Observing

Standard mosaic rotation

The Projects sky preview renders from the viewer's current centre immediately, including before coordinates have been typed or the sky has been moved. The yellow reference camera frame remains visible during mosaic editing.

Every mosaic exposure carries these preferences through project saves, copies, imports and calendar payloads:

Field	Default	Meaning
`mosaic_square_by_camera_rotation`	`true`	Align the pointing grid with the measured camera axes.
`mosaic_custom_rotation_enabled`	`false`	Override camera alignment with an absolute grid angle.
`mosaic_custom_rotation_deg`	`0`	Custom angle in degrees, using the same convention as `image_rotation`.

Negative angles and multiple turns are accepted. Custom rotation takes priority when both checkboxes are selected. Turning both off selects a zero-degree RA/Dec grid. These settings rotate the grid and its requested outline; camera frames retain their measured orientation. They do not command a hardware rotator. SmartStack FOV remains a preview of the RA/Dec-aligned crop inside each frame.

The blue rectangle is fitted inside the union of the displayed panel footprints. It excludes uncovered gaps, holes and concave cutouts; it does not use the convex outer envelope. Its size and orientation are numerically estimated, so it may be smaller than the mathematical maximum. Disconnected panels can yield a rectangle within just one connected portion. This coverage diagnostic does not block a requested grid. Pointing counts are determined by the overlap planner below.

Width and height retain their existing meanings before grid rotation: width is an RA coordinate span, height a Dec span. Convert width to an on-sky angle at the declination in the requested interval closest to the equator. Panel counts and overlap use these dimensions. The planner uses grid-aligned coverage cells inside the full camera frames. Adjacent cells overlap by the requested fraction along each grid axis and their union covers the requested rectangle. Camera/grid angle differences reduce the usable cell dimensions and can require extra rows or columns. Every cell is checked against its projected spherical camera frame; counts refine as needed for changes in the local meridian and projection. This calculation neither crops the acquired detector frame nor changes the requested overlap. RA wrap and pole crossings use spherical projection. Panel order and progress indices remain column-major.

CodexOBS implements the matching transformation in mosaic_grid.py and applies it at each slew using the current mosaic centre, including cycle dithers and recentering. The plan is calculated after centering and cycle dither. If the camera has no measured rotation, OBS uses its existing centering exposure to measure it. A missing angle after that is reported as a planning failure: overlap cannot be calculated without the camera's orientation, even when the grid uses a custom angle or zero-degree RA/Dec alignment.