Halden Tarn 92 Maksutov-Cassegrain, tube only

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

Every configuration of the Halden Tarn 92 Maksutov-Cassegrain shares the same optics; the choices below change how the telescope is mounted, finished, and equipped. This is the optical tube on its own, so you need to supply a mount that can carry its weight steadily. Without a mount in the package, check the tube weight against the payload rating of your mount, with some margin to spare. The gloss white tube reflects sunlight, which helps it stay closer to air temperature while you set up before dusk.

No accessory kit is included, which suits observers who already own eyepieces and want to reuse them. This is the 2023 edition of the Tarn line, the earliest version that is still listed in the catalog. Check the specifications table for the optical figures, since they stay the same across every configuration of the Tarn line.

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

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.