Optec Inc. Optec-DSI 3.5x24tpi Extension Tube Set, 4-piece

Innovative Collaboration: Jointly developed by Deep Sky Instruments and Optec, catering to large aperture telescopes.
Astronomy Industry Standard: Complements the Astro-Physics 2.7-inch spacer set, expanding the industry standard to larger apertures.
Enhanced Light Passage: Designed to allow more light through than AP2.7 spacers, avoiding edge ray clipping.
Precision Design: Features male and female threads for seamless integration and ease of stacking.
High-quality Material: Crafted from high strength 6061 aluminum alloy, ensuring durability and stability.
Anti-reflective Features: Black anodized with anti-reflection threading and internal baffles to minimize internal reflections.
Optimized Spacer Combinations: Available in multiple lengths for various telescope configurations. Includes a reference table for optimized spacer combinations.
Versatile Use: Ideal for fixed focal length telescopes, with extensions available in 1/4" (6.35mm) increments.
Professional Finishing: All internal surfaces are painted and baked with a non-reflective coating for enhanced optical performance.
Knurled Bases: Facilitates easy attachment and detachment, adding convenience for astronomers.
Includes: 1/2", 3/4", 1", and 1-1/2"
Possible spacer combinations:
0.5" through 3.25" with 1/4-inch increments plus 3.75" with all spacers combined. (See Table for combinations.)

Specifications