What are RightAscensionRate and DeclinationRate and how are they used?

These read/write properties are used most commonly to provide a mount with a way to track solar system objects such as asteroids and comets. They both apply a constant rate of change to the mount’s RightAscension and Declination, respectively. This FAQ is provided to help clear up misunderstandings that have historically created problems especially for mount developers.

From a user’s perspective, solar system objects have an ephemeris which shows the coordinates at one or more times, as well as the rate of change of the coordinates at that time. The rates described here are those coordinate rates. DeclinationRate should be easy to understand. The mount’s Declination should change by so many (angular) arc seconds per second, depending on sign of the value.

On the other hand, there are several subtle and confusing aspects of RightAscensionRate. Keep in mind that RightAscension is a time coordinate, not an angular one. So seconds of RA are not arc-seconds. They are seconds of time. Furthermore, RightAscension is a sidereal time not a UTC (clock, solar) time. We’ll get back to this in a bit.

If you are a mount developer, it’s easy to get confused by the sidereal tracking rotation of your mount’s mechanical drive and the Right Ascension of the point at which your mount is pointing. If the mount is tracking, its Right Ascension is not changing. That’s what the mount’s job is, to hold the optics at an unchanging RightAscension (and Declination). Suppose the user wants to apply a positive rate of change to Right Ascension. This means the mount’s Right Ascension must increase (later time) at the requested positive RightAscensionRate. RightAscension increases to the East, right? What does this mean down in the mount’s RA drive? It must slow down so the its pointing drifts toward the east in order to have its RightAscension increase. Perhaps counter-intuitive.

Units of RightAscensionRate

Due to an unfortunate early design choice, the units of RightAscensionRate are in (RA) seconds per sidereal second while Ephemerides all specify the rate per unit of UTC (atomic clock) time. Your driver must accept the rate in these (awkward) RA seconds per sidereal second units since we never make breaking changes. To convert the given rate to (the more common) units of sidereal seconds per UTC (atomic clock) second, multiply the incoming value by 1.00273791 (the number of sidereal seconds in a UTC second).

InvalidOperationException When TrackingRate is not driveSidereal

These applied rates of change of equatorial coordinates don’t apply when the mount is not tracking to follow the equatorial coordinate system. Both RightAscensionRate and DeclinationRate must raise an InvalidOperationException if TrackingRate is not set to driveSidereal.