June 7, 2023

VISible Instrument (VIS)

VISible Instrument (VIS)

The VIS instrument is under the responsibility of the Mullard Space Science Laboratory (MSSL, United Kingdom). France—through CEA IRFU and IAS Orsay—is contributing significantly to this instrument.

VIS (VISible instrument) is a wide-field imager operating in the 550-900-nm spectral band, designed to measure distortion in galaxy images as a result of the weak lensing effect induced by dark matter. The measurements it will acquire are detailed in the section on instrument measurements.

VIS consists of five subsystems shared between the Payload Module (cold parts) and Service Module (see instrument locations).

Cold part of VIS in Payload Module

The three systems making up the “cold” part of the instrument in the Payload Module (PLM) with the telescope are:

Focal Plane Assembly (FPA), built by CEA IRFU

The FPA is composed of 36 (6 x 6) CCDs—baseline CCD203-82 made by E2V and packaged optimally for gaps in the field of view—covering the 0.5-deg² visible field of view. These full-frame CCDs with 4096 x 4096 12-µm pixels afford an instantaneous field of view of 0.3 arcseconds and occupy a surface area of about 0.302 cm x 0.336 cm. The architecture of the control and read-out electronics for the 36 CCDs was developed using lessons learned from previous projects, especially Gaia. But unlike Gaia, on which all the CCDs are operated independently, the CCDs in Euclid’s VIS instrument operate synchronously to simplify subsystem design. The CCDs are in groups of three, with each group handled by its own read-out electronics (ROE) unit.

The Focal Plane Assembly (FPA) is therefore composed of some 600 million pixels, equivalent to 300 HD TV sets. It affords a field of view of 0.787 deg. x 0.709 deg. (0.55 deg2), about two and a half times the apparent surface of a full Moon, with a spatial resolution of 0.23 arcseconds: in other words, VIS can pick out a €1 coin from 22 kilometres away.

To ensure the required thermomechanical stability, the FPA is made of silicon carbide (SiC). It weighs 133 kg, and its dimensions are 0.6 x 0.6 x 0.41 m. It is maintained at a temperature close to 130 K.

     

The VIS instrument Focal Plane Assembly (FPA). Credit: CEA

At system level, the FPA houses the satellite’s Fine Guidance Sensors (FGS), which are in the lower and upper parts of the structure, with the CCD plane (the two locations can be seen in the image left). These are four CCDs identical to the FPA’s, with a field of view of 0.11 x 0.11 deg., but operating at a higher frequency. They form part of the AOCS, enabling the satellite’s attitude to be determined with optimal precision, as they are co-located with the FPA, while averting thermoelastic effects as far as possible.

Calibration Unit (CU), under the responsibility of prime contractor IAS Orsay

The role of the Calibration Unit is to provide uniform illumination for the VIS instrument’s focal plane in order to generate monochromatic flat-fields at regular intervals during the mission. It will calibrate the difference in sensitivity between pixels and correct image effects. It operates at cryogenic temperatures close to 150 K (–123°C) and is composed of an integrating sphere powered by six LEDs at wavelengths of 590, 610, 644, 720, 880 and 885 nm. The light beam is shaped by a rectangular mask and an aspherical lens to cover the entire focal plane (30.3 cm x 33.6 cm), while limiting luminous flux outside this zone. The LEDs are commercial-off-the-shelf components rated for space by CNES and Adveotec.

VIS instrument Calibration Unit (CU). Credit: IAS

 

Read-out Shutter Unit (RSU), under the responsibility of a Swiss prime contractor (Astronomy Department of the University of Geneva/APCO Technologies)

The RSU is located before the focal plane. After each integration (a few hundreds of seconds), it is shut to prevent stray light effects during reading of the accumulated signal. The shutter incorporates a moment-compensation mechanism to avoid disturbances to the satellite and NISP instrument when it moves. It opens and shuts in roughly 10 seconds. It has cold-redundant electronics.

VIS instrument Read-out Shutter Unit (RSU). Credit: ESA/Unige/APCO Technologies

Warm VIS electronics in the Service Module

The two systems making up the “warm” part of VIS in the Service Module (SVM) are:

Power & Mechanical Control Unit (PMCU), under the responsibility of CEA IRFU

The role of the PMCU is to distribute electrical power to the Calibration Unit (CU) and Read-out Shutter Unit (RSU), and to control the three cold elements in the Payload Module (PLM)—temperature, voltage, current and position—and collect housekeeping data. The power supply lines are redundant.

VIS instrument Power & Mechanical Control Unit (PMCU). Credit: CEA

  

Control & Data Processing Unit (CDPU), under the responsibility of Italy (IAPS astrophysics and planetology institute in Rome and OHB-Italy)

The CDPU handles and controls the VIS instrument, providing power, controlling operations and compressing and transferring images to the satellite for transmission to Earth. It operates transitions between modes and harmonizes viewing sequences. It collects housekeeping measurements and generates data for transmission. It also collects images, performs lossless compression on them, and generates and stores data packets before transferring them to the satellite’s mass memory unit (MMU). It incorporates a Maxwell SCS750 Power triple-voting processor for image compression and generation of science data packets, a data handling board that keeps a check on the instrument’s status and the power from the PMCU, and a multiplexer board that interfaces with the 12 read-out electronics units (ROE) in each half of the FPA. Lines are redundant, with the exception of the multiplexer board’s.


Control and Data Processing Unit (CDPU). Credit: IASP/ OHB-I

The five subsystems are interconnected and the PMCU and CDPU interface electrically with the satellite.

VIS functional architecture

VIS functional architecture. Systems on the left (EPLM zone) are in the PLM, systems in the centre (ESVM zone) in the SVM. The Mass Memory Unit (MMU), Central Data Management Unit (CDMU) and Power Conditioning and Distribution Unit (PCDU) are on the satellite side. Credit: Cropper, M. et al., “VIS: the visible imager for Euclid”, SPIE 2016

In short, VIS will image in one day more sky than Hubble has viewed in 30 years, at the same resolution. Its field of view will enable it to image some 50,000 galaxies with each view. Approximately 520 Gbits of science telemetry will be sent back to Earth every day.