Brightwell Classic 20 mm Plössl eyepiece, with hard case

Specifications

Description

The Classic 20 mm is a four-element Plössl eyepiece with a 50-degree apparent field and a 1.25 in barrel. It sits between the 10 mm and 25 mm members of the Classic line, which makes it a useful step in a set of three or four eyepieces. Each focal length in the series uses the same optical layout, so the view changes mainly in magnification and field width.

A 20 mm eyepiece gives moderate low power in most telescopes: 35x in a 700 mm refractor, 60x in a 1,200 mm Dobsonian, and 102x in a 2,032 mm Schmidt-Cassegrain. At these powers you can frame a large open cluster or a small planetary nebula while keeping the image bright enough to show detail. In a 1,200 mm telescope, the 17 mm field stop gives a true field of about 0.81 degrees, enough to frame the Ring Nebula with the surrounding stars.

The Plössl design uses two pairs of cemented lenses facing each other. It gives sharp stars across most of the field in telescopes slower than about f/6, with some softening at the edge in faster reflectors. The lenses are fully multi-coated, and the edges are blackened to reduce internal reflections.

With 16 mm of eye relief, the Classic 20 mm suits observers who wear glasses when the eye is held at the correct distance. The fold-down rubber eyecup blocks stray light when folded up and gives room for glasses when folded down. The barrel is threaded for standard 1.25 in filters, so a lunar or light-pollution filter can be fitted without an adapter.

At 66 g, the eyepiece balances well on a lightweight focuser and places little strain on the drawtube of a small refractor. Keep the dust caps fitted when the eyepiece is stored, and clean the lens surfaces with a soft brush. Dust on the field lens appears as soft shadows across the view, and it is easier to remove before it builds up.

About this configuration

The Brightwell Classic 20 mm Plössl eyepiece is offered in several configurations, and the notes below describe what this particular one adds. The hard foam-lined case protects the eyepiece from knocks during travel and stacks neatly with other cases. A hard case keeps the eyepiece safe in a busy accessory box at a star party or an outreach event. A winged eyeguard helps you hold your eye at the right position, which makes the edge of the field easier to see.

Black anodizing is the standard finish of the Classic line and matches most focusers. The optics in the 2024 edition match the other years; the changes concern the housing only. To find the magnification, divide the focal length of your telescope by the eyepiece focal length in the specifications.

What's in the box

• Eyepiece with fold-down rubber eyecup
• Top and bottom dust caps
• Plastic storage case
• Hard foam-lined case
• Winged rubber eyeguard

Compatibility notes

• Fits any 1.25 in focuser, diagonal, or Barlow lens.
• The barrel accepts standard 1.25 in filters (M28.5 x 0.6 thread).
• Sharp across most of the field at f/6 and slower; softer at the edge in faster reflectors.