Product Overview
The Lunt Solar 1.25" single-speed rack and pinion focuser is a precision mechanical upgrade designed for telescopes where helical focusers limit optical performance. it is compatible with the Lunt LS40, and LS50. Acting as the mechanical interface between the optical assembly and attached accessories, the focuser plays a critical role in maintaining alignment, focus accuracy, and overall system stability. This model is intended for applications where repeatable focus, higher load capacity, and long-term reliability are required.
Limitations of Helical Focusing Systems
Helical focusers use threaded motion to translate the drawtube, a design that is simple but inherently sensitive to manufacturing tolerances, wear, and thermal expansion. Over time, these factors lead to increased backlash, uneven focus response, and reduced positional accuracy. Under modern accessory loads such as heavier eyepieces, cameras, or filter assemblies, helical mechanisms often exhibit focus drift, binding, or loss of alignment, particularly at higher magnifications.
Rack and Pinion Focusing Mechanism
The Lunt focuser replaces the threaded approach with a precision rack and pinion drive system. Linear motion is generated through a machined rack engaged with a matched pinion gear, allowing mechanical loads to be shared across the gear interface rather than concentrated along a single thread. This results in smoother motion, improved repeatability, and significantly greater resistance to deflection under load.
The rack and pinion assembly is designed to support equipment loads of approximately 6–8 pounds (2.7–3.6 kg) without measurable focus shift, a substantial improvement over typical helical focusers, which often become unstable beyond 2–3 pounds.
Materials and Mechanical Construction
All major components are CNC-machined from aluminum and brass, selected for their strength, corrosion resistance, and stable behavior under changing temperatures. Precision machining and controlled tolerances provide consistent performance over the full range of travel, with positioning accuracy of ±0.002 inches (±0.051 mm) and backlash below 0.001 inches when properly adjusted. These characteristics ensure reliable focus placement for both visual observation and imaging.
Internal Brake and Load Control
An integrated internal brake system is included to manage the effects of gravity and accessory mass on the drawtube. The brake provides adjustable drag, allowing resistance to be tuned to the installed payload. When engaged, it offers positive locking without introducing localized stress or deformation, preventing focus creep while preserving smooth operation. This feature is particularly beneficial in applications where focus stability must be maintained over extended observation or imaging sessions.
Compression Ring Accessory Clamping
Accessory retention is handled by a brass compression ring rather than traditional set screws. This system applies uniform clamping force around the full circumference of inserted 1.25" accessories, reducing the risk of barrel damage and minimizing alignment errors caused by uneven pressure. The result is improved repeatability when changing accessories and more consistent optical alignment.
Travel, Rotation, and Compatibility
The focuser provides 0.5 inches of drawtube travel and features a rotatable body, allowing users to adjust the orientation of eyepieces or imaging equipment without affecting focus position. Its mechanical design makes it well suited for solar telescopes, refractors, and retrofit installations where increased rigidity and focus precision are required.
