CASPIR is designed to operate in the high background conditions
encountered at long wavelengths. It uses four 16-bit 500 kHz
analog-to-digital converters to digitize the data, with the rest of
the data train being capable of sustained data rates of at least 2
s/pixel. The requirement that individual frames be coadded at
this data rate to build up the image means that the data cannot be
input directly into MOPRA. Instead a transputer-based
preprocessor is used.
A transputer is a fast microprocessor chip with considerable in-built parallelism and which uses four fast serial ``links'' for I/O. Each link can be connected to another transputer, or to an external device through a ``Link Adaptor'', which is essentially a bi-directional serial to parallel converter. This architecture has made transputers popular in parallel computing applications.
The CASPIR system uses four transputer link adaptors on the ADC cards to serialise the data at the Cassegrain focus. Four serial lines then bring the signals to the Nasmyth Lab where the transputer preprocessor is located. This consists of four T800 transputer boards which each have 1 Mbyte of memory and each are responsible for processing the data from one serial line. The current frame is DMAd into transputer memory at the same time that the previous frame is being coadded to an accumulation array where the summed image is stored. After the requested number of coadd cycles, the accumulated image is divided by the number of coadd cycles, optionally has a bias frame subtracted and is divided by a flatfield frame, and is copied to MOPRA via a transputer link to Q-bus interface with the four separate data channels correctly interlaced to form the final image.
The result of each sequence of coadd cycles is displayed on MOPRA's workstation screen and is stored on MOPRA's disk. The individual frames from each cycle are not normally saved (this only occurs in the occultation observing mode).