Primary Mirror for Telescope, Sky-Watcher SW-5508 (305 mm F/4.0)

Primary mirror for telescope Sky-Watcher SW-5508 with a 305 mm diameter and an F/4.0 focal ratio (effective focal length 1200 mm). This primary (main) replacement mirror is designed for Dobsonian and Newtonian-style reflectors and functions as the primary light-gathering and focusing element. It restores the optical performance of the telescope by re-establishing the correct focal plane and image formation for both visual observation and astrophotography.

Advantages

The mirror offers a large 305 mm aperture that increases light collection for improved resolution and contrast on faint deep-sky objects. Its F/4.0 focal ratio provides a fast optical system that produces wider fields of view and shorter exposure times for imaging. As an OEM-compatible replacement component, the mirror is manufactured to match the optical geometry required for seamless installation into compatible Dobsonian and Newtonian telescope tubes.

Suitable for

• Repair or refurbishment of Dobsonian and Newtonian reflectors that use a 305 mm, F/4.0 primary mirror
• Deep-sky observation of nebulae, galaxies, and star clusters
• Wide-field and short-exposure astrophotography where a fast focal ratio is required
• Visual planetary and lunar observing with appropriate secondary and focuser setup

Key specifications

• Primary mirror material: optical-grade glass (coated reflective surface)
• Clear aperture: 305 mm diameter
• Focal ratio: F/4.0
• Effective focal length: 1200 mm
• Optical configuration: parabolic surface suitable for Newtonian/Dobsonian reflectors
• Surface finish and coating: high-reflectivity coating optimized for visible wavelengths

How to use

Install the mirror as the primary element within the telescope tube following the manufacturer's mechanical mounting points and cell alignment procedure. Ensure the mirror is seated securely in its cell and that any retaining clips or pads are fitted according to the telescope design. After mechanical installation, perform a coarse collimation using a Cheshire eyepiece or laser collimator, then refine alignment with a star test or collimation routine to achieve optimal on-sky performance. Handle the mirror by its edges and avoid touching the optical surface; use clean gloves or finger cots if adjustment by hand is required.

Usage recommendations

For reliable results, perform final collimation at night on a bright star and allow the mirror to reach thermal equilibrium with ambient air before critical observing or imaging sessions. Use appropriate secondary mirror size and focuser backfocus to maintain correct field illumination and focus. Clean the mirror surface only when necessary and follow gentle, non-abrasive cleaning procedures with suitable optical solutions and lint-free wipes to preserve the reflective coating.