Halden Expanse 21 mm eyepiece, with hard case

Specifications

Description

The Expanse 21 mm is a nine-element eyepiece with a 100-degree apparent field and a 2 in barrel. Its field is wide enough that you have to move your eye to see the edges, which gives the impression of looking through a window rather than into an eyepiece.

In a 1,200 mm Dobsonian, the Expanse 21 mm gives 57x and a true field of about 1.7 degrees. That combines the true field of a 32 mm wide-field eyepiece with about 1.5 times the magnification, which darkens the sky background and shows faint objects better. Large targets such as the Veil Nebula, the North America Nebula, and M31 fit with their surroundings.

Nine elements in six groups correct the field to the edge in telescopes as fast as f/4.5, although a coma corrector is still useful in fast Newtonians. The lenses are fully multi-coated and the internal surfaces are blackened.

The eye relief of 15 mm suits observers without glasses. Because of the wide field, most observers move their head slightly to scan the field; a winged eyeguard helps keep stray light out while you do.

At 1,000 g, the Expanse 21 mm is heavy. It needs a sturdy 2 in focuser and can tip a Dobsonian that is balanced for light eyepieces. Counterweights on the tube or adjustable altitude bearings solve this. The 2 in barrel has a safety undercut and accepts 2 in filters.

The Expanse 21 mm suits observers with medium and large Dobsonians who want a single eyepiece for finding and viewing large deep-sky objects.

About this configuration

This listing covers one configuration of the Halden Expanse 21 mm 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 twist-up eyecup sets your eye at the right distance from the lens, with or without glasses.

A color ring near the top of the body marks the focal length, which helps when you swap eyepieces in the dark. This is the 2025 edition, which added a safety groove to the barrel and a firmer eyecup detent. 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
• Foam-lined hard case
• Hard foam-lined case

Compatibility notes

• Needs a 2 in focuser or 2 in star diagonal that can hold 1 kg.
• The barrel accepts standard 2 in filters (M48 x 0.75 thread).
• Sharp to the edge at f/4.5 and slower; a coma corrector helps in fast Newtonians.