What are the Angles that Rotator uses, and How Do They Relate?

An ASCOM instrument rotator is intended to position an imager at a given angle on the sky in the equatorial coordinate system. Adding an instrument rotator to a telescope effectively turns it into a 3-axis system, with the imager being positioned in Right Ascension, Declination, and Position Angle. Refer to this figure:

[Q] What is Equatorial Position Angle?

[A] Looking from behind the imager, Position Angle (PA) is the angle from North (in the equatorial coordinate system) rotating in a counterclockwise direction. It is always positive.

[Q] What is the difference between Position and MechanicalPosition?

[A] It is difficult to mount a camera within a rotator so that the camera’s position angle is exactly the same as the mechanical angle of the rotator. Therefore an ASCOM rotator keeps an angular offset from its MechanicalPosition to the equatorial Position (PA) that the imager sees at that mechanical position. Thus the device’s client app can write the equatorial PA to Position and get the PA it wants, regardless of the mechanical angle at which the imager is mounted in the rotator or the 0-position of the rotator as configured. See the next question.

[Q] What is the purpose of Sync()?

[A] This allows the client app to tell the rotator at what PA it is currently positioned. The client app will typically do a plate solution on an image, yielding the true equatorial PA, then immediately call Sync() with this PA. This establishes the angular offset from its MechanicalPosition to its equatorial Position (PA).

Note

The rotator should store this offset internally, allowing any application to set the camera to the desired PA.

Rotator on an Alt-Az Mount

The above assumes that the rotator is mounted to a telescope on an equatorial mount (fork, German, etc.). When a telescope is carried on an alt-az mount [1], imaging on the sky will result in image field rotation becaused the optics are not aligned with the celestial sphere. Such mounts will often include a field derotator which will attempt to compensate for this field rotation.

This derotator will be an integrated part of the mount control system because its angle and angular rate are related to the current RightAscension and Declination by a transform from equatorial to local horizontal coordinates. As the mount “tracks” its azimuth, altitude, and field rotation angle continuously change in order to keep the RA, Dec, and PA constant (still against the celestial sphere).

It’s beyond the scope of this specification to go into more detail on this. However, there are two consequences that affect the ASCOM Rotator operation in alt-az mounts. Refer to this diagram:

1. The derotation system must operate below the ASCOM Rotator. At all times Position must be the target equatorial PA, and it must remain constant against the sky while the derotator is turning continuously to provide tracking.

2. The MechanicalPosition property is called field position and it must be relative to the optics. This is what the client needs to do flat fields. With an equatorial mount, the optics stay at a fixed equatorial PA so the mechanical offset is a static value. With an alt-az mount, the offset between MechanicalPosition (field position) and Position (equatorial PA) is the sum of the equatorial PA offset resulting from calling Sync() and the continuously varying angle needed to remove the field rotation.

[1]

This refers to a mount that is physically horizontal with the ground, whose axes are vertical and horizontal. Don’t confuse this with operating the mount in alt-az coordinates as oppposed to equatorial coordinates. All astronomical mounts, regardless of their mechanical geometry, can be positioned with alt-az or equatorial coordinate inputs, and can display their current position in either coordinate system.