New electronic read-out design for astronomical detectors

Current-generation imaging electronics are large, heavy, and dissipate a lot of power. These drawbacks are becoming ever more important, particularly for wide-field instrumentation using mosaicked detector arrays. We have recently developed new visual and IR electronic read-out systems aimed at implementation in astronomical cameras. The modular design allows configuration of the electronics for a wide range of currently available IR detectors and CCDs. The new read-out concept can be applied to single or multiple detector systems with up to 144 input channels. The high data-transfer rate, small size (33×13×31cm³), and low heat dissipation make these electronics ideal for use in relatively large focal-plane detector arrays. The first instrument employing the new system is the Panoramic Near-Infrared Camera (PANIC) at the 2.20m (diameter) telescope at Calar Alto Observatory in Spain (see Figure 1).

Our main goal was to design a system which could easily be configured to control both IR and visible detectors. The electronics should also be capable of controlling read-out systems associated with different types or a mosaic of detectors. These requirements were met by developing standard hardware boards and an arbitrary-pattern generator. Different types of detectors are supported by exchangeable interface boards. Systems can be extended by adding more standard components, allowing the construction of detector mosaics or simply to increase the number of input channels. Different back planes are used to connect transfer signals to the corresponding system boards.

Figure 1. New read-out electronics for the Panoramic Near-Infrared Camera at the 2.20m (diameter) telescope at Calar Alto Observatory in Spain.

Figure 2. Simplified block diagram of the controller board. The green devices collect digitized pixel values and transmit data to the work station. The violet pattern generator on the right generates complex signal sequences with deterministic programmable timing. CAN: Controller area network. Cmds.: Commands. FPGA: Field-programmable gate array. Reg.: Register. RS232: Recommended standard-232 (for binary-data signal transmission commonly used in computer serial ports). SRAM: Static random-access memory.

The standard system^1,2 includes detector-protection circuitry and operates with up to four detectors, such as HAWAII2-RG (Rockwell Science) or Fairchild CCD486 devices. It runs on low power (<50W) and is characterized by low read-out noise (2.1 electrons for doubly correlated read-out). Window techniques and on-chip guiding are supported for a number of different read-out modes, including doubly correlated, line-interlaced, multiple end-point, and ‘up the ramp’ sampling.

Figure 3. Optoisolated cluster interconnect hardware board with two fast fiber-optic data interfaces.

The heart of the electronics system is the read-out controller board (see Figure 2 for a simplified block diagram). The central processing unit contains two fiber transmitters and one microcontroller module. A programmable pattern generator combined with 8Mb static random-access memory is responsible for the detector interface-control sequences. Another unit reads the digital pixel values and triggers data transmission to the workstation over two fiber links. The microcontroller module handles communications with the outside world via either a serial or an Ethernet link. An optoisolated cluster interconnect board (Figure 3) receives the incoming data stream. The serial data is converted into parallel words and written into the corresponding first-in first-out memory queue. If an adjustable threshold is reached, a direct-memory-access request is generated, and data is transferred into memory at a rate of 250Mb s⁻¹.

To construct complex read-out systems one must implement powerful built-in diagnostic tools to test all functions at the different system levels. A graphical user interface was developed to configure the hardware for different detectors and optimize all detector parameters. The new read-out electronics constitute a powerful, future-oriented system. The flexibility and modular architecture allow the system to adapt to many different visual and IR detectors in a wide range of applications in astronomical instrumentation.² We will next install the new system design into the Large Binocular Telescope's (LBT) LINC (LBT Interferometric Camera) NIRVANA (Near-IR/Visible Adaptive Interferometer for Astronomy) beam combiner.