Guide Scope

Specifications

Observing parameters

Overview

A guide scope is a short, small refractor bolted alongside the main imaging telescope. A guide camera on it locks onto a star and the autoguider sends tiny correction pulses to the mount whenever that star drifts, compensating for periodic error, polar alignment error and flexure.

The key number is the ratio between the two. If the guide scope's focal length is much shorter than the imaging telescope's, guiding errors get magnified in the main image. A common rule is to keep the guide focal length at roughly a third to a fifth of the imaging focal length, and to make sure the mechanical connection between them is rigid — a guide scope on a flimsy shoe flexes, and flexure looks exactly like drift.

Off-axis guiders avoid flexure entirely by picking off light after the main optics, but they need brighter guide stars and a steadier hand.

Where it fits

Practical notes

• Mount the guide scope rigidly on rings, not on a camera shoe — flexure ruins guiding invisibly.
• Focus the guide scope carefully; a defocused guide star gives noisy correction.
• Choose a guide exposure long enough to average the seeing but short enough to correct drift.
• Keep guide cables routed so they do not snag as the mount slews — a tug shows up as a spike in the graph.

Pairs with

• Autoguider
• CMOS Camera
• Dew Heater
• Equatorial Mount