125mm achromatic doublet lens

Product Description

125mm Achromatic Doublet Objective Lens – ~500mm EFL, Plano-Convex Configuration (BK7/F2 Matched Pair)

This is a premium, large-aperture cemented achromatic doublet in a plano-convex configuration, designed for superior color correction and sharp imaging across the visible spectrum. With a 125mm clear aperture and approximately 500mm effective focal length (EFL), it delivers an impressive f/4 focal ratio, making it an outstanding objective for telescope builders, optical experiments, or laser systems.

Key Specifications:

• Diameter: 125 mm (~5 inches)

• Effective Focal Length (EFL): ~500 mm (f/4 focal ratio)

• Total Edge Thickness: 22.5 mm

• Glass Pair: BK7 crown (low-dispersion) + F2 flint (higher-dispersion) – classic visual achromat

• Design: Cemented doublet (plano-convex overall shape)

• Coating: Likely uncoated or single-layer MgF₂ (minimizes reflections)

• Matching Marks: Both elements etched with “5” – indicating a matched pair for optimal performance

Why This Doublet Is Exceptional:

• Superior Color Correction: The BK7/F2 pairing significantly reduces chromatic aberration compared to a singlet, delivering sharper, truer color images.

• Large Aperture & Fast f/4 Ratio: Provides excellent light-gathering power in a relatively compact optical tube.

• Matched Pair Assurance: The etched “5” on both elements confirms they were factory-matched for optimal alignment and performance.

Applications:

• Refractor Telescope Objective: Ideal for building a high-performance 5-inch (125mm) f/4 amateur or educational telescope.

• Laser Beam Expansion/Collimation: As a large-aperture output lens for expanding or collimating visible/NIR laser beams.

• Imaging & Machine Vision: Primary lens for custom long-focal-length imaging systems or specialized optics.

• Optical Research & Education: Perfect for university labs teaching aberration theory, lens design, or hands-on optical system assembly.

Mounting Note:

Due to its size and weight, a sturdy cell with adjustable supports is recommended for telescope use to avoid strain and maintain alignment.