1. Resolve the target and check it's observable

A target name is resolved to RA/Dec (or you can type coordinates directly). Before anything else, the target's declination is checked against the chosen instrument's site latitude: if the target can never climb above 20° altitude there, the run stops immediately with an error instead of spending time on a catalog query for a field that can never be observed.

2. Query Gaia DR3 around the target

All catalog stars within a circle around the target are pulled from Gaia DR3 — wide enough to cover the instrument's footprint even when the optimizer ends up pointing off-target. The official ESA archive is tried first; if it's unavailable, a VizieR mirror (I/355/gaiadr3) is used instead. Because Gaia photometry at a given sky position doesn't change, successful lookups are cached, so re-running the same target (after tweaking the margin or magnitude filter, say) skips the network round trip.

3. Score every star as a comparison candidate

Each catalog star (other than the target) gets a usefulness weight:

• Magnitude: peaks for stars a touch fainter than the target (ΔG ≈ +0.3 mag is ideal) and falls off smoothly on either side. Stars brighter than the target by more than 1 mag are penalized hard, since a bright comparison caps the exposure time needed to avoid saturating it — which shortens the target's own exposure and hurts its SNR.
• Color: stars with a similar Gaia BP−RP color to the target get a bonus, since similar colors track atmospheric extinction more alike and make for cleaner differential photometry.
• Crowding: a star with a comparably bright neighbor within 12″ is downweighted, since a close neighbor blends into the same aperture.
• Your Gmag range (or delta-Gmag) and margin settings are then applied, and anything below a minimum usefulness floor is dropped entirely.

If an "avoid brighter than" limit is set, any star brighter than it (other than the target) becomes a hard constraint in the next step: no pointing may place it inside the footprint.

4. Search for the best pointing and orientation

The field center doesn't have to sit exactly on the target. The optimizer grid-searches candidate field-center offsets together with a position angle (skipped if "allow field rotation" is unchecked, which fixes the field at PA=0°) — a square footprint repeats every 90°, so only that range needs to be searched. For every candidate pointing that still keeps the target inside the footprint (respecting your target margin) and avoids any "too bright" star, the comparison stars that land inside the footprint (respecting the comp margin) are summed by weight. The pointing and position angle with the highest total score wins.

If no candidate pointing can satisfy every constraint — usually because a bright star can't be dodged anywhere in reach — the optimizer reports that explicitly rather than silently returning a compromised field.

5. Result

The winning field center, position angle, and offset from the target are reported alongside the selected comparison stars (magnitude, color, separation, and weight) and the footprint polygon, which is overlaid on the Aladin sky view for a visual check before you commit to a pointing.

Footprint size is read from data/FOV_*.vot.xml; the polygon shown reflects the optimized pointing and position angle. Pan/zoom freely — “Recenter view” returns to the optimized field.

Proper motion arrows (when enabled) point in each star's direction of motion, colored by category (target vs. comparison). Arrow length is exaggerated and scaled relative to the fastest-moving star shown — only useful for comparing direction and relative speed between stars, not the true angular scale. A comparison star with an arrow parallel to, and about as long as, the target's suggests a common proper-motion (co-moving) companion.