Introduction

This document describes the GBT Antenna Control Unit (ACU) scan data FITS files. Each file contains a primary header describing various co-ordinate transformation related parameters, followed by a binary table of positions in one of three possible formats, depending on the telescope optics mode.

The reported antenna positions are derived from the values read back from the encoders. The binary table contains three versions of each position; MOUNT coordinates, FK5 J2000 co-ordinates, and the co-ordinate system in which the antenna was commanded.

This document rather briefly summarizes the structure of the ACU FITS files. More information on the philosophy and details of the co-ordinate positions and transformations represented in the files are described in GBT/SPN/015, which should be read in conjunction with this note.

Please refer to GBT/SPN/015 for some caveats related to the validity of co-ordinate transformation corrections and the positions recorded in the ACU FITS files under certain circumstances.

Note: Antenna files with FITSVER equal to '1.6' have beam offset tables written with offsets relative to the axial center of the receiver mount.