Halden Cascade 20 mm wide-field eyepiece, with hard case

Specifications

Description

The Cascade 20 mm is a seven-element wide-field eyepiece with a 70-degree apparent field and a 2 in barrel. Its 23.5 mm field stop sets a wide true field, and its 16 mm of eye relief lets most observers, including many who wear glasses, see the full field without pressing the eye against the lens. The eyepiece is intended for telescopes with a 2 in focuser or a 2 in diagonal.

A 20 mm eyepiece gives moderate magnification in most telescopes: 50x in a 1,000 mm f/5 refractor, 30x in a 600 mm reflector, and 100x in a 2,000 mm telescope. In the 1,000 mm telescope, the 23.5 mm field stop gives a true field of about 1.35 degrees, enough to frame open clusters and the bright core of a large nebula with the surrounding stars in view. The wide apparent field also keeps the stars near the edge of the view, which helps you keep your bearings while you move across the sky to a target.

The optical design has seven elements with fully multi-coated surfaces to reduce reflections and light loss. Image sharpness is strongest near the center of the field. In telescopes faster than about f/5, expect slight softening and field curvature toward the edge, because the light cone reaching the eyepiece is steeper. In slower instruments, from about f/8 to f/10, the stars remain sharp across most of the field.

The eyecup folds down for use with glasses and folds up to block stray light from the eye side of the barrel. The 2 in barrel weighs 520 g, a moderate load for a 2 in focuser. The barrel accepts standard 2 in filters for use with light pollution or moon filters.

About this configuration

This listing covers one configuration of the Halden Cascade 20 mm wide-field eyepiece, defined by its case, eyecup, finish, and edition year. 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. The fold-down rubber eyecup blocks stray light, and you can fold it back if you observe with glasses.

Silver anodizing has the same wear resistance as the black finish, with a lighter look. This is the 2023 edition of the Cascade line, the earliest version that is still listed in the catalog. Check the barrel size in the specifications against your focuser or star diagonal before you choose a configuration.

What's in the box

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

Compatibility notes

• Fits any 2 in focuser or diagonal.
• Accepts standard 2 in filters.
• Sharp across most of the field at f/8 and slower; softer at the edge in faster telescopes.