Last Updated Jan 2001

Copyright © 2001 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

The Guide Star Automatic Measuring MAchine (GAMMA) is a laser-illuminated multi-channel scanning microdensitometer, modularly built upon the substrate of the modified PDS used in earlier ST ScI work. A rebuild of the x- and y- servos is based on a HP 5507 laser transducer system with a custom card to implement the PDS "1/N" functionality. The light source is a spatially filtered 2~mW HeNe laser beam, expanded to 1 mm (FW 1/e^2 ), moved with a TeO(2) acousto-optic deflector (AOD), and finally imaged on the plate as a 42 µ Gaussian spot. This size gives low aliasing with the adopted 15 µ sampling.

The table moves in x at 100 mm/sec, while the AOD steps the laser beam through a small number of channels, typically 5, in the y -direction. The AOD is controlled by a frequency synthesizer operated from a digital signal processor (DSP) which cycles through a channel-to-frequency table, synchronized by pulses from the servo 1/N logic. The DSP also removes y -position errors due to residual servo auto-lock disequilibrium by applying a small frequency correction to the AOD, based on a linear function of the error signal from the y-servo.

The light collecting optics consists of a standard microscope, an exit slit, an integrating sphere to make the response insensitive to the channel number, and a photomultiplier. Pixel times are about 25-50 µsec, and the data conversion is done with a floating point ADC. A separate photodiode to monitor the laser flicker is also provided for photometric normalization.

A dedicated VAXStation with IEEE-488 and CAMAC interfaces controls the GAMMA. Separate programs operate each subsystem (servo, data collection, laser monitoring, AOD, console), and communicate with each other via shared memory and common event flags. This matching of the software and hardware architectures results in easily maintainable code.