Optec DSI 3.5" Extension Tube Set - 5 Pieces

The Optec DSI 3.5" Extension Tube Set is used to achieve the correct back focus distance between a telescope's corrector or focuser and the imaging camera sensor. This five-piece set allows you to build a wide range of precise lengths from 0.5 to 6.75 inches to match the requirements of your specific optical train, which might include cameras, filter wheels, and off-axis guiders.

To achieve 2.25 inches of spacing, you would combine two tubes from the set. Stacking the 1-1/2 inch tube and the 3/4 inch tube together (1.5 + 0.75) gives you exactly 2.25 inches. The set is designed to create any length from 0.5" to 6.25" in 1/4" increments by combining different tubes.

The DSI 3.5-inch x 24 TPI (threads per inch) standard was developed by Deep Sky Instruments and Optec for large-format imaging systems that require a wide, unvignetted light path. Optec uses it, for example, on the camera side of the Gemini Focusing Rotator through the #19830 adapter.

Yes. All tubes in the Optec DSI 3.5" set feature internal anti-reflection threading to trap stray light. The largest 3-inch extension also includes internal baffles , which help minimize internal reflections.

Yes, all five tubes can be stacked together to achieve the maximum possible extension of 6.75 inches (171.45 mm). Optec's drawings show a self-centering contact surface on each tube; when stacking multiple components, it is important to ensure each connection is firmly tightened to maintain rigidity in the imaging train.

The five-piece set includes the following extension tube lengths:

• 1/2 inch (12.7 mm)
• 3/4 inch (19.05 mm)
• 1 inch (25.4 mm)
• 1-1/2 inches (38.1 mm)
• 3 inches (76.2 mm)