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Telescope camera thread standards: T-thread, M42 and SCT

Attaching a camera or an accessory to a telescope runs through a small set of standard threads, and knowing their pitches is what lets you build a chain that actually screws together.

The T-thread, or T2

The T-thread, usually called T2, is the workhorse camera connection in astronomy. It is a male thread of 42 millimetres diameter with a 0.75 millimetre pitch. A T-ring specific to your camera's lens mount screws onto the T2 thread and adapts the camera body to the telescope side.

Because the pitch is 0.75 millimetres, the T2 thread is distinct from the similarly sized M42 photographic thread even though both are 42 millimetres across.

M42 photographic thread

The old M42 lens mount, sometimes seen on vintage camera adapters, is also 42 millimetres in diameter but with a coarser 1 millimetre pitch. It will not mate correctly with a 0.75 millimetre T2 thread. When buying rings and adapters, confirm the pitch, because a 42 millimetre thread alone does not guarantee a fit.

In practice most modern astronomy accessories use T2 at 0.75 millimetre pitch, so match to that unless an item explicitly states M42 at 1 millimetre.

The SCT rear-cell thread

Schmidt-Cassegrain telescopes carry a standard thread on the rear cell, the 2 inch, 24 threads per inch SCT thread. Visual backs, focal reducers and camera adapters screw straight onto it. This is why so many Schmidt-Cassegrain accessories are cross-compatible: they share this common rear-cell thread.

Larger Schmidt-Cassegrains sometimes use a wider rear thread, so on big tubes confirm whether the rear cell is the standard 2 inch thread or a larger one before buying reducers and adapters.

Building a connection chain

A typical imaging chain runs from the telescope's rear thread or focuser, through any reducer or Barlow, to a T2 thread, then a camera-specific T-ring, then the camera. Each junction has to match in both diameter and pitch. Working out the thread at each end and filling the gaps with the right adapter is the whole task of connecting a camera to a telescope.

Find the adapter your telescope and camera need

Open the Fit My Telescope gear finderIdentify your telescope's rear thread and the T-ring to match.Find T2 T-ring telescope camera adapter on eBayaffiliate

Work out your telescope's specsRear thread and back focus Choosing a BarlowBarlow in the imaging chain Celestron NexStar 8SESCT rear cell Celestron C6SCT rear thread

Sources: Celestron SCT rear cell thread documentation · T-thread and T-ring standard reference