The intuitive mistake
The intuitive reasoning goes: a longer focal length guidescope gives a larger image scale, which means guide star position changes are larger in pixels, which means finer tracking errors can be detected. This sounds correct — and for ideal conditions, it is. But in practice the opposite is often true.
Why short wins in practice
Flexure
A longer, heavier guidescope introduces more flexure. As the mount slews, the guide scope shifts slightly relative to the imaging scope. This differential flexure appears as a guiding error — the guide camera thinks the star has moved, sends a correction, and the imaging scope's actual pointing drifts. The longer and heavier the guidescope, the worse this problem becomes.
A short, lightweight guidescope — 60mm aperture is typical — has far less flexure. Mounted correctly with a good dovetail or rings, a 60mm guide scope effectively eliminates differential flexure as a source of guiding error.
Atmospheric seeing
In conditions with poor seeing, a long focal length guidescope amplifies atmospheric scintillation of the guide star. The guide star image wanders more in pixels, the guiding software overcorrects, and the result is more movement in the imaging train, not less. A shorter focal length produces a smaller image scale — so the same seeing moves the guide star fewer pixels — and the guiding loop remains stable.
Guide star availability
A short guidescope has a wider field of view, which makes it much easier to find a suitable guide star anywhere in the sky. A long focal length guide scope may struggle to find a bright enough star within its narrow field without rotating it or repositioning the imaging setup.
The QHY5 guide camera
The QHY5 became the standard guide camera for a generation of astrophotographers for exactly the same reasons the short guidescope works: it was small, affordable, highly sensitive, and produced clean frames fast enough to drive an aggressive guiding loop. With the right guide software (PHD2 is now standard) and a 60mm guidescope, RMS values consistently under 1 arcsecond are achievable on a well-polar-aligned mount — even with a 250mm or larger imaging scope.
As a rule of thumb, the guide scope's focal length should be roughly 1/4 to 1/3 of the imaging scope's focal length. For a 1000mm imaging scope, a 250–330mm guide scope is appropriate. A 60mm f/4 guide scope (240mm focal length) works well for most setups up to around 1500mm imaging focal length.