Conventional telescope focusers have some form of sliding bearing to allow the draw tube to move linearly with respect to the telescope tube. A rack and pinion gear is used to adjust the focus. Making linear sliding bearings like this requires precision engineering, so good quality focusers can be very expensive. The focusers supplied with cheap telescopes are often poorly made, with a lot of wobble and backlash that makes focussing difficult.
This alternative design uses rotary bearings to support the eyepiece on a parallelogram. Unlike a slider arrangement, the eyepiece now moves in an arc instead of a straight line. To compensate for this, the diagonal mirror is mounted on another parallelogram, offset by 45 degrees and a factor of sqrt(2) smaller.
In this diagram the green part is fixed to the telescope tube, the blue part carries the eyepiece, the purple part carries the diagonal mirror and the red “elbows” hold everything together. The white circles represent bearings and the dashed line shows the optical axes of the telescope and eyepiece.
Any practical focuser based on this idea will probably be more compact – the diagram is intended to show how it works, not how it should be built. A major problem will be putting it in a light proof enclosure.