Halden Cascade 20 mm wide-field eyepiece

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

All configurations of the Halden Cascade 20 mm wide-field eyepiece share the same optical design; the options change the eyecup, the case, and the finish. This configuration comes without a case, which suits observers who keep their eyepieces in a shared foam box. No case is included, so the eyepiece ships in a simple box with its two dust caps fitted. The twist-up eyecup sets your eye at the right distance from the lens, with or without glasses.

The black body with a color ring lets you tell focal lengths apart quickly under a red flashlight. The optics in the 2024 edition match the other years; the changes concern the housing only. The true field in your telescope is roughly the apparent field divided by the magnification you reach with this eyepiece.

What's in the box

• Eyepiece with fold-down rubber eyecup
• Top and bottom dust caps
• Padded storage 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.