Question on meridian flip logic


I recently decided to try a night of imaging where I had two targets in a sequence file. The first target was acquired east of the meridian and the sequence performed an automated meridian flip with camera rotation properly and the target finished about 2AM. The scope was pointing to the north west at the end. The second target required the scope to point back to the east of the meridian and the scope did a “reverse” meridian flip, acquired the target (with camera rotation) and began tracking. However, immediately, the guiding began a “run away” so I stopped the sequence; performed a new calibration of PHD2 and resumed the imaging run without further problems.

Being 2AM, I did not take the time to trouble shoot the guiding issue but thinking about it later, it seems like PHD2 did not recognize that a reverse meridian flip had occurred and did not flip the calibration data.

I know that during the first meridian flip, SGP told PHD2 to reverse its calibration but I am thinking that when the second target was acquired, SGP may have not told PHD2 to reverse its calibration again.

PHD2 is connected to both the rotator and the mount but the option in PHD2 to flip the calibration data is unchecked since SGP is telling PHD2 to flip.

I did not bring any of the log files home with me but it seems like SGP did not tell PHD2 about the second meridian flip.



Hi Charlie,

If you have your mount connected in PHD2 with an ASCOM connection, then messages from SGP to PHD2 telling it about meridian flips are essentially ignored since PHD2 will make its own determination about side of pier and rotator angle independently of SGP.

Are you using an OAG so the guide camera rotates when you move the rotator? If so, then for rotation and pier flip to work automatically in PHD2 you’ll need the following:

  • make sure your mount is connected in PHD2 via ASCOM (not just ST4; if using ST4 must also connect mount as Aux Mount)
  • make sure your rotator is connected in PHD2 using its ASCOM driver
  • make sure you have the “Reverse Dec output after meridian flip” option set correctly in PHD2. The PHD2 help file describes how to make sure you have this set correctly.
  • make sure you have your “Reverse sign of angle” setting set correctly in PHD2. You can confirm this by stopping guiding, moving the rotator by a significant amount, like 90 degrees, then starting guiding – no runaway guiding indicates you have the setting correct.

You should be able to test this all standalone in PHD2 without involving SGP. If you see any problems, please post your PHD2 guide log and debug log in the PHD2 forum.




Thanks for the response. I have PHD2 connected via ASCOM to my rotator (Optec Gemini) and my mount (AP1100GTO). I thought SGP told PHD2 to “Reverse Dec Output” when it did the meridian flip. If not, then I must have PHD2 properly configured: Reverse Dec Output is unchecked and Reverse sign of angle is unchecked. The automatic meridian flips, which include camera rotation, work perfectly when doing a normal east to west meridian flip. Of course, the camera is doing a 180 degree rotation. When the reverse meridian flip occurred, the camera rotation was not 180 degrees since the new target was at a different rotator setting from the first target.

So, if PHD2 is making the dec reversal decision on its own, then it would have known that the OTA had moved back to the west side of the pier, pointing east and that the camera had been rotated to a new angle.

You have provided good information and I will get a chance to test this later this week. I will specifically test the “Reverse sign of angle” option, since I have never done that. I will also make sure to get the logs if it fails again while going to the second target.




I had an opportunity to retry having SGP end one target and then start a second target automatically. The first target ended with the scope pointing to the SW and the second target required the scope to slew to the NE in order to acquire the target and perform all tasks needed to do the imaging run. This all happened with no problems. The slew back to the NE required a major change to the rotator position. After that change, PHD2 picked a guide star and began guiding properly. That imaging run lasted 4.5 hrs with no problems. So, the previous issue appears to have been an anomaly and not a problem with either SGP or PHD2.

Again, thanks for your time in responding to my question.



Charlie, that’s great to hear. Thanks for the update.