Optec Gemini Rotator Position Angle Difficulties

Hi, everybody.

I’m having a bit of trouble getting SGP to play fully nicely with my Optec Gemini Focusing Rotator, though the basics seem to be sound and I think I may be one misconfiguration or misunderstanding away from happiness. My setup on the hardware side is:

  • EdgeHD 1400, running at f/11
  • Paramount MX+
  • Optec Gemini Focusing Rotator
  • STL-11000M, using internal guider

Software-wise I’m using:

  • SGP version
  • PHD2 2.6.4
  • Windows 10 Fall Creators Update

I notice that @johnnyK and @chasmiller46 are using the Gemini as well, and have worked through some issues that sounded similar to what I’m seeing, but I’m either facing something slightly different, or I’m not faithfully following what was discussed in the following related thread (Optec Gemini Focuser & Rotator - Equipment Compatibility - Main Sequence Software).

What is working for me:

  • I can connect the Focuser and Rotator parts of an SGP equipment profile to the Gemini.
  • The Gemini powers up and can move focuser and rotator without difficulty.
  • SGP autofocus works perfectly.
  • SGP can make the rotator turn, though not necessarily where I want it to go, or where SGP wants it to go when doing a plate-solve driven precise centering operation.

The problem I’m having is that I can’t get seem to get SGP to establish the proper offset between the hardware angle reported by the Gemini (the value that I believe ends up in the POSANGLE FITS header) and the plate solved East-of-North position angle (which ends up in the ANGLE FITS header).

The inability to get these values synchronized means things like plate solve driven centering and the like never work because SGP ends up sending the rotator on crazy excursions that never manage to get the camera oriented the way it’s supposed to.

What I’ve been trying is the following:

  1. Power up and connect all the gear
  2. Home the rotator by pressing the ‘Zero’ button in the ‘Rotator’ dockable module
  3. After the rotator finishes going home (typically pressing ‘Zero’ sends it to a reported angle of 360.0), I take an image using ‘Frame and Focus’ and then right click to plate solve the image using Plate Solve 2. I see the RA and DEC image center and the solved position angle in the Plate Solve 2 UI after the solve completes successfully.

It was my possibly wrong understanding that doing the above would be sufficient to get SGP rationalized in a way that the angles it would report for the rotator in the UI, and any user or SGP chosen positions requested of the Gemini would henceforth all be expressed in sky position angles and things would work happily thereafter, but this doesn’t seem to be the case.

Can anyone point to me to what I may be doing wrong with either:

  • Any configuration I’m likely to have forgotten, or got wrong, in either SGP or any Gemini-specific configurable state?
  • Any ritualized steps I need to do at the start of a session, after powering up the gear, to get SGP’s understanding of the rotator into a good state?

Thanks in advance for any pointers…



The Gemini and SGP work together just fine but you have to observe some “protocol” in the way things are initialized. I don’t currently have access to my Gemini but as I remember, in the “settings” dialog, there is an option in the “Rotator Configuration” dialog box that says “Reverse Direction.” You must have this option checked. Otherwise, SGP will move the rotator in the wrong direction when moving to a specified position angle.

When you startup the Gemini Commander software, the first thing to do after connecting to the rotator is to home the rotator. This will cause the Gemini to move from its current position to an internal position of 75 degrees; then it will reverse and move to 0 degrees internal position angle. From now on the only thing you will ever do with Gemini Commander is to enable / disable temperature compensation.

Now connect SGP to your rotator. SGP will retrieve the Gemini’s internal position angle (0) and display that.

Point the scope somewhere and do a Solve and Sync. This will sync both the mount and the rotator. SGP will now display the camera’s sky position angle. At this point, SGP knows how to map the camera’s sky position angel to the Gemini’s internal position angle.

In SGP, enter the position angle you want the camera at and click set. SGP will then move the rotator to that position. If you now look at the position angle reported in Gemini Commander, it will be completely wrong. Forget about it; don’t attempt to make the sky pa in Gemini Commander be “correct.” The angle reported in SGP is correct.

You can move the Gemini to the desired position either by entering the sky pa in SGP or put the sky pa in the Target Settings dialog box and check “Rotate Camera To” option. SGP will move the rotator to the specified position when the target starts.

But its important to remember not to ever use the Gemini Commander software to move the rotator to some specific position angle. Only move the rotator with SGP. It is also important to do that first Solve and Sync to give SGP the beginning sky pa.


I had the same issue with the Gemini. First thing I did was to include the rotator with PD2 as part of the additional accessories.

I used the Gemini basic software to “reverse the direction”. This seemed to do the trick.

I start up the Gemini and it homes to a zero position when electrical connection is applied. I start up PhD with “don’t reverse on meridian flip” option.

Start up SGP and attach all equipment and it worked from there on. Didn’t need to plate solve or any of that extra processes. I do set my initial rotator position to 0.0 in the star wheel option in SGP so the plate solving can find its own way when acquiring an image.

I have attached a file that included all the discussions on the Gemini but I used it just for reference. The reverse direction and including the Gemini in PhD make the changes that allowed it to work. Charles Miller was the one who gave me the hints to fix it.


Hi, @johnnyK, thanks much for the quick reply. Following your advice i’ve set the reverse direction box in the Gemini basic software. There’s still a couple of hours before sunset when I’ll try putting things through their paces.

One question, what is the “star wheel option in SGP” that you’re referring to and where is it in the UI?

I didn’t spot an attached file in your forum post… did you perhaps forget to stick it in?


Hi, @chasmiller46… Thanks very much for the quick reply and clarifications. I should be able to put this stuff to the test in a couple of hours. I definitely did not have the reverse direction checkbox set before, and wasn’t using “Solve and Sync” because I was afraid of messing up the Paramount’s T-Point model based on some tales I’d read in other forum posts.

I’ll give it a spin tonight and let you guys know.

Thanks again @JohnnyK and @chasmiller46… I greatly appreciate your insights.


I think the TSX people do have concerns about the Solve and Sync procedure messing up the TSX pointing model but a TSX user will need to clarify that for you.

You might need to use the Rotator module’s “Manual Sync” option instead of the Solve and Sync to avoid causing problems with TSX. If so, take a sub and then plate solve (might need to do a blind solve, if scope is not accurately pointing) and get the camera’s sky position angle. Click the Manual Sync button in the Rotator module (or in Control Panel) and enter the sky position angle reported in the plate solve. That value should show up in the “Current Position” field. After that, entering a position angle in the “Set position” control and clicking Set, should do it.

Note – I have not used the Manual Sync option but I think I have described it correctly.


I was referring to the gear wheel in the “target list” where you can set your rotator angle. I always leave it unchecked and let the plate solve in conclusion with the rotator to fix the angle.

I guess the file didn’t attach. I could send it to you via direct e-mail. It is just a compilation of responses associated with the Gemini.

The file probably doesn’t make sense to be in the Forum since it is just a reiteration of the embedded data already submitted.

Ah yes, thanks for clarifying.

Thanks @johnnyK and @chasmiller46! Followed your suggestions and everything is working like a charm. In the hour or so between sunset and astronomical dusk, the rotator is now properly reporting sky position angles, centering and rotating are working like a charm, and it was a snap to pick up the location and orientation of an object that’s been a work in progress since March and that’s been worked on the last two nights with more manual fussing about than the near zero that was required tonight.

Thanks again, guys.