Inspect in this order, because the terminal faults are quick to check and the cosmetic ones are not worth time.
Look at the coatings, not the dust
Dust costs almost nothing. Look for patches where a mirror coating has lifted or clouded, and for fungus in a sealed refractor or a pair of binoculars — a branching pattern inside the glass. Those are the terminal faults.
Work the focuser through its whole travel
Feel for roughness, for sideways play in the draw tube, and for a lock that shifts focus as it tightens. A worn focuser is repairable and the repair is a real cost.
Move every mount axis with the clutches loose
Tight spots, grinding and backlash you can feel by hand are all reasons to negotiate. On a GoTo mount, ask to see it slew and align — electronics are where used equipment fails.
Check that the fittings are standard
A telescope with a proprietary eyepiece fitting, or the old narrow barrel, cannot be improved later whatever its condition. That changes what it is worth.
Count what is missing
A mount without its counterweights, a tube without its dovetail, a Dobsonian whose base has been replaced with something homemade — each of those is a purchase the price has to reflect.
The questions that keep arriving
Is a scratched mirror coating serious?
A few small marks scatter a negligible amount of light. Widespread deterioration of the coating — patches where it has lifted or gone milky — is a recoating job, and that cost has to come off the price.
What about a marked eyepiece barrel?
Cosmetic, and universal on eyepieces used in a thumbscrew diagonal. What matters is the glass and whether the eyecup mechanism still works.
Last reviewed 17 September 2026