Moving from Fork mounted to GEM

Hi Group
Soon I will receive a CGE Pro mount to replace my old LX200 Classic used for imaging for the last 12 years. The wait has given me time to dwell on several questions.
I will need to learn how to deal will meridian flip (which before was not an issue). It’s occurred to me that after flip, the image in the scope will be inverted. If true …

  1. Can PHD2 put me back on target to continue imaging?
  2. Is it standard practice to then re-orient the post flip images with the processing software?
    I would prefer not to add a rotator to the optical train, which I think would obviate the above. I would also hope to be able to continue using the Mosaic and Framing wizard.
    I know this is basic stuff. If you can point me in the right direction, that would be appreciated.

Thanks for your thoughts,

Hi Mark,

Congratulations on the new mount.

SGP can be setup to do the meridian flip automatically. I use the auto meridian flip with my CEM60. There are several things that need to be setup to make this work correctly.

  1. SGP must be setup to do the meridian flip before your mount initiates the flip. For example with my iOptron CEM60 I setup the mount (option in hand controller) to delay the meridian flip until it is 10 minutes past the meridian. However, I tell SGP to do the meridian flip 1 minute past the meridian. This way SGP will initiate the flip.
  2. You should have one of the plate solvers setup and working in SGP so that SGP can re-center the target after the flip. I have found that PlateSolve2 (free and built into SGP) works almost flawlessly. You do need to download and install the catalogs as described in the SGP manual).
  3. After the flip SGP will recenter the image using the plate solver. When that is complete, PHD2 will acquire a new star and begin guiding.
  4. Your image will be rotated 180 degrees after the meridian flip. This is not a problem because most post processing software can correctly align the photos during the alignment process. I use PixInsight which is able to align the images from both sides of the meridian without any manual adjustment from me.
  5. You do not need a rotator.
  6. I use the Framing and Mosaic to enter my targets. I have not used it to shoot mosaics at this point, However, I find it extremely useful for entering my sequence targets.

I hope this helps.


Thanks Fred,
all that is consistent with how I hoped it would be.



Flips are easy with the CGE Pro, but you need to set a few things up first to make sure things work well. The first thing you want to set up is the RA Limits. The default limit is 0 degrees, which means that the mount will stop when it reaches the meridian. You want the RA limits set up so the mount will track as far past the meridian as it can without risk of a pier crash. The maximum is 20 degrees past the meridian. Allowing the mount to track past the meridian gives you some latitude in setting up the flip.

The other thing to set up is “Meridian” (both RA Limits and Meridian are in the Scope Setup menu). I like to set mine up to “Favor West”. This means that if the target is accessible from both sides of the meridian within the RA Limits you defined above, the mount will slew to it as though it were on the west side of the meridian (thus with the OTA on the east side).

By setting these two options, you will open a flip window that spans 20 degrees before the meridian (or your RA Limits if they are lower) to 20 degrees past the meridian. That gives you almost a 3-hour window during which SGP can successfully flip the mount. That can come in very handy for planning the flip if you are doing long exposure narrowband imaging.

As Fred mentioned, you will want to set up plate solving for centering after the flip. I use PlateSolve 2 as the primary solver and ANSVR as a blind failover. Once the flip has completed, SGP will center the target using plate solving.

Once centered, SGP will tell PHD2 to restart. It will automatically find a new guide star, since the old pre-flip guide star is now on the other side of the frame. The CGE Pro correctly reports side-of-pier to PHD2 so there is no need to tell PHD2 to reverse DEC output after a flip.

As Fred mentioned, stacking software will have no trouble aligning the pre- and post-flip images.


Thanks Tim,
all good info.
I’m fairly certain that I’ll need 2 sets of flats now. One for pre flip and one for post, since (I think) the image will invert.

You only need one set of flats. The flats are relative to the optical train not the sky so unless you have a rotator one set is enough.

Thanks DesertSky.
Sounds good, but having a little trouble visualizing this. My experience is with the fork mount, so is this effectively equivalent? No doubt, when I see the action, then it will be obvious to me.
Thanks again,

Think of a light panel mounted to the front of your telescope. Now turn the telescope upside down. The physical relationship between the light box and the telescope has not changed. The same relationship applies even if you are using a twilight sky.
What the flats are measuring is the optical train which does not change when you flip unless a rotator is used. Your processing software will get it right even if some images are flipped.

Thanks DesertSky,
I’ll take that as gospel.
It takes me a bit longer nowadays to work through problems, but I usually eventually get there.

In other news the mount just showed up, so let the fun begin.