Optec-DSI 3.5-inch Telescope Extension Rings, Adapters and Retaining Rings

Optec-DSI 3.5-inch Telescope Extension Rings, Adapters and Retaining Rings - View 3

Optec-DSI 3.5-inch Telescope Extension Rings, Adapters and Retaining Rings - View 4

Optec-DSI 3.5-inch Telescope Extension Rings, Adapters and Retaining Rings - View 5

Optec-DSI 3.5-inch Telescope Extension Rings, Adapters and Retaining Rings - View 6

$120.9 ~~$195~~ In stock

Category: Astrophotography Accessories

Description

Rich Simons of Deep Sky Instruments and Optec have jointly developed a new, larger diameter standard spacer for large aperture telescopes. Like the Astro-Physics 2.7-inch spacer set, the Optec-DSI 3.5-inch spacers are available in a five different lengths.

The AP 2.7-inch thread has become an astronomy industry standard thanks to the tireless efforts of Roland and his team at Astro-Physics. Optec has produced a complementary extension set and a few new adapters in an effort to extend this industry standard. However, there are times when the 2.5" clear aperture of the AP2.7 spacers will not allow enough light through clipping the edge rays.

Optec-DSI 3.5-inch extension tubes feature female threads on one end with male threads on the opposite end with knurled bases to help attach and detach these tubes. The extensions can be combined and stacked. We believe our four and five piece sets will provide all the needed extension for fixed focal length telescopes with 1/4" (6.35mm) increments using a minimum number of pieces.

All extensions and mounts are made from high strength 6061 aluminum alloy and black anodized. Anti-reflection threading and internal baffles on the largest extension tube help minimize internal reflections. All internal surfaces are painted and baked with a non-reflecting ultra-flat black paint.

Specifications

Spacer Lengths Available: 1/2", 3/4", 1", 1-1/2", and 3"
Clear Aperture: 3.25" (82.5mm)
Back-focus Usage: Variable (see TABLE below)

Extension Tube Sets

Stock #19889 DSI 3.5 Extension Tube Set, 4-piece set 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 below for combinations.)
Stock #19890 DSI 3.5 Extension Tube Set, 5-piece set Includes: 1/2", 3/4", 1", 1-1/2", and 3" Possible spacer combinations: 0.5" through 6.25" with 1/4-inch increments plus 6.75" with all spacers combined. (See table below for combinations.)

Individual Extension Tubes and Retainers

Stock #19891 DSI 3.5 Extension - 1/2" Length Overall Height: 0.88" (22.4mm) Clear Aperture: 3.25" (82.5mm) Weight: 1.0 oz. (30 g) Back-focus Usage: 0.50" (12.7mm)
Stock #19892 DSI 3.5 Extension - 3/4" Length Overall Height: 1.13" (28.7mm) Clear Aperture: 3.25" (82.5mm) Weight: 1.4 oz. (40 g) Back-focus Usage: 0.75" (19.1mm)
Stock #19893 DSI 3.5 Extension - 1" Length Overall Height: 1.38" (35.0mm) Clear Aperture: 3.25" (82.5mm) Weight: 1.8 oz. (55 g) Back-focus Usage: 1.00" (25.4mm)
Stock #19894 DSI 3.5 Extension - 1-1/2" Length Overall Height: 1.87" (47.5mm) Clear Aperture: 3.25" (82.5mm) Weight: 2.4 oz. (70 g) Back-focus Usage: 1.50" (38.1mm)
Stock #19895 DSI 3.5 Extension - 3" Length Overall Height: 3.38" (85.8mm) Clear Aperture: 3.25" (82.5mm) Weight: 8.2 oz. (230 g) Back-focus Usage: 3.00" (76.2mm)
Stock #19896 DSI 3.5x24tpi to AstroPhysics 2.7" female Adapter	No Picture Available
Stock #19897 DSI 3.5 Retaining Ring Overall Height: 0.14" (3.5mm) Clear Aperture: 3.45" (87mm) Weight: 0.4 oz. (15 g) Back-focus Usage: 0.10" (2.5mm) Anodize: Gold

Target Extension	Spacers Required	AP2.7 Spacer Stock Numbers	DSI 3.5x24 Stock Numbers
0.50”	½”	19882	19891
0.75”	¾”	19883	19892
1.00”	1”	19884	19893
1.25”	½” + ¾”	19882, 19883	19891, 19892
1.50”	1-½”	19885	19894
1.75”	1” + ¾”	19884, 19883	19893, 19892
2.00”	1-½” + ½”	19885, 19882	19894, 19891
2.25”	1-½” + ¾”	19885, 19883	19894, 19892
2.50”	1-½” + 1”	19885, 19884	19894, 19893
2.75”	1-½” + ¾” + ½”	19885, 19883, 19882	19894, 19892, 19891
3.00”	3”	19886	19895
3.25”	1-½” + 1” + ¾”	19885, 19884, 19883	19894, 19893, 19892
3.50”	3” + ½”	19886, 19882	19895, 19891
3.75”	3” + ¾”	19886, 19883	19895, 19892
4.00”	3” + 1”	19886, 19884	19895, 19893
4.25”	3” + ¾” + ½”	19886, 19883, 19882	19895, 19892, 19891
4.50”	3” + 1-½”	19886, 19885	19895, 19894
4.75”	3” + 1” + ¾”	19886, 19884, 19883	19895, 19893, 19892
5.00”	3” + 1-½” + ½”	19886, 19885, 19882	19895, 19894, 19891
5.25”	3” + 1” + ¾” + ½”	19886, 19885, 19883, 19882	19895, 19893, 19892, 19891
5.50”	3” + 1-½” + 1”	19886, 19885, 19884	19895, 19894, 19893

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Customer Reviews (5.0 / 5 · 5 reviews)

Victoria S. ★★★★★ Oct 21, 2026

These spacers made dialing in the exact back focus on my large Newtonian so much easier. The threads are buttery smooth and the anodized finish matches my other gear perfectly. Exactly what I needed for my imaging train.

Abigail P. ★★★★★ Oct 20, 2026

The retaining rings fit perfectly on my wide-field setup, and the threading is noticeably smoother than my older adapters. It saved me from having to shim or jury-rig spacers during precise collimation.

Sofia V. ★★★★★ Oct 19, 2026

I’ve been looking for a larger standard spacer for my big aperture scope, and this set fits the bill perfectly. The threads are clean and mate up without any binding, making my image train rock solid.