I had a situation this evening where an exposure started after my meridian flip was due.
When SGP checked whether the flip was needed, it was just before the deadline:
[9/17/2015 1:47:51 AM] [DEBUG] [Sequence Thread] Checking if Meridian Flip is needed [9/17/2015 1:47:51 AM] [DEBUG] [Sequence Thread] Telescope is on the West side of the mount [9/17/2015 1:47:51 AM] [DEBUG] [Sequence Thread] Meridian Flip not needed, Hour Angle < Degrees Past To Flip: -0.0965208333333345 < 0
but then SGP did an AF (temperature triggered):
[9/17/2015 1:47:51 AM] [DEBUG] [Sequence Thread] TEMP - Current Event3: 0 [9/17/2015 1:47:51 AM] [DEBUG] [Sequence Thread] Waiting for AF to complete... [9/17/2015 1:47:51 AM] [DEBUG] [Camera Thread] SGM_FOCUSER_AUTO_FOCUS message received... [9/17/2015 1:47:51 AM] [DEBUG] [Camera Thread] Checking for auto focus... [9/17/2015 1:47:51 AM] [DEBUG] [Camera Thread] Auto focus required (temperature change trigger). Last temp: 27.9541666666667; Current temp: 27.3541666666667
and by the time the AF was done, the Meridian flip deadline had passed, yet SGP still started the exposure. I was doing 20 min subs so the exposure did not finish until about 25 minutes past my desired meridian flip time.
It would have been nice if SGP had checked for the meridian flip right before the exposure started and then it would have seen it was too late to start the exposure.
I think you want auto focus to run right before taking the image for a couple of reasons
For SCTs, if you focus prior to the flip you’ll have considerable mirror flop.
Running a meridian flip can potentially take a while, especially if you have the “wait for meridian” option checked and the temp could change.
If your mount can’t track past the meridian for your exposure length plus a little wiggle room, then it’s likely best to use the “wait for meridian” option so you can guarantee the flip happens at your specified position.
I will play devils advocate on the first point… for SCT’s it seems like they should be using “AF after centering action” to cover this scenario. The temperature-triggered AF is non-deterministic, an SCT user can’t know when the AF will fire relative to the flip.
And for your second point, I agree, checking for AF right before the image seems important especially if there was any time delay.
I don’t know if it’s too complicated to add an additional a check for meridian flip right before the exposure, just throwing that out for consideration.
I can work around this on my end now that I better understand what to expect – I need to plan for tracking exposure+AF past the meridian, and if that is not possible, I need to select “wait for meridian”.
One more thought… it seems like I may need to choose “wait for meridian” or not depending on my exposure duration. If I’m doing narrowband, I need to select it; if I’m doing shorter LRGB exposures, I can omit it. It would be cool if I could just tell SGP how far I my mount can track past the meridian and let SGP manage things automatically. The “wait for meridian” option could be replaced by a “mount can track X minutes past meridian”, and SGP would schedule things accordingly. Is that a reasonable feature request?