How do I build a flat master for a night?

Where you start

Open a session and scroll to Flats this night. Click a filter’s row to open a dock beside the strip, scoped to that one night. The same dock opens from the rig’s flat timeline: click any night on the strip and it opens identically, with tabs for every filter that shot flats.

The dock’s tabs switch filters without leaving the dock. A green checkmark on a tab means a master already exists for that filter this night; a pulsing dot means a build is running.

When the most recent build attempt for the open filter ended in failure, a banner sits at the top of the dock reading ✗ build failed, or ✓ master · last rebuild failed where an earlier attempt already produced a master, with the error text and a Retry button. It stays until an attempt succeeds.

Building one filter

Inside a filter’s tab, click Build flat master. This opens the build panel, scoped to that filter’s flats for that config. While a build for that filter is in flight, the button’s place is taken by a pulsing Queued… or Building…, with a Cancel beside it, so a second click cannot start a duplicate. If evening and morning flats were both shot, the panel offers All, Evening, and Morning as separate checkboxes, each showing its frame count. Check one, two, or all three to queue that many masters.

Building every filter at once

Click Build all filters at the top of the dock to open the same panel scoped to every filter shot that night. It checks off every filter and sky-batch combination that does not already have a master, so one click catches up a night you have not touched yet. Combinations that already have a master start unchecked, so a repeat click does not duplicate work.

Screenshot: Flat night dock with filter tabs and the Build all filters button, one tab showing a completed master and another showing a build in progress.

The evening/morning split

A night’s flats are shown as one evening batch and one morning batch whenever a large enough gap in capture time separates them. A single unbroken run gives you only the All option. Where a split exists, building All combines both batches into one master; building Evening or Morning separately keeps them apart. Flats shot hours apart under a changing sky do not always blend cleanly into one master, so the split lets you build whichever grouping matches what you actually shot.

Rotation groups

When a filter’s flats for the night were shot at more than one rotator angle, the panel offers Build per rotation group, naming the groups and their angles, with the measured agreement between their surfaces printed beneath it. Checking it queues one master per rotation group instead of one combined master, and each master’s file name carries its group’s angle label (for example pa074). In the label, pa is position angle, the rotator angle recorded on the frames, and the number is that angle in degrees padded to three digits: pa074 is the 74° group. Frames with no recorded rotator angle form their own group, labeled noang.

Which way the checkbox starts depends on what the surfaces measured. When the groups agree, the combined master is the default, since it is the same surface with more frames behind it, and the split stays one click away. When they measure as different surfaces (a Rotation groups verdict), the split arrives pre-checked: one master per group is the plan for that night, on this panel and on Build all filters alike, unless the night is set to Merge anyway. If it is, the panel says so next to the checkbox.

Culling before you build

Flats render raw in their contact sheet, and the batch’s flat verdict sits above it. If the verdict flagged frames, reject them before building, either with the verdict’s own Reject N flagged action or by hand in the contact sheet. Frames already surface-flagged as suspect (dew, cloud, flicker) are excluded from the build by default; a checkbox in the panel’s footer opts them back in if you disagree with the flag.

Screenshot: Flat verdict banner above a night’s flat contact sheet, with one frame rejected by hand.

The bias picker

Below the frame selection, the panel lists every bias master that matches this flat’s camera configuration: instrument, gain, offset, readout mode, binning, and sensor dimensions. The most recently built matching bias master is pre-selected and marked best match. Pick a different one from the list, or choose No bias correction.

Choose a matching bias master when one exists: subtracting it removes the sensor’s bias pedestal before the flat frames combine. Choose No bias correction only when nothing in the list matches your camera configuration, which the panel tells you outright, or when you deliberately want an uncorrected flat.

If no bias master matches at all, No bias correction is the only usable option, and the panel explains which fields did not match your flats’ camera signature so you know whether you need a new bias master or a signature adjustment.

Screenshot: The flat master builder with the bias picker open and the evening batch selected.

Running the build

Click the build button and a pre-flight check runs first: it verifies the source flats are ready and, if you picked a bias master, that it is ready too and its geometry matches. A clean pre-flight submits immediately. A pre-flight with only warnings, such as an unusually small or large frame count, asks for one more click before it submits; anything blocking, such as a bias master whose geometry does not match, keeps that unit from submitting until you fix it or drop the bias.

Once submitted, the panel switches to a live progress view, one row per master queued: preparing source frames, splitting into tiles, combining, then saving the finished master as XISF. A completed row links straight to the Stacks page; a failed one tells you to check there for details.

Submitting a build that matches one already queued, the same frames and the same settings, tracks that job rather than starting a second one, and its row is marked · already queued. Submitting a build that conflicts with one already queued for the same slot lists it under This master was not started with the reason, and offers Cancel the queued build and submit this one. Where the queued build has already started combining, the panel says so and points you to the Stacks page to cancel it there.

Screenshot: Build panel showing live progress bars for two masters, one preparing and one combining.

The result

A flat master is built for one rig, one camera configuration, one filter, and the batch you built it for: all, evening, morning, or a single rotation group. A master built from one rotation group records that group’s angle, which is what lets calibration assignment hand each light the master nearest its own angle. It registers in your library the same way any master does, with no separate step: it appears immediately under Masters and on the dock’s tab as a green checkmark. From there, how calibration is assigned covers how EigenFrame decides which lights it applies to.