Recent European atmospheric imaging missions have seen a move towards the use of CMOS sensors for the visible and NIR parts of the spectrum. These applications have particular challenges that are completely different to those that have driven the development of commercial sensors for applications such as cell-phone or SLR cameras. This paper will cover the design and performance of general-purpose image sensors that are to be used in the MTG (Meteosat Third Generation) and MetImage satellites and the technology challenges that they have presented. We will discuss how CMOS imagers have been designed with 4T pixel sizes of up to 250 μm square achieving good charge transfer efficiency, or low lag, with signal levels up to 2M electrons and with high line rates. In both devices a low noise analogue read-out chain is used with correlated double sampling to suppress the readout noise and give a maximum dynamic range that is significantly larger than in standard commercial devices. Radiation hardness is a particular challenge for CMOS detectors and both of these sensors have been designed to be fully radiation hard with high latch-up and single-event-upset tolerances, which is now silicon proven on MTG. We will also cover the impact of ionising radiation on these devices. Because with such large pixels the photodiodes have a large open area, front illumination technology is sufficient to meet the detection efficiency requirements but with thicker than standard epitaxial silicon to give improved IR response (note that this makes latch up protection even more important). However with narrow band illumination reflections from the front and back of the dielectric stack on the top of the sensor produce Fabry-Perot étalon effects, which have been minimised with process modifications. We will also cover the addition of precision narrow band filters inside the MTG package to provide a complete imaging subsystem. Control of reflected light is also critical in obtaining the required optical performance and this has driven the development of a black coating layer that can be applied between the active silicon regions.
As part of a strategy to provide increasingly complex systems to customers, e2v is currently developing the sensor solution for focal plane array for the WSO-UV (World Space Observatory – Ultraviolet) programme, a Russian led 170 cm space astronomical telescope. This is a fully integrated sensor system for the detection of UV light across 3 channels: 2 high resolution spectrometers covering wavelengths of 115 – 176 nm and 174 – 310 nm and a Long-Slit Spectrometer covering 115 nm – 310 nm. This paper will describe the systematic approach and technical solution that has been developed based on e2v’s long heritage, CCD experience and expertise. It will show how this approach is consistent with the key performance requirements and the overall environment requirements that the delivered system will experience through ground test, integration, storage and flight.
PLATO is a candidate mission for an European Space Agency M-class launch opportunity. The project aims to detect
exo-planets from their transits across host stars and to characterise those stars by studying their oscillations, hence the
name PLATO for, PLAnetary Transits and Oscillations of stars. In order to achieve this aim the mission proposes to fly a
satellite with a focal plane of up to 34 mini-telescopes, each containing 4 large area back illuminated Charge-Coupled
Devices (CCDs) to provide ultra high precision photometry. If successful, the satellite will have nearly 0.9 m<sup>2</sup> of image
sensors and will be by far the largest composite detector focal plane ever flown. To meet the mission requirements e2v
have developed the CCD270 which has 4510 by 4510 pixels, each pixel is 18 μm by 18 μm, in a development funded by
the European Space Agency. This large area (81 mm x 81 mm) full frame image sensor is intended for precision
photometry with a dynamic range in excess of 30,000. The CCD270 has been manufactured with a thinner gate dielectric
and a higher buried channel dose than standard devices to increase the full well capacity in the image area. The
additional advantages of the thinner gate are lower power dissipation, smaller clock voltage swing for standard channel
doses and higher tolerance to ionising radiation. This paper describes the imager sensor in detail and focuses on the novel
aspects of the device, package and interface.
The success of the next generation of instruments for 8 to 40-m class telescopes will depend upon improving the image
quality (correcting the distortion caused by atmospheric turbulence) by exploiting sophisticated Adaptive Optics (AO)
systems. One of the critical components of the AO systems for the E-ELT has been identified as the Laser/Natural Guide
Star (LGS/NGS) WaveFront Sensing (WFS) detector. The combination of large format, 1760x1680 pixels to finely
sample (84x84 sub-apertures) the wavefront and the spot elongation of laser guide stars, fast frame rate of 700 (up to
1000) frames per second, low read noise (< 3e-), and high QE (> 90%) makes the development of such a device
extremely challenging. Design studies by industry concluded that a thinned and backside-illuminated CMOS Imager as
the most promising technology. This paper describes the multi-phased development plan that will ensure devices are
available on-time for E-ELT first-light AO systems; the different CMOS pixel architectures studied; measured results of
technology demonstrators that have validated the CMOS Imager approach; the design explaining the approach of
massive parallelism (70,000 ADCs) needed to achieve low noise at high pixel rates of ~3 Gpixel/s ; the 88 channel
LVDS data interface; the restriction that stitching (required due to the 5x6cm size) posed on the design and the solutions
found to overcome these limitations. Two generations of the CMOS Imager will be built: a pioneering quarter sized
device of 880x840 pixels capable of meeting first light needs of the E-ELT called NGSD (Natural Guide Star Detector);
followed by the full size device, the LGSD (Laser Guide Star Detector). Funding sources: OPTICON FP6 and FP7 from
European Commission and ESO.
The purpose of this paper is to give an overview of the state of the art wavefront sensor detectors developments held in
Europe for the last decade.
The success of the next generation of instruments for 8 to 40-m class telescopes will depend on the ability of Adaptive
Optics (AO) systems to provide excellent image quality and stability. This will be achieved by increasing the sampling,
wavelength range and correction quality of the wave front error in both spatial and time domains.
The modern generation of AO wavefront sensor detectors development started in the late nineties with the CCD50
detector fabricated by e2v technologies under ESO contract for the ESO NACO AO system. With a 128x128 pixels
format, this 8 outputs CCD offered a 500 Hz frame rate with a readout noise of 7e-.
A major breakthrough has been achieved with the recent development by e2v technologies of the CCD220. This
240x240 pixels 8 outputs EMCCD (CCD with internal multiplication) has been jointly funded by ESO and Europe under
the FP6 programme. The CCD220 and the OCAM2 camera that operates the detector are now the most sensitive system
in the world for advanced adaptive optics systems, offering less than 0.2 e readout noise at a frame rate of 1500 Hz with
negligible dark current. Extremely easy to operate, OCAM2 only needs a 24 V power supply and a modest water cooling circuit. This system, commercialized by First Light Imaging, is extensively described in this paper. An upgrade of
OCAM2 is foreseen to boost its frame rate to 2 kHz, opening the window of XAO wavefront sensing for the ELT using 4
synchronized cameras and pyramid wavefront sensing.
Since this major success, new developments started in Europe. One is fully dedicated to Natural and Laser Guide Star
AO for the E-ELT with ESO involvement. The spot elongation from a LGS Shack Hartman wavefront sensor
necessitates an increase of the pixel format. Two detectors are currently developed by e2v. The NGSD will be a 880x840
pixels CMOS detector with a readout noise of 3 e (goal 1e) at 700 Hz frame rate. The LGSD is a scaling of the NGSD
with 1760x1680 pixels and 3 e readout noise (goal 1e) at 700 Hz (goal 1000 Hz) frame rate. New technologies will be
developed for that purpose: advanced CMOS pixel architecture, CMOS back thinned and back illuminated device for
very high QE, full digital outputs with signal digital conversion on chip. In addition, the CMOS technology is extremely
robust in a telescope environment. Both detectors will be used on the European ELT but also interest potentially all giant
telescopes under development.
Additional developments also started for wavefront sensing in the infrared based on a new technological breakthrough
using ultra low noise Avalanche Photodiode (APD) arrays within the RAPID project. Developed by the SOFRADIR and
CEA/LETI manufacturers, the latter will offer a 320x240 8 outputs 30 microns IR array, sensitive from 0.4 to 3.2
microns, with 2 e readout noise at 1500 Hz frame rate. The high QE response is almost flat over this wavelength range.
Advanced packaging with miniature cryostat using liquid nitrogen free pulse tube cryocoolers is currently developed for
this programme in order to allow use on this detector in any type of environment. First results of this project are detailed
These programs are held with several partners, among them are the French astronomical laboratories (LAM, OHP,
IPAG), the detector manufacturers (e2v technologies, Sofradir, CEA/LETI) and other partners (ESO, ONERA, IAC,
GTC). Funding is: Opticon FP6 and FP7 from European Commission, ESO, CNRS and Université de Provence,
Sofradir, ONERA, CEA/LETI and the French FUI (DGCIS).
Gaia, funded by ESA with EADS Astrium as the prime contractor, is an ambitious space observatory designed to
measure the positions of around one billion stars with unprecedented accuracy and is currently planned for launch in
2011. The Gaia instrument will feature a focal plane containing 106 large area CCD91-72s manufactured by e2v
technologies. This will be the largest CCD focal plane ever flown in space covering an area of 0.286m<sup>2</sup>. To ensure that
the devices meet the required high specification, they undergo significant testing before being accepted by the end user.
This involves geometrical, mechanical, environmental, endurance, electrical and electro-optical testing. With the flight
phase contract for Gaia requiring the delivery of 130 flight grade devices (plus another 40 engineering devices of
various grades), the volume of testing is an order of magnitude greater than and of similar timescale to, the typical space
programmes e2v technologies are involved with. This paper will begin by providing an overview of the Gaia mission
and the custom CCD91-72 that e2v technologies have designed for it. Next the various phases of the Gaia programme
will be outlined and how e2v approached the test requirements for each stage. Problems encountered, lessons learned,
and technical and logistical solutions implemented at each stage will be presented, to discuss how e2v technologies
improved the quality of the test data whilst reducing the test times. There will be particular emphasis on the electro-optical
testing and the test cameras on which this is performed.