20mm Orion Expanse Telescope Eyepiece | Orion Telescopes: Eyepieces

Design

Design

Different eyepiece designs allow for a wide range of features, depending on the needs of the observer. Orthoscopic and Plossl designs can provide a great quality image with a standard field of view, but may be difficult to use at higher powers while wearing glasses due to the short eye-relief. Long eye-relief designs have more elements and can be more expensive, but work very well with glasses, even at higher powers. Wide angle designs provide a larger view of the sky at the same power as a standard eyepiece. More complex designs can combine all these attributes together in a single eyepiece.

Ultrawide
Eyepiece focal length

20.0mm
Barrel size

1.25"
Apparent field of view

Apparent field of view

The edge-to-edge angular diameter of the light emerging from the eyepiece as seen by the eye. It is an inherent specification for a given eyepiece type or design. The apparent field of view of an eyepiece is directly related to the true field of view seen through the telescope; for a given focal length eyepiece, the greater the eyepiece's apparent field of view, the greater area of sky will be seen.

66.0°
Eye relief

Eye relief

The distance, measured in millimeters, between the observer's eye and the eyepiece lens in which the entire field of view remains visible. Eye relief varies with the optical design but generally increases with decreasing magnification. Long eye relief is advantageous for observers who wear glasses, as they don't have to put their eye directly on the eyepiece to see the entire field of view, nor do they have to remove their eyewear.

18.0mm
Coatings

Coatings

Lenses are coated with an antireflective material to ensure that as little light as possible is reflected away, so more gets to your eye. Good lenses are at least "fully coated," with a single layer of magnesium fluoride applied to each air-to-glass lens surface. Multiple layers of coatings are even more effective; the term "multi-coated" means one or more lens surfaces have multiple coatings. "Fully multi-coated" is even better, meaning that all lens surfaces are multiple-layer coated. Mirror star diagonals are coated with a reflective material similar to telescope mirrors in order to reflect the light into the eyepiece. Standard aluminum coatings reflect between 88%-92%, and enhanced aluminum reflects 97%. Dielectric coatings use a different process to deposit a coating that reflects 99% of the light. In addition to the higher reflectivity, Dielectric coatings are much more durable than aluminum coatings, so they last longer and can be cleaned with less risk of damage.

Multi-coated
Number of elements

4
Filter threads

Yes
Parfocal

Parfocal

Parfocal refers to an attribute of an eyepiece set where little to no focusing is required when swapping out one eyepiece for another.

No
Field stop diameter

Field stop diameter

The field stop is a mechanical ring in the eyepiece which provides the sharp edge to the field of view when looking through the eyepiece. The field stop diameter greatly affects the apparent field of view of the eyepiece; the larger the field stop diameter, the larger the apparent field of view is.

23.5mm
Barrel security recess

Barrel security recess

The barrel security recess is a shallow groove or depression cut into the 1.25" or 2" barrel of the eyepiece. In the event the thumb screw holding the eyepiece in the focuser becomes slightly loose, the eyepiece will not fall out because the tip of the thumb screw will catch on the lip of the security groove.

Yes
Blackened lens edges

Blackened lens edges

Blackened lens edges in eyepieces and refractor lenses prevent unwanted light scatter inside the tube. This results in higher image contrast.

Yes
Rubber eyeguards

Yes
Body material

Aluminum
Case

No
Weight (oz.)

4
Dimensions

2.3 in. x 1.3 in.
Warranty

One Year

Why Buy From Orion

Design

Apparent field of view

Eye relief

Coatings

Parfocal

Field stop diameter

Barrel security recess

Blackened lens edges

Why Buy From Orion

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About Orion Telescopes & Binoculars

Why Buy From Orion

Design

Apparent field of view

Eye relief

Coatings

Parfocal

Field stop diameter

Barrel security recess

Blackened lens edges

Why Buy From Orion

Why Buy From Orion

Shop Our Catalogs

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About Orion Telescopes & Binoculars