Hall's Harbour Observatory,   Hall's Harbour, NS

DomeSync 1.1

24-Oct-2020 12:20 ADT 24-Oct-2020 15:20 UTC

What DomeSync is and why it exists

DomeSync is a utility application to synchronize an observatory dome with a German equatorial telescope mount. It requires an ASCOM driver for the telescope mount and an ASCOM driver for the dome. DomeSync is a solution to a widespread problem in the astronomy software ASCOM community.

Most applications that perform dome synchronization rely on the ASCOM value "SideOfPier" in their calculation of the required dome azimuth. Unfortunately, for historical reasons, the terminology surrounding this value is confusing. Although the ASCOM group has attempted to clear up the confusion in their documentation, the outdated terminology remains grandfathered into the standard. The result is that new device drivers are still being written today that incorporate an incorrect interpretation of SideOfPier.

The result is that if you have two software products that use SideOfPier, chances are about 50-50 that they will miscommunicate. If you have more than two software products using the value, chances are close to 100% that you will have a problem. The problem shows up in dome synchronization as an incorrect dome azimuth for the mount position. Although the software combination may get the dome azimuth right for most of the sky, where it typically misbehaves is in the poleward side of the sky, i.e. hour angles 6-18.

DomeSync addresses the issue by calculating the dome azimuth without reference to SideOfPier, using instead only the mount's hour angle and declination and some reasonable assumptions about GEM behaviour.

How it works

DomeSync makes the assumption that a GEM will always prefer to be in a counterweight-down orientation. This assumption resolves most ambiguities about which orientation is being used. Because it does not refer to SideOfPier, it does not know for certain that the mount is in this orientation. However, it is a reasonable assumption in most cases. Given the confusion in the industry about the meaning of SideOfPier, knowing this value does not, in practice, resolve the orientation anyway.

There are two cases in which a GEM might reasonably be in a counterweight-up orientation. If the mount continues to track an object across the meridian, it will be in a counterweight-up orientation after meridian passage. DomeSync detects this situation and makes the corresponding adjustment to its dome azimuth calculation. In order to detect this situation, the mount must track into the counterweight-up orientation; slewing to an object in that part of the sky will cause it to use the default counterweight-down assumption. DomeSync will cease to accept the counterweight-up orientation once the hour angle exceeds +1:00:00, or when the mount slews, as it will in a meridian flip.

The second situation in which a counterweight-up orientation is reasonable is if the operator anticipates a meridian passage and presets the mount to the counterweight-up orientation in order to avoid a flip at the meridian. As of version 1.1, DomeSync now handles this situation.

DomeSync v1.1 is now "supposed to" handle southern hemisphere operation. It has not been tested in the southern hemisphere, so feedback from southern users is requested.


This software is distributed free of charge, with no warranty, as a public service. If you have difficulty with it, I will do my best to help you, but I do not have the resources to offer a technical support department.

You can download the package here: DomeSync v 1.1 (297kb)