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ITelescopeV3SideOfPier Property
Indicates the pointing state of the mount.

Namespace: ASCOM.DeviceInterface
Assembly: ASCOM.DeviceInterfaces (in ASCOM.DeviceInterfaces.dll) Version: (
PierSide SideOfPier { get; set; }

Property Value

Type: PierSide
PropertyNotImplementedExceptionIf the property is not implemented.
InvalidValueExceptionIf an invalid side of pier is set.

For historical reasons, this property's name does not reflect its true meaning. The name will not be changed (so as to preserve compatibility), but the meaning has since become clear. All conventional mounts have two pointing states for a given equatorial (sky) position. Mechanical limitations often make it impossible for the mount to position the optics at given HA/Dec in one of the two pointing states, but there are places where the same point can be reached sensibly in both pointing states (e.g. near the pole and close to the meridian). In order to understand these pointing states, consider the following (thanks to Patrick Wallace for this info):

All conventional telescope mounts have two axes nominally at right angles. For an equatorial, the longitude axis is mechanical hour angle and the latitude axis is mechanical declination. Sky coordinates and mechanical coordinates are two completely separate arenas. This becomes rather more obvious if your mount is an altaz, but it's still true for an equatorial. Both mount axes can in principle move over a range of 360 deg. This is distinct from sky HA/Dec, where Dec is limited to a 180 deg range (+90 to -90). Apart from practical limitations, any point in the sky can be seen in two mechanical orientations. To get from one to the other the HA axis is moved 180 deg and the Dec axis is moved through the pole a distance twice the sky codeclination (90 - sky declination).

Mechanical zero HA/Dec will be one of the two ways of pointing at the intersection of the celestial equator and the local meridian. In order to support Dome slaving, where it is important to know which side of the pier the mount is actually on, ASCOM has adopted the convention that the Normal pointing state will be the state where a German Equatorial mount is on the East side of the pier, looking West, with the counterweights below the optical assembly and that pierEast will represent this pointing state.

Move your scope to this position and consider the two mechanical encoders zeroed. The two pointing states are, then:

Normal (pierEast)Where the mechanical Dec is in the range -90 deg to +90 deg
Beyond the pole (pierWest)Where the mechanical Dec is in the range -180 deg to -90 deg or +90 deg to +180 deg.

"Side of pier" is a "consequence" of the former definition, not something fundamental. Apart from mechanical interference, the telescope can move from one side of the pier to the other without the mechanical Dec having changed: you could track Polaris forever with the telescope moving from west of pier to east of pier or vice versa every 12h. Thus, "side of pier" is, in general, not a useful term (except perhaps in a loose, descriptive, explanatory sense). All this applies to a fork mount just as much as to a GEM, and it would be wrong to make the "beyond pole" state illegal for the former. Your mount may not be able to get there if your camera hits the fork, but it's possible on some mounts. Whether this is useful depends on whether you're in Hawaii or Finland.

To first order, the relationship between sky and mechanical HA/Dec is as follows:

Normal state:

  • HA_sky = HA_mech
  • Dec_sky = Dec_mech

Beyond the pole

  • HA_sky = HA_mech + 12h, expressed in range ± 12h
  • Dec_sky = 180d - Dec_mech, expressed in range ± 90d

Astronomy software often needs to know which which pointing state the mount is in. Examples include setting guiding polarities and calculating dome opening azimuth/altitude. The meaning of the SideOfPier property, then is:

pierEastNormal pointing state
pierWestBeyond the pole pointing state

If the mount hardware reports neither the true pointing state (or equivalent) nor the mechanical declination axis position (which varies from -180 to +180), a driver cannot calculate the pointing state, and *must not* implement SideOfPier. If the mount hardware reports only the mechanical declination axis position (-180 to +180) then a driver can calculate SideOfPier as follows:

  • pierEast = abs(mechanical dec) <= 90 deg
  • pierWest = abs(mechanical Dec) > 90 deg

It is allowed (though not required) that this property may be written to force the mount to flip. Doing so, however, may change the right ascension of the telescope. During flipping, Telescope.Slewing must return True.

This property is only available in telescope InterfaceVersions 2 and 3.

Pointing State and Side of Pier - Help for Driver Developers

A further document, "Pointing State and Side of Pier", is installed in the Developer Documentation folder by the ASCOM Developer Components installer. This further explains the pointing state concept and includes diagrams illustrating how it relates to physical side of pier for German equatorial telescopes. It also includes details of the tests performed by Conform to determine whether the driver correctly reports the pointing state as defined above.

See Also