What one thread standardises
A male thread of 42 mm diameter with a 0.75 mm pitch, written M42 × 0.75. It came from the T-mount, a camera adapter system designed so that one lens could serve many camera bodies through a cheap interchangeable ring, and astronomy adopted it wholesale because it solved the same problem in the other direction: one telescope, many camera bodies.
What follows is that the thread appears at both ends of nearly everything behind the focuser. A T-ring converts a camera bayonet to it. Extension tubes, reducers, filter wheels and camera noses carry it. An imaging train is assembled by screwing those together in a chosen order.
The convention fixes the thread and, with it, the flange focal distance of 55 mm between the thread face and the sensor. That second figure is what makes the whole system arithmetically predictable — a T-ring exists precisely to make up the difference between 55 mm and whatever distance a particular camera bayonet sits at.
What it does not promise
It does not promise that an assembled train reaches focus. Every component consumes optical path, and the telescope offers a finite amount behind it; a train that screws together perfectly can still fall short.
It does not promise correct spacing either. A reducer or a flattener demands a specific distance to the sensor, and the thread will happily hold the wrong one. Stars stretch outward at the corners when the spacing is long and inward when it is short — a symptom that looks optical and is arithmetic.
And it says nothing about diameter of the light path. A T2 thread has a clear aperture that will vignette a large sensor regardless of how well it is assembled, which is why the wider threads exist alongside it.
What it works with
The T2 thread is the junction between the telescope side and the camera side. It works with T-rings on one end and with noses, extensions, reducers and filter wheels on the other, and every one of those has to be counted in the back focus budget before ordering.
On this site it appears wherever a camera is involved: T-ring adapters, extension tubes, focal reducers, planetary cameras, filter wheels and the guide scopes that carry a camera of their own.
The questions that keep arriving
Is T2 the same as the M42 screw mount on old camera lenses?
The diameter is the same and nothing else is. The old screw lens mount is M42 × 1.0 with a flange distance of about 45.5 mm; T2 is M42 × 0.75 at 55 mm. They will start on each other and then bind, and even if one could be forced on, the 9.5 mm difference in flange distance would put the focus in the wrong place. Any listing that treats them as interchangeable is wrong twice over.
Why do imaging trains use threads instead of push-fit barrels?
Because a thread holds a fixed distance and cannot tilt. A barrel clamped by a screw can sag under a camera and shift between exposures, and a shift of a fraction of a degree shows as elongated stars at the corners.
Does a T-ring exist for my camera?
There is one for most common bayonets, but the mirrorless and reflex mounts of the same brand are different fittings with the same brand name. The ring has to match the exact bayonet on the body, not the maker.
Last reviewed 17 September 2026