Request for a "Re-centre every x frames" trigger for the new sequencer

Superb work on the new sequencer. What a great effort, and a most-welcome addition to NINA.

I went to the NINA discord today for the specific purpose of proposing the addition of this feature, as a trigger in the new sequencer, however, I see that @NickHolland has already proposed it, so I thought I might formalise the request here.

The lack of a recovery mode in NINA is about the only obstacle that keeps me from using it across several imaging trains. I tried NINA last year for about three months but often woke to hours of ruined subs due to PhD walking the mount off-target when a cloud passed through (a nearly-guaranteed event here on any given night, unfortunately). I couldn't find (and still cannot find) any setting in PhD that would reliably stop it from detecting clouds as stars, so I reluctantly gave up on NINA and went back to the other sequencing software I had been using, which has a recovery mode. These false-positive detections are still sometimes frequent enough to "trick" the sequencer into thinking that guiding is ok, and thus no recovery mode is actioned, so on occasion I still discard hours of off-target subs.

If NINA could implement a "Check centring every x frames" then that would negate the need for a recovery mode to handle this problem. I would like to propose that the required plate solve could optionally use the most recently-download image. This would save the time of having to do the "change to the L filter, take a short image, then solve" routine. Sequencer images are usually of much longer duration, and thus should provide a high SNR source for the plate solver, resulting in less chance of plate solve failures. If the plate solver was ASTAP then the time taken for confirming centring would only be a few seconds of additional time, considering these images are almost always well-centred, so even checking every downloaded frame would not cost too much imaging time over the duration of a full night.

In terms of parameters, a centring tolerance in pixels would be required, but that parameter could perhaps be the same as the parameter currently used by NINA's centring function. Another parameter could be a search radius (-r option in ASTAP I think). Because we only need to confirm whether the target centring is still within tolerance or not (a few 10's of pixels usually), the search radius can be very small, and a small search radius will make the plate solve lightning fast too, so this parameter would be most likely independent of the equivalent in the Plate solving tab.

In terms of operation, if centring is confirmed to be within tolerance, the sequence simply continues. If plate solve succeeds but the centring is out of tolerance, run a re-centring action and continue the sequence. If the plate solve fails ... I'm not sure what to do here; it would depend on the failure mode:

(1) image centre is too far off target (outside search radius);
(2) clouds prevented plate solve;
(3) mount is on-target, no clouds, but plate solve simply failed on that image.

If (1), we would want re-centring to occur immediately, but if (2) we would not want immediate re-centring; centring should be retried after a period of time has passed (e.g., retry every M minutes). If (3), just continue imaging. Unfortunately the cause of a plate solve failure cannot be determined. All of these three failure modes are rare events anyway, so perhaps an OK compromise would be to simply pass the image over to the blind solver. If it succeeds then re-centre (if necessary) and continue the sequence. If it fails, clouds are most likely the cause, so retry every Y minutes via the current centring function.

Please feel free to critique the above; I'm sure these is much room for improvement.

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