A&G Box Geometry

 

Some aspects of the A&G box geometry should be tested or measured occasionally in order to ensure the optimum performance of the INT.

• The position of the axis of rotation of the A&G box turntable (the rotator centre position) in the coordinate system of the TV camera or guide probe. They serve to inter-relate the coordinate systems of various A&G box components.
• The offset from the rotator centre of the reference position used for the SNAFU, the nominal aperture, which is usually defined as some point on the instrument or detector. This reference position does not coincide with the rotator centre and its offset must be known to the system, so that the position of an object placed on the reference position is invariant with rotator position angle.

Different techniques are required for measuring these quantities at Cassegrain and Prime. At Cassegrain the probes can reach the on-axis position but at Prime they cannot, and furthermore at Prime the detector itself is mounted off-axis.

A simple quick-look test to check if the geometry is set up correctly consists of placing a star on the reference position and rotating the turntable. The star should remain stationary on the TV probe (at Cassegrain) or the CCD (at Prime) during the process.

• Geometry at Cassegrain
• Geometry at Prime
• Rotator Mechanics
• A&G box coordinate system