This paper will discuss and compare recent refractive and catodioptric imager designs developed and manufactured at SunSpace for Multi Sensor Satellite Imagers with Panchromatic, Multi-spectral, Area and Hyperspectral sensors on a single Focal Plane Array (FPA). These satellite optical systems were designed with applications to monitor food supplies, crop yield and disaster monitoring in mind. The aim of these imagers is to achieve medium to high resolution (2.5m to 15m) spatial sampling, wide swaths (up to 45km) and noise equivalent reflectance (NER) values of less than 0.5%. State-of-the-art FPA designs are discussed and address the choice of detectors to achieve these performances. Special attention is given to thermal robustness and compactness, the use of folding prisms to place multiple detectors in a large FPA and a specially developed process to customize the spectral selection with the need to minimize mass, power and cost. A refractive imager with up to 6 spectral bands (6.25m GSD) and a catodioptric imager with panchromatic (2.7m GSD), multi-spectral (6 bands, 4.6m GSD), hyperspectral (400nm to 2.35μm, 200 bands, 15m GSD) sensors on the same FPA will be discussed. Both of these imagers are also equipped with real time video view finding capabilities. The electronic units could be subdivided into the Front-End Electronics and Control Electronics with analogue and digital signal processing. A dedicated Analogue Front-End is used for Correlated Double Sampling (CDS), black level correction, variable gain and up to 12-bit digitizing and high speed LVDS data link to a mass memory unit.
Hybrid sensors performance critically depends on the performance of the analog read-out electronics. The analog design methodologies are very well known and documented provided the operating temperature stays above temperatures where carrier freeze-out occurs. Even though the behavior of individual MOS transistors at low temperature, i.e. below 30 K, has been studied in detail, this has not yet led to design guidelines for the design of building blocks and/or complete systems that will operate satisfactorily at these low temperatures. The main challenges are the design of amplifiers with high open-loop gain, low electrical power consumption and low noise. By providing cryogenic design guidelines, this paper tries to bridge the gap that exists between the information available on the MOS transistors and the needs of design engineers confronted with practical problems. The issue of transistor dimensioning using simple models of the MOS transistor will be discussed.
The Photoconductor Array Camera and Spectrometer (PACS) will be equipped with two sensor arrays consisting of 16 X 25 pixels each. Arranged in linear arrays of 16 detectors the sensitivity of the sensors is tuned to the wavelength ranges 60 micrometers to 130 micrometers and 130 micrometers to 210 micrometers , by applying different levels of stress to the Ge:Ga crystal utilizing a special leaf spring which is part of each of the 25 modules. The electronics of the sensors are mounted on the same module but thermally isolated from the sensor level which is at a lower temperature of about 2 K. The sensors are read out by a specially developed integrating and multiplexing cryogenic read-out electronics. With a fore optics made of light cones in front of the detector cavities a 100% filling factor is achieved and a high quantum efficiency close to 0.5 is expected. In order to achieve extremely good stress uniformity in all detectors and therefore equal cutoff wavelengths, a high degree of the quality of the Ge:Ga detectors and of the assembling components used for this dedicated stress mechanism is required. The first 6 engineering modules have been successfully manufactured and tested afterwards. The relative responsivity of a set of pixels has been determined and a good performance has been demonstrated for the sensors, which are very close to fulfilling the requirements for PACS aboard the infrared spectra telescope FIRST.