Need help configuring Meridian Flip with AP900

I recently purchased an AP900 and with the weather being unsuitable for real observations, I’ve been trying to workout as many issues as I can indoors.

I am using the AP900 with a GTOCP2 controller. Hand controller is not attached and the latest ASCOM driver V5.09.07. SGP connects to the mount fine. I simulate pointing to the sky by using CDC. SGP shows time to meridian flip but when the moment arrives, there is no action. SGP pier flip info on the info bar goes from time remaining to NA and the AP driver shows that “pierside” goes from East to West. The mount continues to track past the meridian without flipping as is the case with AP mounts. I have the setting for tracking past meridian to “0” in the AP driver so it should in principle flip immediately.

One thing I have noticed is that the RA and DEC values dsiplayed by SGP (J2000) are different than those displayed by the AP driver (JNOW?). Is that the problem? I have done meridian flips routinely with an EQ6+EQMOD before and I think I have all the SGP settings done correctly. Could someone please post the settings? Are there any gotchas?

One other thing I noticed was that the telescope nudge controls on SGP seem grayed out. Could that be the problem? Am I forgetting a simple setting since I switched to the new mount?


i don’t think SGP will attempt the slew that ends up causing a flip unless it is in the middle of a sequence. i assume you’re just idle since you’re running indoors…?


That’s right. There’s no camera and no real sequence running. Well, that’s good to know. Thanks Rob.

I could bet a penny or two that I did something like this to figure out how to do the flips with EQMOD. I may have used a simulator for the camera. I could also be remembering incorrectly that it ever worked with a simulator.


well, all mounts are different… there may be mounts out there that can be explictly told to flip, or ones that flip on their own, which could make the SGP behavior different. i’ve never used SGP with anything but an AP mount, so i can’t say.

AP mounts won’t flip until the mount is past the meridian (real or delayed) and then it is commanded to slew to a target that is on the other side of the meridian.

Two ways to Meridian flip the mount:

  1. When mount crosses the Meridian, manually send (using hand controller, planetarium, or SGP’s Slew Now command, etc.) the slew command of same object and the mount should auto flip.

  2. While SGP sequence is running and you have “Use Auto Meridian Flip” checked under Telescope tab of Control Panel to perform Meridian Flip. Click on “Set” button next to “Use Auto Meridian Flip” check box to setup Meridian Flip settings like “Minute Past Meridian to Flip” (I set it to 4 minutes). I also checked “Auto Center After Meridian Flip”.

SGP uses J2000 coordinates. A-P mounts uses JNow coordinates. SGP parses your mount’s EPOCH via ASCOM and your mount will return JNow so SGP will automatically convert from J2000 to JNow coordinates before sending proper (JNow) coordinates to your mount. No problem.

I have A-P1100 and works great with SGP. A-P V2 ASCOM mount driver is extremely robust.



Thank you for those tips. I was trying with the settings you outlined in method 2. The only thing I did not have was a sequence running. Will try as Rob and you have pointed out with a sequence.



I believe I made a mistake describing J2000 and JNow for A-P mounts. I corrected my previous post. I incorrectly said that SGP converts from JNow to J2000 and sends J2000 coordinates to A-P mounts.


Peter, I should confess that I skimmed over the post and did not catch the error. Thanks for the correction and the notification.


i should amend this statement:

if you are using APCC, one of the meridian limit features is to automatically flip the mount when the limit is reached. so if you are just using the CP3+driver the mount will not flip but if using APCC and it is configured to do so, the mount can flip on its own. of course SGP would not like this, as it wants to do the meridian flip.

No worries there. I am using a CP2 and SGP. I am going to stick with just SGP for a while.

OK I was able to achieve a successful flip in the house with the mount and sims.

I used ASCOM simulators for camera, focuser and filterwheel. The mount was driven by the AP ASCOM-V2 driver 5.09.07 (latest as of this post). Both CDC and SGP connect to the ASCOM driver simultaneously. I used Cartes du Ciel to point the scope and then set the target in SGP to the position where the scope was pointing in the telescope control panel. I made up a sequence and ran it. The camera sim saves an 800x600 image of M42 as it runs. I pointed the scope to a star about 14 minutes away from the “meridian” and waited. The mount moved past the meridian but was still in the process of “imaging” the target. As soon as the image was completed, the mount flipped(!) from the West to the East side of the pier.

So what made this work? I recently switched from a GTOCP2 to a GTOCP3 controller. Apparently pointing calculations are more accurate with the CP3 controller; I remember reading something to that effect in the GTOCP3 chip revision document on the AP site. So now I can configure all my hardware bits like cables and such and be prepared.



The AP900GTO will perform a meridian flip when a “goto” is issued by SGP and the OTA is on the west side of the mount and the target has crossed the meridian and is also on the west side of the mount. When SGP is imaging and the target approaches the meridian, it will initiate a meridian flip depending on the settings you have chosen in the Meridian Flip Options:

If “Wait For Meridian” is checked, SGP will determine if the target will cross the meridian during the next exposure. If it will, SGP will simply wait for the meridian crossing and then issue a “goto” for the target, triggering a meridian flip. After the flip, SGP resumes imaging. If that option is not checked, SGP checks for meridian crossing after every exposure. When an exposure ends and the target has crossed the meridian during the exposure, SGP will issue a goto for the target and your AP900GTO will then perform a meridian flip.



Thanks.Those are exactly the settings I used. However, the point made by Rob and Peter was that the flip will not work unless there was a real sequence running. So I attempted running with simulators for the camera, filterwheel and focuser indoors while doing the pointing with CdC. The flip did not work with the CP2 controller. I read elsewhere a post by either Jared or Ken that simulators like CdC may not be sufficiently accurate to cause a flip to happen.

It does work with a CP3 controller and with the steps I outlined above. So what use is running a mount indoors? Well while the weather outside is still crappy, I can configure cables and such and make sure the mount flip will work without problems when I eventually get a chance to image outside.


The thing to understand is that a “Meridian Flip” is a slew command sent to the mount to slew to the target. Since the mount knows which side of the meridian it is on, it slews to the opposite side to facilitate tracking it better.

What SGP is doing (I assume) is checking the Hour Angle to to see if it meets the requirements set for sending that command. If it meets those requirements, a slew command is sent.

Yes – an actual sequence is required to be running, otherwise SGP is effectively idle. If it were slewing peoples mounts while idle that would be a problem. Thankfully it doesn’t.

So what made this work? I recently switched from a GTOCP2 to a GTOCP3 controller.

With all due respect, no it didn’t. The concept of tracking objects hasnt changed that much from CP2 to CP3.