Astronomical Research Cameras, Inc. has developed a set of versatile controllers designed
to operate a wide variety of optical and infrared imaging arrays. Originally designed for
operating CCDs in a slow scanned readout mode, they have been broadened to operate
CCDs and infrared arrays read out at medium speeds. CCD and IR arrays use all the same
controller elements except for the video processor and its associated readout software.
The camera electronics described here is targeted to operate one or more CCD or infrared arrays, each containing one or more readout circuits. From one to 64 readouts can be operated efficiently with one set of camera electronics, coming from multiple arrays, multiple readouts per array, or both. The large dynamic range of CCDs and IR arrays is maintained through the use of low noise techniques that produce detector limited noise levels, and 16-bit A/D converters at speeds of up to one million pixels processed per readout per second. Considerable flexibility has been built into the design to allow operation of a variable number of readouts, as well as operation of a wide variety of both optical and infrared arrays in such modes as staring, drift scan, shuttered, windowed, binning and frame transfer. They can be programmed by the user to meet a wide variety of applications, while several flavors of readout source code are available to handle simple configurations and to provide the user with a starting point for customizing their systems.
These systems are suitable to be used in high end scientific applications where low readout noise, high dynamic range, and repeatable and reliable operation are all important. They can be used in mosaic systems containing more than one array operating synchronously in a single focal plane. They can be used with almost any, or perhaps any, CCD or IR array. The CCD or IR array can be mounted several meters distance from the controller, allowing for compact detector heads to be mounted in space constrained locations and the relatively large controller to be placed where space permits. The controllers are particularly well suited for programs that involve experimentation or optimization with one or more flavors of detectors because they can be used with almost any, or perhaps any, type of CCD or IR array, while allowing the user to adjust the clocking, biasing, or readout methodology relatively easily.
A typical CCD camera system is shown in the figure. The major elements of the system are as follows:
The large, nearly cylindrical gold colored object on the right is a liquid nitrogen dewar
held under vacuum that is used to keep the CCD cold during operation, manufactured by
IR Labs of Tucson, Arizona. Off the figure on the right side on the flat plate perpendicular
to the dewar's axis is a large window behind which is located a CCD with 2048 x 4096
pixels. It is mounted to a housing containing controller boards, shown with two of its panels
remove for visibility and access. Connecting wires between the CCD and the controller
boards can be seen to the far right of the figure, entering the board on top through two black
and silver colored DB connectors. This is the CCD video board that processes two readout
channels of the CCD, converting the analog video streams into digital streams with the two A/D
converters, visible towards the back of the board as two gold plated objects with blue
lettering. Behind this video board are located a clock driver board and a timing board, with
all three boards plugged into the controller backplane. The grey box to the left is the power
supply, supplying the DC power to the controller through the large black cable with
circular aluminum connectors at each end. The small board in front is the PCI interface that
accepts digital image data from the controller over the black fiber optics cable shown coiled up
and writes them to the host computer. In use, the dewar and controller would be mounted on a telescope or
other source of images in a light-tight enclosure, and the PCI board would be put
in a computer located some distance away.
This section describes the major system components in moderate detail, including alternative video processors that are used with infrared arrays. The electronics board are described first, followed by the mechanical, power supply, dewar and host computer software components.
The ARC-22 250 MHz fiber optic timing board provides digital timing (or sequencing) signals for controlling the array, and communicates with the PCI interface board over the fiber optic cable. Its heart is the Motorola DSP56003, a monolithic, integer digital signal processor with a 24-bit data word. It has a 24-bit address space, a fast ALU, extensive onchip peripheral support and a Reduced Instruction Set Computer (RISC) architecture. It writes 16-bit data words as often as every 40 nanosec over the backplane to the clock driver board, which controls the array clocks needed to move the charge around the CCD or select the IR pixel to be read out. It also controls the video processor functions and supervises the transfer of image data to the PCI board, as well as overall controller operation.
The timing board executes a program that generates clocking patterns and sets the DC bias voltages specific to the detector of choice. The source code files for this program are resident in the host computer, where assembler and linker tools are used to generate a DSP object code file that is downloaded from the computer. This operation of editing the source code files, assembling, linking and downloading the object code is quite fast, allowing relatively efficient development of readout and control software.
The ARC-32 clock driver board supplies a total of 24 clocks for driving CCD or IR arrays over a range of up to +13 to -13 volts. The clock voltages are set by 8-bit digital-to-analog (DAC) converters. Two DAC output voltages are used by each clock driver circuit, with a fast analog switch selecting one of these two voltages for amplification by a fast op amp. Low noise and fast circuit techniques produce 20 nanosec switching times for a 10 volt swing. RC networks can be installed by the user to slow down these switching times, and resistor networks can be altered by the user to change the maximum and minimum voltages output by the board in the interest of protecting the detecor.
The ARC-45 CCD video processing board has two input amplifiers, one single-ended with a gain of x4 for direct connection to CCDs, and a unity gain differential stage for connection to preamplifiers mounted next to the CCD. After some gain, a polarity reversing amplifier drives a resettable integrator to implement a correlated double sample integrator. A fast amplifier operates at x2 gain to drive the A/D converter and provide offset adjustment. The video processor operates at a maximum rate of 1.0 microseconds per pixel, limited by the conversion time of the A/D converter. Two complete video processors are provided on each board, operating simultaneously and read out separately over the backplane under control of the timing board. A set of 12-bit DACs provide 14 DC bias voltages for powering the CCD(s).
The ARC-42 infrared video processor board is similar to the CCD video processor, but has simpler analog circuits because a digital correlated double sample is done digitally in the computer. It supplies six DC bias voltages for driving the IR array.
The ARC-50 utility board provides a miscellany of support functions that are not directly involved with readout of the detector. These include, but are not limited to, exposure timing, array temperature control, and system voltage and temperature monitoring. Based around a DSP56001, it is supplied with programming to support these functions, and can be programmed by the user to support other functions (such as an additional temperature controller, dewar level and ID, shutter status, LED driving for status, switch monitoring for direct system control without a host computer) by programming the use of a number of uncommitted andlog and digital I/O circuits.
The ARC-73 power control board conditions the DC power to protect the detectors from transient or incorrect voltages. This board switches power from the power supplies to the backplane under command from the timing board after its DSP software has been downloaded so that incorrect or transient voltages are not passed on to the analog boards or detectors. A bank of comparators examines the incoming power and turns off these switches in the event of a power supply failure after the power has been switched on.
The ARC-64 PCI and the ARC-65 PMC interface boards provide a communication path between the fiber optic link on the timing board and the host computer. They send commands from the host computer to the timing board following the 24-bit protocol of the serial link, accept image data from the timing board and write them to the host computer application memory using an on-board DMA (Direct Memory Access) controller without intervention from the host processor. This allows non real-time operating system such as UNIX and Windows to be used in the host computer, and permits concurrent operation of the host computer during image readout. Exposures are initiated by the host computer, timed by the timing board, and written into memory by the interface board, after which the interface board signals to the host processor that the image is available in memory so it can be processed, displayed and stored by the host computer.
The ARC-80 (large) and ARC-81 (small) units supply the DC power needed to operate the controller. Utilizing 40 watt (large supply) and 20 watt (small supply) switcher modules a bank of inductors and capacitors on a filter board suppresses the ripple and noise inherent in switcher modules to an acceptable level for lower noise detector operation. The large power supply resides in a box about twice as large as the small power supply, and produces about twice the powr. Additionally, the small power has a driving circuit suitable for operating solenoids on iris-type shutter.
The ARC-70 (6-slot) and ARC-72 (12-slot) controller housings consist of machined aluminum frames with removable front, back and side panels for easy access to the controller boards, a backplane to accommodate either six or twelve controller boards and an ARC-73 power control board mounted behind the backplane.
The ARC-46 (IR) and ARC-48 (CCD) 8-channel video processor boards can be profitably used in systems containing a large number of readout channels. They are similar in concept to their two channel counterparts, but provide 8 channels of simultaneous readout. They also supply the DC bias voltages for powering CCD and IR arrays.
The "voodoo" and "owl" host computer image acquisition and control programs are user interface programs for communicating with and controlling the systems from a host computer running either Sun Solaris, Red Hat Linux or Windows XP/Vista operating system. They both initialize the controller DSPs with downloaded software, set user parameters, issue exposure sequence commands, allow individual manual commands to be issued and writes image to disk in the FITS or TIFF format. They are both written in Java and perform many of the same tasks, while "owl" is a newer and more powerful program targeted for eventually replacing "voodoo".