Explore Scientific 3-inch Self-Centering Focuser

A precision 3-inch rack-and-pinion focuser with a self-centering compression system, 9:1 reduction ratio, and 17-pound lifting capacity, built for astrophotographers who need rock-solid focus with heavy imaging trains.

The self-centering compression ring applies uniform pressure around the entire circumference of the drawtube, which Explore Scientific says prevents flex, tilt and image shift. The helical rack-and-pinion mechanism with a 9:1 reduction ratio turns coarse motion into fine, precise control, which matters when focusing at long focal lengths with a camera. Stainless steel ball bearings ensure smooth travel across the full 84.5mm of drawtube movement. A locking clutch holds focus without disturbing the image.

Fourteen internal baffles suppress stray light and internal reflections, keeping contrast high for both imaging and visual use. The assembly is machined from black anodized aluminum with stainless steel hardware.

The focuser takes 3-inch accessories directly and 2-inch accessories through the included 3-inch to 2-inch twist-lock adapter, and the M82 x 1.25 thread on the drawtube accepts adapters. Tool-free adjustment knobs set tension to match your equipment weight. The design is compatible with electric focuser motors for automated focusing.

Three configurations available:

• ES-FOC3 — Focuser only, for Explore Scientific 152mm refractors and other-brand refractors (no back cell adapter)
• ES-FOC3-BC100127 — Focuser with back cell adapter for the FCD100-127-CF (carbon fiber tube)
• ES-FOC3-BC100127T — Focuser with back cell adapter for the white tube ED127 and FCD-100 series. Explore Scientific notes that white-tube telescopes with threaded back cells must be shipped to Explore Scientific to have the back cell adapter installed.

Key Features

• Self-centering compression system — zero tilt, zero image shift
• 9:1 helical rack-and-pinion with stainless steel ball bearings
• 17 lb lifting capacity — supports heavy imaging trains
• 84.5mm (3.32") drawtube travel
• 14 internal baffles for stray light suppression
• M82 x 1.25 drawtube thread for adapters; 3"-to-2" twist-lock adapter included
• Locking clutch and tool-free tension adjustment
• Electric focuser motor compatible

Ideal For

• Astrophotographers with heavy imaging trains — 17 lb capacity handles cameras, filter wheels, and off-axis guiders without flex
• Owners of Explore Scientific 120mm+ refractors — an upgrade from the stock focuser; see the configurations above for the back cell adapter your tube needs
• Visual observers who want fine focus control — the 9:1 ratio makes fine focus effortless at high magnification