Quick take: An optical tube that produces a sharp center does not necessarily cover a larger camera sensor at the edges. Read the full checklist and compare the linked original resources before making a purchase or a high-stakes decision.
Start with the illuminated and corrected field
Manufacturers may describe image circle and corrected coverage in different terms. Look for star shapes and illumination at the corners of the sensor you intend to use.
Know the role of correctors and spacing
Reducers, flatteners and coma correctors can change effective focal length and field quality. Check the intended spacing and compatibility with a camera, filter wheel and adapter stack.
The mount remains critical
Even a fast astrograph cannot correct tracking errors. Include the imaging train and accessories in the weight and balance calculation, and consider cables and flexure.
Use image scale as a planning tool
Calculate sampling and field of view for your proposed combination, then compare those numbers with typical seeing and the objects you hope to photograph. Avoid treating a single specification as the entire buying decision.