The capabilities and performance of focal plane subsystems used in astronomical space telescopes have increased significantly over the last 45 years. Significant gains have been made in format size, sensitivity, and spectral range coverage. This paper outlines the history of UV, visible, and IR astronomy missions occurring over this time period and describes the focal planes used. We also discuss the progression of the associated detector and focal plane technology during this timeframe.
Although there have been significant gains over the last 45 years, there is still both the promise and need for continued improvement. Several missions over the next few years will use new and innovative technologies. In this paper, we describe upcoming missions and how technological breakthroughs in detectors, focal plane packaging, and readout electronics will extend the reach of science. Finally, we conclude by describing how these technologies will mature over the next 10 years.
There is little doubt that the Hubble Space Telescope is one of the most important astronomical instruments of the late 20th and early 21st centuries. Spectacular imagery has been sent back to earth-bound observers leading to a much greater understanding and appreciation of the universe around us. Much of this ability to image has been due to an extraordinary detector: the charge-coupled device (CCD). The intent of this paper is to review the history of the CCDs that have been employed on Hubble and consider some of the lessons learned.
The Corot project, developed in the framework of the CNES small satellite program with a wide European cooperation, will be launched in 2006. It is dedicated to seismology and detection of telluric planets. It will perform relative photometry in visible light, during very long (150 days) observing runs in the same direction. Both programs are running simultaneously and about 50.000 stars will be observed during the 3 years life. The concept of the instrument is based on an off-axis telescope (27 cm pupil, 3° square field of view), a dioptric objective images the stars on a focal plane. The focal plane is made of 4 CCDs, 2k*4k pixels, AIMO and frame transfer, at -40°C, two for each scientific programs. Electronics boxes manage the CCD readout, the thermal control and house-keeping, onboard software makes pre-processing and data reduction. As the expected signal is made of very small fluctuations expressed in ppm (part per million) a specific calibration of all the photometric chain and sub-systems is necessary. We have developed a specific test bench to calibrate CCDs. The manufacturer (E2V) provided us 10 CCDs and we realized calibration tests on them to be able to choose 4 CCDs for the flight focal plane (with different optimizations for the two scientific programs). Thereafter the camera sub-system has been integrated and calibrated on a specific test bench. This sub-system is made of a dioptric objective, focal plane, electronics boxes, mechanical and thermal equipment. We will present the camera sub-system (constraints and design), the test bench, and the results of the different tests : CCD calibration, radiation effects, Focal-Plane integration, optical setup, thermal balance and Camera calibration.
ESA's Gaia astrometry mission is due for launch in 2011. The astrometric instrument focal plane will have an area of up to 0.5m2 and will contain more than 100 CCDs. These will be operated in Time Delay and Integration mode in order to track and observe sources whilst the telescopes continuously scan the sky. Gaia's target for astrometric precision of a few millionths of an arc second, places extreme demands on focal plane thermo--mechanical stability and electronics performance. The CCDs themselves are large area, back illuminated, full--frame, four phase devices. They require maximum efficiency for observing the majority of (faint) objects, yet must simultaneously be able to handle very bright objects that will regularly cross the field of view. Achieving the final astrometric precision will also require excellent noise performance and MTF. In addition to demanding excellent performance from each CCD, they will need to be produced in large numbers which raises production and yield issues. When analyzing Gaia data it will be essential to understand and calibrate CCD behaviour correctly, including the expected performance degradation due to radiation damage. This is being addressed through comprehensive testing and the development of CCD models.
The James Webb Space Telescope (JWST) will have several on-board instruments, of which the Mid-Infrared Instrument (MIRI) will cover imaging and low resolution spectroscopy in the 5-28 micron region. To achieve the many science goals for MIRI, the detector arrays must be capable of achieving high sensitivity. The primary obstacle to high sensitivity is the total noise. The total noise is often dominated by two main parts: the read noise of the multiplexer and the shot noise of the detector array's dark current. We present recent results of the measured read noise for several candidate multiplexers from the first Si-foundry run.
1K × 1K Si:As Impurity Band Conduction (IBC) arrays have been developed by RVS for the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI). MIRI provides imaging, coronagraphy, and low and medium resolution spectroscopy over the 5 - 28 μm band. The IBC devices are also suitable for other low-background applications. The Si:As IBC detectors have a pixel dimension of 25 μm and respond to infrared radiation between 5 and 28 μm, covering an important Mid-IR region beyond the 1 - 5 μm range covered by the JWST NIRCam and NIRSpec instruments. Due to high terrestrial backgrounds at the longer Mid-IR wavelengths, it is very difficult to conduct ground-based observations at these wavelengths. Hence, the MIRI instrument on JWST can provide science not obtainable from the ground. We describe results of the development of a new 1024 × 1024 Si:As IBC array that responds with high quantum efficiency over the wavelength range 5 to 28 μm. The previous generation's largest, most sensitive infrared (IR) detectors at these wavelengths were the 256 × 256 / 30 μm pitch Si:As IBC devices built by Raytheon for the SIRTF/IRAC instrument1. Detector performance results will be discussed, including relative spectral response, Responsive Quantum Efficiency (RQE) vs. detector bias, and dark current versus temperature. In addition, Sensor Chip Assembly (SCA) data will be presented from the first Engineering SCAs. The detector ROIC utilizes a PMOS Source Follower per Detector (SFD) input circuit with a well capacity of about 2 × 105 electrons. The read noise of the "bare" MUX is less than 12 e- rms with Fowler-8 sampling at an operating temperature of 7 K. A companion paper by Craig McMurtry (University of Rochester) will discuss the details of SB305 MUX noise measurements2. Other features of the IBC array include 4 video outputs and a separate reference output with a frame rate of 0.36 Hz (2.75 sec frame time). Power dissipation is about 0.5 mW at a 0.36 Hz frame rate. Reset modes include both global reset and reset by row (ripple mode). Reference pixels are built-in to the output data stream. The 1K × 1K IBC is packaged in a robust modular package that consists of a multilayer motherboard, SiC pedestal, and cable assembly with 51-pin MDM connector. All materials of construction were chosen to match the thermal expansion coefficient of Silicon to provide excellent module thermal cycle reliability for cycling between room temperature and 7 K.
Electron multiplying CCD (EMCCD) technology has found important initial applications in low light surveillance and photon starved scientific instrumentation. This paper discusses the attributes of the EMCCD which make it useful for certain space instruments, particularly those which are photon starved, and explores likely risks from the radiation expected in such instruments.
A scientific camera system having high dynamic range designed and manufactured by Thermo Electron for scientific and medical applications is presented. The newly developed CID820 image sensor with preamplifier-per-pixel technology is employed in this camera system. The 4 Mega-pixel imaging sensor has a raw dynamic range of 82dB. Each high-transparent pixel is based on a preamplifier-per-pixel architecture and contains two photogates for non-destructive readout of the photon-generated charge (NDRO). Readout is achieved via parallel row processing with on-chip correlated double sampling (CDS). The imager is capable of true random pixel access with a maximum operating speed of 4MHz. The camera controller consists of a custom camera signal processor (CSP) with an integrated 16-bit A/D converter and a PowerPC-based CPU running a Linux embedded operating system. The imager is cooled to -40C via three-stage cooler to minimize dark current. The camera housing is sealed and is designed to maintain the CID820 imager in the evacuated chamber for at least 5 years. Thermo Electron has also developed custom software and firmware to drive the SpectraCAM SPM camera. Included in this firmware package is the new Extreme DRTM algorithm that is designed to extend the effective dynamic range of the camera by several orders of magnitude up to 32-bit dynamic range. The RACID Exposure graphical user interface image analysis software runs on a standard PC that is connected to the camera via Gigabit Ethernet.
We report on the measurement results for the STA model 1046 CCD, designed to support requirements for USNO's proposed Astrometric Mapping Explorer (AMEX) space astrometry mission. The STA1046 is found to meet AMEX requirements for operating speed, read noise, charge transfer inefficiency (CTI) and dark current after exposure to a high-energy proton fluence of 5 x 109 p+ cm-2 (@63.3 MeV). The lab measurements provide the basis for developing a validated performance model of the STA1046 which we use, along with USNO's Radiation Effects Simulation (RES), to predict on-orbit centroiding performance of an STA1046-based instrument.
Silicon-based hybrid CMOS visible focal plane array technology is emerging as a viable high performance alternative to scientific CCDs. The progress is attributed to the rapid advances in CMOS technology, mature precision flip-chip hybridization of large size and fine pixel arrays, and detector array performance improvements. Its technology readiness level (TRL) for space applications is being enhanced by relevant environmental tests and in-depth characterization of sensor performance. In this paper, we present recent results of Rockwell Scientific's hybrid CMOS silicon focal plane array technology, including large format arrays up to 2048x2048, broadband QE, sensor noise improvement, high radiation hardness, and the higher degree of system integration through on-chip ADCs and companion ASICs.
The digital focal plane array (DFPA) project demonstrates the enabling technologies necessary to build readout integrated circuits for very large infrared focal plane arrays (IR FPAs). Large and fast FPAs are needed for a new class of spectrally diverse sensors. Because of the requirement for high-resolution (low noise) sampling, and because of the sample rate needed for rapid acquisition of high-resolution spectra, it is highly desirable to perform analog-to-digital (A/D) conversion right at the pixel level. A dedicated A/D converter located under every pixel in a one-million-plus element array, and all-digital readout integrated circuits will enable multi- and hyper-spectral imaging systems with unprecedented spatial and spectral resolution and wide area coverage. DFPAs provide similar benefits to standard IR imaging systems as well. We have addressed the key enabling technologies for realizing the DFPA architecture in this work. Our effort concentrated on demonstrating a 60-micron footprint, 14-bit A/D converter and 2.5 Gbps, 16:1 digital multiplexer, the most basic components of the sensor. The silicon test chip was fabricated in a 0.18-micron CMOS process, and was designed to operate with HgxCd1-xTe detectors at cryogenic temperatures. Two A/D designs, one using static logic and one using dynamic logic, were built and tested for performance and power dissipation. Structures for evaluating the bit-error-rate of the multiplexer on-chip and through a differential output driver were implemented for a complete performance assessment. A unique IC probe card with fixtures to mount into an evacuated, closed-cycle helium dewar were also designed for testing up to 2.5 Gbps at temperatures as low as 50 K.
Burst noise (also known as popcorn noise and random telegraph signal/noise) is a phenomenon that is understood to be a result of defects in the vicinity of a p-n junction. It is characterized by rapid level shifts in both positive and negative directions and can have varying magnitudes. This noise has been seen in both HAWAII-1RG and HAWAII-2RG multiplexers and is under investigation. We have done extensive burst noise testing on a HAWAII-1RG multiplexer, where we have determined a significant percentage of pixels exhibit the phenomenon. In addition, the prevalence of small magnitude transitions make sensitivity of detection the main limiting factor. Since this is a noise source for the HAWAII-1RG multiplexer, its elimination would make the HAWAII-1RG and the HAWAII-2RG even lower noise multiplexers.
The results of total ionizing dose and proton fluence characterization of hybrid Si P-i-N focal plane arrays are reported. The focal plane arrays consist of a silicon P-i-N detector array bump bonded to 128 x 128 CMOS readout integrated circuit (ROIC). The FPAs were characterized in total ionizing dose and proton fluence radiation environments. Full radiometric characterizations were performed at each radiation dose level to determine the impact of the radiation on dark current, noise, responsivity, sensitivity, and dynamic range. Results from the total ionizing dose experiment demonstrate an unexpected increase in the visible P-i-N detector dark current. The median dark current increased more than two orders of magnitude from pre-radiation to 300 krad(Si) and the magnitude of the dark current was found to be a strong function of detector bias. No appreciable change in responsivity or noise was observed for wavelengths above 400 nm up to a total ionizing dose of 750 krad(Si). Results from the proton radiation experiment show no appreciable change in responsivity was observed up to a 63 MeV proton fluence of 3 x 1012 protons/cm2 (400 krad(Si) of total ionizing dose). The median dark current increased approximately two orders of magnitude, but even at this higher level, the dark current did not contribute significantly to the median noise at an integration time of 10 ms. The dominant degradation mechanism, in both the total ionizing dose and proton fluence environments, is an increase in dark current in the Si P-i-N detectors.
As spacecraft missions increase in scope and duration, the need for established focal planes to fulfill these mission requirements increases proportionally. The visible hybrid HAWAII-2RG (HgCdTe Astronomy Wide Area Infrared Imager with 2k x 2k resolution, reference and guide mode) has a venerable terrestrial-based telescope history. These focal planes are considered candidates for space applications. As a candidate focal plane, the responses of the HAWAII-2RG under nominal operating conditions in an ionizing debris gamma environment are discussed. Measurements in dark current frame captures and voltage bias currents are delineated.
With the introduction of the Raytheon 2.5 micron HgCdTe VIRGO detector array, some astronomy programs, such as the VISTA Program, are turning to Raytheon for near infrared detector arrays. We characterize one VIRGO detector array and provide results of measurements at low backgrounds including dark current, read noise, total noise, quantum efficiency, and operability. The Raytheon VIRGO HgCdTe detector arrays are excellent candidates for many low background astronomical programs, including space-borne telescope missions.
Proton induced luminescence in the HgCdTe detectors for the Wide Field Camera 3 instrument has been investigated. A radiation experiment has been conducted to localize the source of the luminescence. Conclusive evidence is shown that the luminescence originates in the CdZnTe substrate and propagates toward HgCdTe photodiodes as ~800 nm radiation. Luminescence is proportional to the proton energy deposited in the substrate. Subsequent testing of detectors with the substrate removed confirmed that substrate removal completely eliminates proton induced luminescence.
We present results of CCD radiation testing for a proposed Jovian mission. Samples of two candidate star tracker CCDs were irradiated with 10-MeV and 50-MeV electrons at Rensselaer Polytechnic Institute's Gaerttner LINAC. Differences in displacement damage effects on CCD parameters and star tracker performance are discussed for these two energies. Dark current, charge transfer efficiency (CTE), hot pixels, and flat-band voltage shifts are examined. Our electron data is compared to proton irradiation data taken by other experimenters. 10-MeV electron-induced transient data are also discussed.