AATSR is the latest in a series of instruments designed to measure global sea-surface-temperatures to an accuracy of 0.3 K and to monitor global vegetation coverage and cloud properties. It forms part of the payload on ESA's ENVISAT mission due to be launched in 2000. Features new to AATSR include a different cooler system, an improved mechanical structure, and a corrected visible calibration system, VISCAL. The methods and results of the instrument's pre-launch calibration tests are described. These include the field-of- view, visible and infrared radiometric calibrations. The radiometric responses of the visible/near infrared channels were measured, and the in-flight VISCAL unit was calibrated. The calibrations of the thermal infrared channels were verified over a range of target temperatures between 210 K to 315 K and corrections derived for detector non-linearity. Tests were also performed to verify freedom from any significant scan dependent variations or effects due to changes in the thermal environment. Data from the initial calibration run identified a major fault with the instrument's optical alignment, and was vital in establishing the solution for the eventual repair. In addition, the calibration of the visible channels revealed important characteristics affecting the accuracy of the scientific products that would otherwise have been overlooked.
Solar diffuser based monitors are the preferred method for on- board calibration for short wavelength regions of Radiometric Earth Remote Sensing instruments where spectral matching and long term stability are paramount. This paper describes an aluminum integrating sphere, with internal photo-diode monitoring, being developed for the on-board short wavelength, (0.32 - 4 micrometer) calibration monitor of the GERB instrument. GERB will image the earth surface from geostationary orbit over a bandwidth of 0.32 - 30 micrometer and is mounted on the Meteosat Second Generation (MSG) spin stabilized satellite resulting in a very rapidly rotating field of view of GERB (100 RPM). The adopted arrangement for the integrating sphere is described and its performance illustrated with supporting test data and optical modeling. Comparisons with the ATSR-2, MS20 flat tile system are made and recommendations for future calibration systems, drawn.
An in-flight visible calibration system, VISCAL, is used to calibrate the visible/near infra-red channels of ATSR-2. The in-flight monitoring of the VISCAL is described and results presented. Data shows that the visible channels are affected by condensation effects as well as some long term degradation. The accuracy of the measured ground reflectances is dependent on the long term stability of the calibration. An area within the Libyan Desert has been used to determine calibration drift rates of 0.5% (1.6 micrometer), 1.8% (0.87 micrometer), 0.8% (0.66 micrometer) and 1.7% (0.56 micrometer) per year.
The Rutherford Appleton Laboratory Photon Counting Detector (RALPCD) is a highly adaptable intensified imaging system with applications in the x-ray, EUV and visible wavelength regions. The detector comprises commercially available high gain microchannel plate intensifiers fiber optically coupled to CID or CCD cameras, to form a modular detector arrangement. Frames of data from the cameras are detected and centroided in a transputer parallel processor array where correction algorithms using look up tables are used to produce pattern free images at high resolution. Data from the applications are used to illustrate the performance and future advances are discussed.
We describe an imaging photon counting system for the EUV and its application to the measurement of the imaging quality of CDS EUV telescope for SOHO. The detector system uses an open window micro-channel plate intensifier coupled to a CCD with a fiber-optic taper. The digitized data are processed using a transputer based processing system which resolves the channel pores in the front micro-channel plate. The pores are used to register the EUV image. Dithering of the detector is used to enhance the resolution.
Photoemissive photon counting tubes of 40 mm diameter are routinely made, and 75 mm tubes are in development. When these tubes are coupled to a suitable scintillator, substantial quantum efficiency can be achieved from soft rays through to gamma energies. Available scintillators and their performance are described in this paper. The advent of fast PC compatible computers has enabled the electronics readout to be improved, simplified, and reduced in cost. The new electronic readout system is described, and the image processing capability outlined. The pulse height distribution is available to the user, and to a limited extent, this allows the user to measure the energy of the input photons to the system. Systems of this type are used for x-ray crystallography, and could find wider applications in nuclear medicine and gamma ray photography.
The requirement for ever improved resolution at lower and lower photon levels has lead researchers to a number of various possible systems. Since we make image tubes on a custom basis, we have made several different photon counting tubes with different readout systems for a variety of laboratories. These readout systems are reviewed in our paper, and we attempt to analyze the state of the art with these different readout systems. Since all these systems use micro channel plate electron amplification, the fundamental properties of the channel plate effects them all to some degree. Advanced micro channel plates are being assessed, and the progress made is reviewed. Advanced electronic readout systems based upon the latest components, including transputers, promises increased resolution and count rates in systems optimized for X-ray and UV applications.
Many space applications of photon counting detectors (PCDs) are particularly challenging due to the requirements for high local and global count rates and for sensitivity down to the soft x ray region. The count rates per pixel required may be as high as several hundred counts/second and overall count rates as high as 1,000,000 counts/second. At the Rutherford Appleton Laboratory (RAL) we have been developing a range of modular detector systems in the visible and UV which are built specifically with solar UV observing in mind. The high resolution/high count rate imaging performed pushes the technology to the limit and shows up, in some detail, problems in intensifier and microchannel plate (MCP) manufacture. Close cooperation with the manufacturers of the intensifiers used, Photok Ltd., has led to improvements in the intensifier design. In the following paper the performance of these detectors is discussed in detail along with plans for their future development.
A prototype 40-mm-diameter proximity-focused microchannel-plate intensifier intended for photon-counting applications in both ground-based and space astronomy is described. The intensifier described is a sealed-window device and is also well suited to open-window ultraviolet applications in space astronomy. The tube makes use of a combination of an unfilmed curved-channel plate (C-plate), to prevent ion feedback, and a single straight-channel plate, aligned to prevent optical feedback. The tube has excellent cosmetic quality and shows a counting efficiency superior to that of filmed plate devices reported previously, giving an overall detective quantum efficiency at least equal to that of the best four-stage magnetically focussed intensifiers.
The MIC, a 40-mm intensified microchannel-plate photo-counting detector being developed for the Anglo-Australian, Isaac Newton, and William Herschel telescopes, is described and illustrated with diagrams and sample spectra. The MIC is linked by optical fibers to a fast-scanning CCD detector, and an accurate centroiding technique is applied to yield an effective maximum of 3104 x 2304 10.6-micron pixels, for field-averaged resolution 27 microns FWHM. Applications include high-resolution spectroscopy, especially in the blue, and Fabry-Perot and speckle interferometry.
A prototype microprocessor-based electronic processing system is described which performs the on-line data processing in an image photon-counting detector. The detector head consists of a microchannel-plate intersifier optically coupled to a CCD with rapid-scanned readout. The digitized data are passed to a group of transputers which separate out the events, calculate the event centers to subpixel accuracy and accumulate the results in an image buffer. In the prototype, all processing is done in software for evaluation purposes. This paper discusses the basic design of the system, the centroiding algorithm and fixed patterning correction, and the implications of the results for future systems.