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Afocal clamp for e.g. 36mm o.d. Nikon trinocular UW head eye tubes
SVBONY SV158 2 inch Extension Tube Telescope Extension Tube Optical
ordered from Amazon; received 30 July 2020
Intended for Astrophotographic Applications (75mm length), 48mm internal thread for about 35mm

Newer SV158s have 2 thumb screws at right angles; 2 were added with about 120 degree spacing.

Spacer ring cut from 16mm delrin using hole saws and cleaned up with sandpaper

48mm (male) to 42mm step-down without a flange

Initially used for chasing extension tube 48mm internal treads after cutting.
48mm i.d. section will be cut short,
so that 48-49mm step-up and 52-49mm lens step-down rings will be close to 30mm eyepiece.
A perhaps easier alternative from AliExpress: M48x0.75 - 2'' telescope, 49mm long
More nearly the correct length:

Seemingly the same from Amazon or StarDikor

The SV158 was cut to 42mm:

Real men would use a lathe and parting tool to cut it, but a 65mm chuck, drill press and hacksaw blade sufficed:

My SV158 had only a single uncomfortable thumb screw;  replacements ordered.


10 Aug 2022
Amazon thumbs screws arrived;
thread matches and more comfortable than stock SVBONY...

The clamp backside could be notched
to clamp farther down the eye tube.
This clamp will facilitate comparing ocular fields and afocal image quality among various UW trinocular schemes.

photomacrography.net:  Eye tube clamp for Nikon Optiphot/Labophot UWhead afocal imaging

Afocal clamp for CT phase telescope
The SVBONY SV158 48mm section lopped off, along with 48-42mm step-down and 1.25 clamp,
enable 40mm f/2.8 pancake lens with 52-48mm step-down to take afocal images from the CT phase telescope.

This was used to document a damaged PL 60x 160/0 objective:
maintained by blekenbleu