Observatory Dome "Sync" button


Can someone please explain the appropriate manner in which to use the Dome Sync button? Preferably someone with similar equipment (Foster Systems Dome AstroMC, EQMOD, EQ8). This option is found under Control Panel - Other - Observatory section “Sync” button (then asks for “Angle”).

For instance, do I jog the dome shutter opening to best align with OTA, then select “Sync”? What angle do I use? Should “Slave to telescope” be checked or not at this point?

I’ve tried different combinations but can’t seem to get anywhere. Or should I ignore all this and do the sync in Foster Systems application?

I’ve tried finding specific documentation on this but no luck - sorry if I’ve missed something obvious.



Sync has nothing to do with slaving to your telescope. Sync (like a telescope sync) allows you to correct the angle that your dome is reporting. For instance if your dome is pointed due north (0 degrees) but is reporting 30 degrees, you can use “Sync” and input 0 to correct this. The dome will then be synced to 0 degrees at its current position.

Not all mounts support this, and you probably don’t need this behavior unless your dome isn’t pointing where it should be pointing.



Thanks Jared for the info. That is the problem I am having (ie. dome is not pointing where it needs to point). It is close, but not close enough :=)

Sounds like I’m better off using AstroMC to make adjustments to this sync.



In case you have forgotten…double check & adjust your numbers here…

I had to play around with the numbers a bit (initially) to get where the dome followed the scope exactly how I wanted.


My understanding (and I’ll freely admit it is limited :grinning:) is that these settings are required to account for the funky geometry created by the realities of an EQ mount (and to lesser extent, Alt-Az mount). However, there still needs to be an initial alignment or synchronization between the mount and dome. That is what the “Sync to Scope” button does in AstroMC (Foster Systems). And I think that is what the SGP “Sync” button under Control Panel - Dome does as well.

I wonder if the AstroMC “sync” setting still has effect if SGP is slaving the dome (and the “AstroMC calculates Az” is turned off in AstroMC application). -I’ll try testing this next chance I get.



I am not sure that is the way to look at it! I get the feeling that you are syncing the dome to the telescope angle when you start for the night…though I might be very wrong with that assumption.

You need to look at the dome and scope (read mount) as completely separate…it is just the funky geometry that brings the two together for the night.

Say you have your scope pointing low at 179° (almost south). Picture how a GEM mount might be sitting…your scope will not be pointing out from the center of the obs…but will be off to one side. So the angle reported by the dome will not be 179°…it has to take into account the funky geometry as you call it.

The dome (software) needs to know where the shutter opening is…so the dome needs to be aligned with NSEW before it ever tries to follow the mount. It must have it own reference point - not just be aligned with the scope when starting out.
So my question at this point is… have you got the dome home or park position properly fed into the dome software? Does the dome “know” what actual angle it is pointing at? (Not just lined up with where the scope is pointing). If you have that OK then the ‘funky geometry’ will ensure that it points to the right ‘offset’ from the angle that the scope is pointing to.

The ‘sync’ button is used if you notice that there has been any slippage between the dome drive and the dome actual angle. E.G. if your dome park/home position id definitely 350° but you notice it is showing say 355° when you park it up…then it is easy to sync it back to 350° (using that button) and things will be correct when you start up again the next night. You would not normally use the sync button to sync the dome to the mount angle…at least that is how I have worked things with my GE Mount and once I got the funky geometry number right…I have never had a problem since.

The hard part in all of this is getting the dome angle right…


Thanks for the detailed response and insight. I understand the nature of the EQ geometry especially when pointing near meridian or zenith etc. (ie. dome azimuth offset from OTA azimuth)

I think some of the confusion comes from my use of the Foster Systems dome control package (AstroMC software). It’s the only one I have any experience with. They have defined a “Sync to Scope” process to “align” the dome opening to the OTA. Here are their instructions:

“Move the dome so that the shutter is facing approximately due East or due West. Then move your telescope so that the OTA is centered in the shutter opening at an altitude of approximately 45 degrees. Then click SYNC-TO-SCOPE.”

I assume they pick due east/west and 45deg Dec because this is one orientation where the dome and OTA azimuth would be the same?

What I’m not sure about is whether this Foster System Sync-to-Scope even applies if I have “AstroMC calculates Az” turned off (and SGP is set to slave the dome).



@dnube As I am not familiar with Foster Systems I can’t be very specific about how to set things up. The only two things I would be confident about: 1) Trying to mesh AstroMC & SGP for controlling the dome is sure to lead to problems. Use either one or the other…and 2) SGP had a few problems initially but all sorted now. SGP has the ability to control the dome in tandem with the mount once all the numbers are fed in correctly…and you should be OK for full automation.

Again…not being familiar with AstroMC, I would not be surprised if the E or W exercise is just to give your dome an orientation angle…it will take it from the mount when there are no offsets between how they are pointing. If that gives you an angle in the dome software then that is a ‘sync’. I would mark that dome position, note the angle…then you will always have a reference point to sync your dome on should the need arise to check anytime in the future…but once done, I would say let SGP take it from there.

I hope someone else is on here with similar dome to you - perhaps they can let you know for sure how they set things up.

As above: I think that is just allowing AstroMC to get an angle from the mount…Hopefully if you do that…you will be able to see that dome angle somewhere in that software…then do as I suggested…mark the spot, note the angle and you will always have a reference angle to sync the dome to…even when just using SGP.