Every catalog star sits at a fixed direction on the sky. For each pair of stars simultaneously in the field of view, the tracker measures the angular separation between them. A pair is only useful for identification if that separation is unique among all currently visible pairs — a duplicated separation is ambiguous and can't be resolved to one catalog match.
- A star counts as identified once it anchors at least two unique-distance pairings — enough to fix it uniquely in the catalog.
- Three or more identified stars let the tracker solve a single rigid rotation that maps catalog directions onto the observed image: lock.
- Sparse sky regions, glare blocking part of the field, or fast slew (which blurs faint stars below the effective detection limit) all shrink the usable star set — below three identified stars, the solution is undetermined: lost lock.