Camera angle reporting and plate solving


I am having issues with plate solving since I installed a new computer in my observatory. I have tracked this down to variations in the camera angle reported when plate solving an image. I would have thought that the camera angle reported when doing a plate solve should be fixed but I am finding that it changes depending which side of the meridian I am on. For example, for subs that I am taking of M83, it is reporting a camera angle of 356.73 degrees when the OTA is on the west side of the pier and pointing east. It is then reporting a camera angle of 176.87 degrees for the same object after the meridian flip (when then OTA is on the east side of the pier and point west).

I tested this tonight by moving the scope around and executing a blind solve at various position. Objects on the eastern side of the meridian are reporting approximately 357 degrees and objects on the west side of the meridian are reporting approximately 177 degrees. This is causing a problem and I cannot select ‘centre now’ on an object from the home position as the plate solve fails because it thinks that it has the wrong camera angle (aprox 180 degrees instead of aprox 360 degrees).

My set up is a Skywatcher EQ8 running EQMOD for mount control.The control computer is running windows 7.

Maybe I don’t understand how the camera angle measurement works but my assumptions was that the camera angle is what it is and shouldn’t change. I don’t recall having this problem either before I had to install a new computer and reload all of the software.

Any thoughts on this to assist would be much appreciated.

Thanks in advance,


Hi Rodney,

It seems this is the way it should be. When changing side, the geometry of the mount induces a 180° rotation of the camera in respect to the sky. That is reflected in the solved values: 357 = 177 + 180.



Thanks for the response Horia. It is apparent that I have an issue with Plate Solving at the moment which I need to look at. Since I replaced my computer in the observatory the initial plate solve executed from the home position of the mount is failing. Needs more analysis but I don’t think it is related to the camera angle thing.


If the “home position” of your mount is counterweight down with the scope pointed north or south (in the southern hemisphere), then I would NOT recommend syncing and plate solving in this position as bad things can happen.



Thanks for this advice Jared as I didn’t realise that this would be an issue. I am in the Southern Hemisphere and have been doing exactly as you described, solving and synching with the mount in the home position. I am now a little puzzled as I am sure that it used to work ok. Notwithstanding this, I now wonder how I am supposed to move to my target. This is the process that I had been using:

  1. Power on mount and all equipment
  2. Open SGP and connect to all equipment
  3. Open the sequence for my imaging target
  4. Click on ‘centre now’
  5. The mount should then plate solve its current (home) position, slew to the target, plate solve to confirm new position etc.

Is this correct or should I be following a different process?


Some mounts it may work ok on (they likely just toss out the sync data when it’s so close to the pole). On my mount it almost always causes it to get completely lost as the RA angle can’t accurately be determined. Since you’re at the center of rotation all angles are tightly converged and the cone error in your mount or even your polar alignment could cause your mount to think that it’s in a “counter weight up” position.

If you want to manually invoke the slew/center, I would recommend using the “Slew To Target” option on the target if you’re pointed at the south pole, you can then Center from a more reasonable point. Otherwise you can keep using the “center now” option if you’re not at the pole.

You can also do this as part of the sequence. Just check both the Center and Slew options for the target (and naturally the coordinates :slight_smile: ):

The slew will always happen first. Then the Center will have a very close local sync. We typically recommend selecting both options because of this.



Very clever. Thanks Jared as I didn’t realise that one could click both the slew and centre now options. I have been doing this in two steps, clicking the slew option then enacting the centre now option after the mount had arrived at its new location.

Importantly, I want to make sure the automation works properly as I like to be able to pre set the start time for the sequence and then leave it to kick off automatically at the allotted time. This is useful if waiting for the moon to set or the target to be at an appropriate elevation.

Rodney Watters
Mob: 0412 171 704
Int: +61 412 171 704


Wow useful info thanks!