The module consists of a pair of large-diameter prisms which cover the entire field of view acccessible to LRIS. The separation of these prisms is varied as a function of telescope elevation to produce refraction which is almost perfectly opposite to the refraction induced by the Earth's atmosphere, thus yielding a net zero differentual refraction for all elevations from zenith to 2 airmasses.

Schematic depiction of Cass ADC
dispersion compensation

The following figure shows the physical appearance of the module (without its cladding).

Mechanical rendering of the Cass ADC

Motion

The positioning of the two prisms is controlled by a single motor which moves the prisms in opposite directions. When the prisms are at their minimum separation ("nulled position") they induce nearly zero refraction. At their maximum separation they produce enough refraction to correct well to almost 30° elevation.

Rotation

The Cass ADC prisms do not rotate; they are fixed and LRIS rotates relative to them. This means that as the LRIS physical drive angle changes (for example, to maintain a constant sky position angle for LRIS) the observer will be looking through a different part of the ADC prisms.