Sync Behavior - how does it work?

It is my first post, so welcome and hi all!

I am very curious about the option Sync behavior in the Control Panel -> Telescope tab (SGP version beta). I looked into the help and I could not find any information about that. How does it work?
I imagine that during centering now you can select:

  • Sync: then mount will be synced and repositioned
  • Target/Scope offset: mount is not synced, but position error is calculated and is used to move scope to compensate this error

But what is the difference between target and scope offset option?
There is similar option in Maxim 6 - Refine via dRA, dDec, and I found it very useful for my setup with EQ6 mount and EQASCOM driver. In this case once the alignment model is done there is no point to add more alignment points using sync command, because it is not getting better. Refining position works very well, and I guess this Sync behavior work in similar way.

Many thanks,

We generally recommend Sync unless there is a reason not to. Allowing your hardware to know and control its location is generally better than attempting to add in some “fudge factor” through software (essentially what the offset does).

Also here are some notes on EQMOD and SGP which I recommend reading:

Thank you,

Thanks Jared for the link,

That makes perfect sense. One last question: what is the difference between Target offset and Scope offset option? I could not find it out :frowning:


Jared I believe that EQMOD is in the class of mount interfaces that do not like to by sync’d. This has caused problems for SGP/EQMOD users - and the normal solution is to turn off the mount model in EQMOD - which disables many of the features of EQMOD that give it good pointing accuracy in the first place. I don’t know the details of how to do this - but there are many threads on getting syncing and centering to work with EQMOD and SGP - and I think you put EQMOD in dialog mode. A google search on EQMOD sync SGP gets many hits.

Thus EQMOD is like Paramount and other mounts in that Sync is intrusive - and in no way “generally better.” It corrupts the mount model that is one of the valuable features of the mount software.

I would like to point people like EQMOD users to the “Scope Offset” mode of sgp centering - but it currently does not work. Please take a look at the issue with Scope Offset - because until it actually works, there is no way to center Celestron mounts well - and EQMOD users are forced to disable their mount modeling capabilities. I posted a recent thread on it in this forum with fresh results.

At the same time - I think that EQMOD users would be able to use the full mount model along with Sync Target - but I don’t know if anyone has tried it. I believe this avoids syncing the mount during the centering procedure - and from my tests it appears to work as designed - but it is not a solution for Celestron mounts.

The entire centering procedure in SGP amounts to iteratively calculating a local fudge factor - and there is no inherent advantage in the mount doing that calculation. In fact - there is a big disadvantage because for many mounts sync does not behave in the simple way that SGP expects it to - and the best way for SGP to control how sync behaves is for it to calculate the offset itself and not sync the mount during the procedure.


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Thanks Frank, that are also exactly my observations. Once I have 3 or 6 points model in EQMOD then I get pointing accuracy of few arc minutes. But I need to have these points properly selected - i.e. two big triangles around the hemisphere. Adding one more point near the target helps, but then if there are more points added close to the target (with subsequent syncs during frame center), then pointing accuracy does not improve. And sometimes it is even getting worse.
That’s why I loved idea of refining scope position by dRA and dDec that Maxim implemented. It just calculates error between the position mount reports and real plate solved position and moves scope without sync. Then I can point target with fraction of arc minute accuracy. I am sure I also will love Scope offset option then :slight_smile:

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I believe that you can go ahead and use Target Offset and it may work with EQMOD - even with a mount model. It is worth a try - and if it does work then other EQMOD users would probably want to use it. I believe it is doing what you suggest - and SGP keeps track of the offset.

If it doesn’t work then I think you need to disable the EQMOD mount model and just use the normal Sync mode of centering.

Unfortunately target offset isn’t quite enough for Celestron mounts and a different offset calculation is needed - but it is only slightly different - and it is Scope offset - but it current doesn’t work. But once scope offset works for Celestron mounts - I don’t see why it wouldn’t work for all mounts - and I don’t know why any other approach would be preferred.

One problem with using the mount to sync - even if it works - is that if the mount has an adequate but imperfect sky model, syncing in one part of the sky will reduce accuracy in other parts of the sky. So if you center on one target with a sync - and then go to another target far away - it will miss more than it would have - and that would affect the plate solve.

If instead you only calculate the offset for each target and you leave the mount’s sky model alone - the all sky accuracy isn’t impacted - and you can go ahead and use all the mount modeling features so the pointing is accurate in the first place.



I have an Avalon mount using Synscan. I passed on EQMOD after my old EQ6 days and just use the regular ASCOM driver. If I turn the mount on with accurate location, time and polar alignment, and in the home position (which is also the park position), I do not bother with any initial star alignment and it works with the target offset just fine.