What are the equatorial coordinate reference frames?

The EquatorialSystem property refers to the “flavor” of equatorial coordinates that the telescope (mount) uses for slewing input and current coordinate readout.

Not all equatorial coordinates are equal. For aiming a telescope (and neglecting atmospheric refraction), the Right Ascension and Declination refer to the rotational state of the Earth at the current time, and at the location of the observer on the earth. You can think of these as a simple transformation from Alt/Az to RA/Dec using the current local Sidereal Time, the latitude of the observer, and the current tilt of the earth’s axis. The proper name for this flavor of celestial coordinates is topocentric. In EquatorialCoordinateType it is equTopocentric. This is often referred to as “JNow”, but this is a slang term. equTopocentric is the EquatorialSystem that the vast majority of commercial and DIY mounts use.

Catalogs of stars and deep space objects obvioualy cannot refer to topocentric equatorial coordinates since the time and location of the observer are unknown. Therefore, the listed coordinates are instead referred to a specific time and place, which defines the earth rotation state and other things like parallax, light deflection abberration, and space motion of the object. Most common catalogs use a reference system based on the mean pole and equinox for the standard epoch J2000.0 (The Gregorian date January 1, 2000, at 12:00 Terrestrial Time.). In EquatorialCoordinateType J2000 is equJ2000. The other coordinate types are less common.

The most precise system is the International Celestial Reference System (ICRS). For a deep dive into this, see Standards of Fundamental (SOFA) Astrometry Tools (external, PDF) specifically Chapter 2, The Supported Coordinate Systems.

Note

The ASCOM Initiative has published .NET / COM implementations of the high level methods in the full SOFA library (external) and the full Naval Observatory Vector Astrometry Software (NOVAS) (external) library. These are both available in the ASCOM Platform Developer Components (external) and in the cross platform ASCOM Library (external).