A small tube whose only job is to watch one star
A guide scope is a small telescope that never looks at anything worth seeing. It watches a single star, hands the position to a camera and software, and the software nudges the mount to keep that star exactly where it was — which is how an exposure of ten minutes holds round stars on a mount whose gears are not perfect. Because the job is measurement rather than viewing, the specifications invert. A short focal length is usually better than a long one, since a wide field makes it easy to find a guide star anywhere in the sky, and modern guide cameras have pixels small enough to measure a star accurately without magnification. Aperture only has to be enough to see a usable star, and adding more adds weight where weight is unwelcome. The property that decides whether the whole arrangement works is flexure: if the guide scope shifts by a fraction of a degree relative to the main tube during the exposure, the software will faithfully correct for a movement that never happened and smear the image. That is why rigid mounting rings, or a solid shoe with no adjustment screws to creep, matter more than the optics. Read the focuser — a helical focuser that locks is the right kind here — and read the back focus the camera needs. We compare aperture, focal length, mounting method, flexure resistance, focuser type, back focus and weight.
50 mm aperture · 200 mm focal length · helical focuser
A 50/200 guide scope with a brass compression-ring helical focuser, 40 mm of travel including 10 mm of fine focus, supplied with tube rings and a Synta-style dovetail plate.
Best for: Guiding a main telescope of moderate focal length, and doubling as a wide finder
≈ $78 / £88 / €107
60 mm aperture · helical focuser · male T-thread
A 60 mm guide scope with a non-rotating helical focuser and a male T-thread at the eyepiece collar for screwing a guide camera on directly.
Best for: A guide scope that will spend half its life as a finder
≈ $82 / £94 / €100
70 mm aperture · 400 mm focal length · 90 mm focus range
A 70/400 guide scope with a helical focuser offering 90 mm of range including 10 mm of micro-focus, supplied with tube rings, a Synta plate and an adjustable counterweight.
Best for: Guiding a longer main telescope, where a short guide scope would not resolve the correction
≈ $156 / £164 / €189
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