Converting platform rotation to rotation on the sky

 The platform can be rotated by pulling on the tie downs or redistributing weight on the platform. The picture below shows the different parts of this

rotating the platform about the main cables.
    The distance the horn moves increases as the zenith angle increases. The sky angle is the purple distance divided by 435 feet. The platform angle is the distance divided by the blue line. The sky rotation will be about 6 times smaller than the platform rotation because of the ratio of the distances.

    The picture below shows the ratio of SkyRotationAngle/PlatFormRotationAngle as a function of zenith angle:

SkyAngle/PlatformAngle vs za

The ratio changes because the distance from the center of rotation of the platform to the horn changes with za.
The idl routine platrotosky will compute the positions of the horn as the platform rotates as well as the rotation angle on the sky.
processing: x101/070114/
See Also:   the idl routine pnt platrotosky