Halden Tarn 92 Maksutov-Cassegrain, tube with alt-az mount

Specifications

Description

The Tarn 92 is a Maksutov-Cassegrain catadioptric telescope with 92 mm of aperture and 1104 mm of focal length, which gives a focal ratio of f/12. Light enters through a thick meniscus corrector lens, reflects off a spherical primary mirror, and returns to a small aluminized spot on the inner surface of the corrector. From there it passes back through a central hole in the primary and reaches the focuser. The closed tube keeps the optics sealed against dust and keeps the design compact for its focal length.

At f/12 the Tarn 92 gives high magnification with a narrow field. A 25 mm eyepiece with a 50 degree apparent field yields about 44x and a true field of roughly 1.1 degrees, which is enough to frame the Moon, the planets, and close double stars. Under steady air, you can use magnifications above 150x to examine the cloud bands of Jupiter, the Cassini Division in the rings of Saturn, and the phases of Venus. The central obstruction from the secondary spot is modest, but it still reduces contrast slightly on low-contrast targets, so the eyepiece and seeing conditions matter more here than the optical design alone.

Because the corrector is thick, the tube takes longer to reach outdoor temperature than an open-tube reflector of the same size, so allow about thirty minutes before critical viewing. The corrector can collect dew on humid nights, and the included dew cap slows this. At 2.4 kg for the tube, the Tarn 92 sits comfortably on a light alt-az mount or a small equatorial mount. Typical targets also include the Double Double in Lyra, the Pleiades in a wide-field eyepiece, and the Ring Nebula in Lyra, which appears as a small disc at this focal length.

About this configuration

This listing covers one specific configuration of the Halden Tarn 92 Maksutov-Cassegrain, defined by its bundle, finish, accessory kit, and edition year. An alt-az mount keeps setup short: level the tripod, attach the tube, align the finder, and start finding targets. Manual alt-az motion suits visual observing, though you need to nudge the tube now and then to follow objects as the sky turns. The gloss white tube reflects sunlight, which helps it stay closer to air temperature while you set up before dusk.

This configuration comes without extra accessories, so budget for at least one more eyepiece if you are starting out. The optics in the 2024 edition match the other years; the changes concern hardware details only. The optical figures in the specifications table apply to this configuration and to every other version of the Halden Tarn 92 Maksutov-Cassegrain.

What's in the box

• Optical tube with dew cap
• Vixen-style dovetail bar
• 1.25 in star diagonal
• 6x30 finder scope with bracket
• Front and rear dust caps
• Alt-az mount with aluminum tripod

Compatibility notes

• Accepts 1.25 in eyepieces directly through the star diagonal.
• The Vixen-style dovetail bar fits most small and medium mounts.
• The focuser has a T-thread for camera adapters once the diagonal is removed.