AstroGearFit
Compare image scales
The main camera and guide camera do not need identical focal lengths or pixel sizes. What matters is the relationship between their image scales and how well the guide camera can measure star movement.
Use the guiding-ratio calculator as a first check. A ratio that looks reasonable on paper can still fail if the guide scope flexes, the stars are soft, or the guide camera is noisy.
Rigidity beats theoretical perfection
A guide scope that shifts slightly relative to the imaging telescope can make guiding corrections look good while the main camera still shows elongated stars. Attach it firmly, keep the dovetail short where possible, and check the focus after the system cools down.
For longer focal lengths, an off-axis guider may be worth considering because it shares the main telescope’s optical path.
Keep the setup easy to use
A guide camera that is sensitive enough to find stars, supported by stable drivers, and simple to focus is usually a better choice than one with a perfect specification that becomes a nightly struggle.
Important: These calculators are planning aids, not guarantees of equipment performance or mount safety. Check manufacturer specifications and test your own system.