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:
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Diameter: 125 mm (~5 inches)
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Effective Focal Length (EFL): ~500 mm (f/4 focal ratio)
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Total Edge Thickness: 22.5 mm
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Glass Pair: BK7 crown (low-dispersion) + F2 flint (higher-dispersion) – classic visual achromat
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Design: Cemented doublet (plano-convex overall shape)
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Coating: Likely uncoated or single-layer MgF₂ (minimizes reflections)
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Matching Marks: Both elements etched with “5” – indicating a matched pair for optimal performance
Why This Doublet Is Exceptional:
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Superior Color Correction: The BK7/F2 pairing significantly reduces chromatic aberration compared to a singlet, delivering sharper, truer color images.
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Large Aperture & Fast f/4 Ratio: Provides excellent light-gathering power in a relatively compact optical tube.
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Matched Pair Assurance: The etched “5” on both elements confirms they were factory-matched for optimal alignment and performance.
Applications:
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Refractor Telescope Objective: Ideal for building a high-performance 5-inch (125mm) f/4 amateur or educational telescope.
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Laser Beam Expansion/Collimation: As a large-aperture output lens for expanding or collimating visible/NIR laser beams.
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Imaging & Machine Vision: Primary lens for custom long-focal-length imaging systems or specialized optics.
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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.



