What the parameter fixes
Back focus is the distance from the last optical surface of a telescope, corrector or reducer to the plane where the image is formed sharply. When a maker publishes one, it is an instruction: put the sensor there.
Two figures have to be distinguished. The telescope’s own back focus says how much room exists behind it for anything to be inserted at all. A reducer’s or a flattener’s required spacing says exactly where the sensor must sit relative to that element — and unlike the first, it has no tolerance worth speaking of.
Everything in the train counts against it: the flange distance the T-ring establishes, the length of each extension, the optical path a filter wheel adds, and the depth from a camera’s own thread to its sensor.
What it does not promise
Reaching the right spacing does not promise a flat field. A reducer covers a stated image circle, and a sensor with a diagonal larger than that circle will vignette and soften at the corners however precisely it is spaced.
Nor does the printed reduction factor hold away from the specified distance. The factor is a function of the spacing, so a train that is deliberately lengthened produces more reduction than the box states and a shortened one produces less — which is why a measured field of view sometimes disagrees with the specification.
And it says nothing about tilt. A train at exactly the right length, hanging from a clamp that sags, produces corner defects that look identical to a spacing error and are corrected quite differently.
What it works with
Back focus governs every component between the telescope and the sensor, and it is the reason those components are sold in precise lengths rather than in convenient ones. Spacer rings of a millimetre or two exist for exactly this arithmetic.
On this site it appears on focal reducers, T-ring adapters, extension tubes and filter wheels, on the cameras at the end of the train, and on the telescopes whose own back focus decides whether such a train fits behind them at all.
The questions that keep arriving
How do I know whether my train is the right length?
By adding it up before ordering: the flange distance of the T-ring, the length of every extension, the optical thickness the filter wheel adds, and the depth from the camera’s own thread to its sensor. The sum has to equal the figure the reducer’s maker publishes.
My stars are elongated at the corners. Is that spacing?
It is one of the two usual causes, the other being tilt. Spacing errors are symmetrical about the centre and point outward when the train is too long and inward when it is too short; tilt is worse on one side of the frame than the other.
Does a filter change the spacing?
Yes. Glass in the path shifts the focal plane slightly backwards, and a filter wheel adds both its glass and its body. Makers of reducers usually state the figure with a filter assumed or excluded, and which one it is has to be read rather than guessed.
Last reviewed 17 September 2026