The Payload Module (PLM) comprises a telescope and the “cold” part of the two instruments: the wide-field VISible Instrument (VIS) and the Near Infrared Spectro Photometer (NISP). Both instruments are supplied by the Euclid Consortium.
Mathematical model and schematic diagram of Euclid’s PLM. Light rays arrive at the instruments after the M3 mirror. A dichroic lens lets through infrared light to the NISP instrument and reflects visible light to the VIS instrument. Credit: ADS Toulouse
The prime contractor for the telescope, integration and testing of the PLM is Airbus Defence & Space (formerly Astrium) France, Toulouse.
The telescope’s main features are:
- Korsch telescope with three anastigmatic mirrors in a 0.45° off-axis configuration to convey visible and infrared light to the instruments, avoiding the multiple optical distortions induced by spherical or parabolic mirrors. The advantage of using several mirrors lies in the ability to adjust more parameters—mirror curvature, distances between mirrors, etc.—and thus control image distortion generated by the astigmatism.
- Entrance pupil: 1.2 m
- Focal length: 24.5 m
- Field of view: 0.53 deg2 with an angular resolution on the order of 0.2 arcseconds
- M1 concave entrance mirror with a diameter of 1.2 m and a collecting area of a little more than 1 m2 with a field of view of 0.54 deg2, i.e. about two and a half times the apparent surface area of the full Moon
- M2 mirror with a diameter of 0.35 m mounted on a three-degrees-of-freedom mechanism for in-flight focal corrections
- Baffle length (distance between M1 and M2 mirrors): 1.756 m
- M3 concave elliptical mirror (0.535 x 0.406 m)
- FoM1 (358 x 215 mm2) and FoM2 (283 x 229 mm2) wide-bandwidth flat elliptical return mirrors and FoM3 (358 x 215 mm2) flat elliptical return mirror with bandpass filter
- Baseplate structure: mostly silicon carbide (SiC) for thermomechanical stability, cooled to 130 K (–143°C).
- Mass: 570 kg not including instruments, 850 kg with the two instruments and their interfaces
- Dimensions: 2.7 m x 2.8 m and 3.6 m tall
- Visible and infrared light separated by a 117-mm dichroic lens
Left image :Integrating the telescope at ADS in Toulouse. Credit: Airbus Defence & Space
Top right image : Dichroic filter (qualification model). Credit: Optics Balzers Jena
Bottom right image : Euclid’s M1 primary mirror. Credit: Safran Reosc
Its architecture is built around two cavities on either side of a 200-kg baseplate structurally connected to the Service Module (SVM) by three glass-fibre bipods. The upper cavity houses the primary and secondary mirrors, focusing mechanism, structure and baffles. For optimal performance, the Payload Module (PLM) must operate at 130 K (–143°C) with a colder zone to guarantee the proper functioning of the NISP instrument’s infrared detectors, which must be cooled to under 100 K, nominally 95 K (–178°C).
To achieve exceptional thermomechanical stability, the mirrors and a large portion of the structure are in silicon carbide (SiC), a material affording great strength and good thermal conduction. This ceramic material not only withstands the harsh launch environment, but also offers good thermomechanical stability despite the large temperature swings it is subjected to between its departure at ambient temperature on Earth and cryogenic temperatures when operating in space.
To meet these temperature requirements, the PLM is wrapped in low-conductivity multilayer insulation (MLI) and equipped with radiators to reject heat into space. Heating elements maintain the thermal stability of sensitive elements within their optimal operating range. They are also used during decontamination phases. One final thermal control component in the PLM is the large cylindrical baffle around the telescope that also shields it from stray light.
The telescope includes the “cold” part of the VISible Instrument (VIS) and Near Infrared Spectro Photometer (NISP) detailed in the section on Euclid’s instruments.
The telescope for Euclid’s PLM in testing. Credit: ESA