommodate the eyelashes of the observer to avoid discomfort.

The simple negative lens placed before the focus of the objective has the advantage of presenting an erect image but with limited field of view better suited to low magnification.

Huygens eyepieces consist of two plano-convex lenses with the plane sides towards the eye separated by an air gap.

If the lenses are made of glass of the same Abbe number, to be used with a relaxed eye and a telescope with an infinitely distant objective then the separation is given by: where

[c] This optical design is now considered obsolete since with today's shorter focal length telescopes the eyepiece suffers from short eye relief, high image distortion, axial chromatic aberration, and a very narrow apparent field of view.

Cemented eyepieces are traditionally regarded as potentially vulnerable to heat damage by the intense concentrations of light involved.

The lens separation varies between different designs, but is typically somewhere between ⁠ 7 /10⁠ and ⁠ 7 /8⁠ of the focal length of the eye-lens, the choice being a trade off between residual transverse chromatic aberration (at low values) and at high values running the risk of the field lens touching the focal plane when used by an observer who works with a close virtual image such as a myopic observer, or a young person whose accommodation is able to cope with a close virtual image (this is a serious problem when used with a micrometer as it can result in damage to the instrument).

In a Kellner eyepiece an achromatic doublet is used in place of the simple plano-convex eye lens in the Ramsden design to correct the residual transverse chromatic aberration.

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 eyepiece was an obscure design until the 1980s when astronomical equipment manufacturers started selling redesigned versions of it.

This eyepiece is one of the more expensive to manufacture because of the quality of glass, and the need for well matched convex and concave lenses to prevent internal reflections.

This gives the eyepiece a nearly perfect image quality and good eye relief, but a narrow apparent field of view — about 40°–45°.

They are preferred for reticle eyepieces, since they are one of the wide-field, long eye-relief designs with an external focal plane; slowly being supplanted by the König.

Due to their low degree of distortion and the corresponding globe effect, they are less suitable for applications which require an extensive panning of the instrument.

Erfle eyepieces are designed to have wide field of view (about 60°), but are unusable at high powers because they suffer from astigmatism and ghost images.

[15] Edmund Astronomy News (March 1978) called the eyepiece the "Rank-Kaspereit-Erfle" (RKE) a "redesign[ed] ... type II Kellner".

Introduced in 2007, the Ethos is an enhanced ultra-wide field design developed principally by Paul Dellechiaie under Albert Nagler's guidance at Tele Vue Optics and claims a 100–110° AFOV.

In effect, a Nagler is a superior version of a Barlow lens combined with a long focal length eyepiece.

The main disadvantage to Naglers is in their weight; they are often ruefully referred to as ‘hand grenades’ because of their heft and large size.

Long focal length versions exceed 0.5 kg (1.1 lb), which is enough to unbalance small to medium-sized telescopes.