Because focus is a position rather than a setting. The telescope forms its image at a fixed distance behind the optics, and if the sensor cannot be placed there, no amount of turning the focuser helps. An extension tube or a shorter train is the fix.
This is the commonest first frustration in astrophotography, and it is arithmetic rather than a fault.
What focus is
The telescope brings light to a sharp point at a fixed distance behind its optics. The focuser moves the eyepiece or the camera to find that point, and it has a finite amount of travel.
Why a camera changes the problem
An eyepiece forms its image inside the barrel, close to the focuser. A camera sensor sits behind a bayonet, a ring and often a nosepiece, which places it much further back. Many telescopes simply cannot run the focuser in far enough to compensate.
The two fixes
Shorten the train, usually by removing the diagonal, or lengthen it with an extension tube when the problem is the other way round. A tube of the right length is arithmetic, not judgement — measure the shortfall rather than ordering a set.
The design that rarely has the problem
A folded telescope, where focusing moves the primary mirror and the usable distance behind the tube is generous. That is exactly why those tubes adapt so easily to cameras.
The questions that keep arriving
Which way do I need to go?
If the focuser runs all the way in and the image is still not sharp, the sensor is too far out and the train needs shortening. If it runs all the way out, the sensor is too close and an extension is needed.
Does removing the diagonal help?
Often, and it is the first thing to try on a refractor. A diagonal consumes a considerable length of optical path, and taking it out frees that length for a camera.
Last reviewed 17 September 2026