A.

NISP description

NISP instrument is at the beginning of phase D. The goal is to deliver the Flight Model (FM) to ESA and AIRBUS in 2018. Its design is well advanced and described in detailed in [2]. It is made of 3 main assemblies: the NI-OMA (Opto-Mechanical Assembly), composed of the mechanical support structure and the optical elements, the filter and the grism wheels and the calibration unit; the Focal Plane Array (NI-FPA) and the Sensor Chip System (NI-SCS) that compose the Detector System Assembly (NI-DS). It is made of 16 H2RG detectors and associated 16 ASICS (Sidecars), passively cooled at operating temperature (<100K for the detectors; 140K for the ASICS Sidecar). Thermal stabilization of the detector is “naturally” obtained thanks to the very good thermal stability provided by the Euclid PLM at the NISP interfaces; the Warm Electronics Assembly (NI-WE), composed of the Instrument Data Processing Unit and Control Unit (NI-DPU/DCU), and the Instrument Control Unit (NI-ICU).

A Structural and Thermal Model (STM) of NISP has been manufactured, and tests in 2016 at LAM [2]. The STM has shown that the design of NISP instrument was compliant with the requirements in term of mechanical and thermal response. The first TB/TV of NISP STM has been done in ERIOS chamber as shown in Fig. 1 and as explained in subsection III, parts of the Thermal Ground System Equipment (TGSE) used during the STM campaign will be re-used for the Flight Model test campaign.

Fig. 1.

Left: NISP instrument description. Middle: NISP STM in ERIOS chamber for the STM TB/TV test. Right: zoom on the NISP thermal interfaces without MLI.

B.

NISP test campaign objectives

NISP instrument will be tested at LAM in vacuum and cold conditions to mimic the behavior of the instrument during the observations with EUCLID. The goal of the thermal performance test will be to validate the performance of the instrument before delivery to the PLM. The main objectives of the test campaigns are:

The NISP test campaign will take place in ERIOS chamber, already used to test NISP STM. It is required to develop a set of GSE called the Verification Ground System (VGS) to test NISP performance in appropriate conditions.

A.

The NI-TS: the telescope simulator of EUCLID telescope for NISP test

One the main important component of the VGS is the NI-TS (NISP Telescope Simulator) that simulates the F/20 EUCLID telescope beam. It is made of an elliptical mirror positioned on translation and rotation stages that allow the telescope to move in five degrees of freedom to illuminate the entire NISP Field of View (FoV). The main goal of the NI-TS is to provide a stable focal distance to NISP to allow the measurement of the position of NISP object plane. To achieve this goal, the NI-TS is designed with a stable thermal environment done by the thermal flexures made with a low conductivity material between the telescope itself and the motor assembly to provide a thermal isolation between the 2 items. This is one of the main challenges for the NI-TS design as the different parts of the NI-TS operate at different temperatures and in a warmer environment than NISP. The Active Telescope Support unit (ATS) moves the Telescope Simulator (TS), around to illuminate the full NISP FoV. The ATS is an assembly of 5 motors with a large moving range to commit with NISP need and to allow a scanning of the focus to find NISP best: 3 linear and 2 rotational high precision stages with remote controlled motor and absolute encoder to provide position of the unit. These stages are vacuum compatible but working at 293K. They ATS will be covered by MLI blanket to be kept warm and to avoid being cooled down by the cooling shrouds. The ATS is directly in interface with ERIOS bench thanks to 3 shims under the last translation stage baseplate. The TS is made of an off-axis elliptical mirror assembled to a baseplate thanks to optical contact. On the same baseplate, a pinhole is set at the entrance focus to provide an interface for the source. It is a single surface optical design which provides an unobstructed system. The design of the TS is to provide a first focal point at 500mm while the distance from the mirror vertex to the second focal point should be 3000mm. The whole TS assembly is made of Silica to ensure a good stability of the focal distance of the telescope and will operate at 170K to limit impact of the temperature on the stability of the focal distance. The telescope is illuminated through a 2µm diameter pinhole placed in the primary focus and provides uniform illumination of the 160mm diameter aperture. The focus of the telescope should be known with a precision better than 20µm at operational temperature meaning that the distance from the pinhole to the mirror should be stable within 0.5µm. It is also needed to measure the focal distance of the TS at ambient temperature thanks to a Coordinate Measurement Machine (CMM) with high precision to know precisely the focus position with respect to laser tracker targets set on the back surface of the telescope. The TS is designed to provide a WFE lower than 30nm rms over the 160mm diameter aperture to ensure a good image quality on NISP during the performance test. A view of the telescope design is shown in Fig. 3. In order to minimize stray light during NISP test, the telescope will be surrounded by a baffle made of several vanes.

Fig. 3.

Left: the NI-TS without MLI showing the motor assembly and the telescope in its baffle, right: the optical layout of the telescope.

The illumination system used for the NI-TS is made of a fiber connected at the entrance focus and of a monochromator that is fed from a continuum laser source. The output from the monochromator is delivered to a cryogenic fiber through a pair of vacuum windows in interface with ERIOS chamber. A cooled Neutral Density (ND) filter suppresses the thermal background. The flux can be adjusted in large steps via ND filters mounted in a filter wheel also serving as order-sorter for the grating based monochromator, The flux can be fine-tuned using a variable rotating ND filter. A spectral lamp and an etalon are available for wavelength calibration and can be selected using linear motions. The NI-TS will be provided with a Telescope Control Computer (TCC) to control the motors position, the light illumination and intensity.

B.

Thermal and mechanical ground support equipment

The Thermal and Mechanical Ground Support Equipment also called TMVS provides thermal and mechanical interfaces for NISP needed for the performance test. It should simulate the PLM thermal environment. The radiative thermal environment consists of ERIOS liquid nitrogen shrouds. To avoid 300K background coming from the optical windows needed for the metrology measurement (see subsection III-C) a cold baffle is fixed between LN2 shrouds and the windows. In addition, a shutter activated from outside by a ferrofluid feed through is placed between this baffle and the windows to cut light coming from the windows when the metrology is not used during performance test of NISP detectors. The TMVS has also to provide conductive interfaces for NI-OMA, the Warm Electronic (WE) and for the intermediate harness as described in Table 1.

Table 1.

Thermal Test case for NIOMA

To meet these requirements, the TMVS is composed of four different parts:

Fig. 4:

NI-CSS (left) and NI-SSS (right)Thermal interfaces

To allow reaching the good temperature, a liquid nitrogen loop has been designed to fill the tanks under the cold plate and behind NISP detectors assembly. The principle is shown Fig. 5. A supply tank is positioned in the upper part of ERIOS. This tank permits to have autonomy of 30 hours between two fillings. It also permits to manage the cooling down phase in order to keep detectors warmer than the rest of the instrument. In addition to NIOMA, the TMVS has to provide conductive thermal interfaces for the harness and the warm electronic. The harness thermal unit is a stainless steel plate, where the NISP intermediate harness is fixed, with a temperature gradient between operational temperature (130K) to room temperature. The cold part of the unit is linked by thermal braids to the NIOMA cold plate and the warm part is regulated by heaters. The warm electronic thermal enclosure provides thermal interfaces for DPU and ICU electronic boxes between 253K and 303K. The interface plate is an aluminum plate thermally regulated by a heat transfer fluid circulation. The fluid temperature is adjusted by a thermostated bath (Bain Lauda) outside the chamber. To avoid molecular contamination from NI-DPU and NI-ICU, a cold trap will be implemented in front of an aperture in the cover of the enclosure.

Fig. 5:

TGSE LN2 loop

C.

The NI-MVS: Metrology Verification System

In addition to the classical ground support equipment, it is needed to develop a Metrology Verification System (MVS) to provide at operational temperature the measurement of references frames set on the telescope simulator and NISP, the knowledge of the coordinates of the object point source provided by the telescope simulator and the measurement of the angle between the telescope simulator optical axis and NISP optical axis. The MVS concept is based on the use of two devices put outside ERIOS chamber: a laser tracker, which is a portable coordinate measuring machine provided by Leica that allows extreme accuracy over large distances. The measurement with the laser tracker will be done through a window seeing the reflectors into ERIOS. The current configuration foreseen is to use a curved window. More details on the laser tracker configuration can be found in [4]; a theodolite, which is a portable angle measuring system, to measure accurate angle between two mirrors, one put on NISP and one on the NI-TS.

The principle of the measurement of the laser tracker is the measurement of the position of reflectors set respectively on the NI-TS and on NISP instrument. The goal of this measurement is to deliver the position of NISP reference frame R_u with respect to NISP object plane R_nisp at Operational Temperature (OT) (i.e. 130K) with accuracy within +/-150µm to Airbus, the responsible of EUCLID PLM, which will align NISP on the EUCLID payload thanks to this information. To provide this information, a sequence of measurement is needed at NISP instrument level:

Fig. 6.

Schematic of the measurement to be done with the MVS. The frames to be measured are shown

The metrology done during the NISP test campaign will be particularly important as it will provide the knowledge of the position of the NISP focal plane with respect to the telescope simulator. From these measurements, the real NISP focus will be known. The theodolite measurement will be used to monitor the angular position of the TS will respect to NISP with accuracy giving the FoV position seen on NISP detectors.

The concept and the feasibility study of the MVS are fully described in [4]. The laser tracker and the theodolite will be placed out of ERIOS chamber and will do the measurement through a window. One has to take into account the measurement errors introduced by the glass in the laser tracker measurement. A mechanical interface will allow the positioning of the both components on ERIOS chamber door to measured reflectors set inside ERIOS. Fig. 7 provides a view of the MVS for the NISP performance test. The beam sights are shown to validate the position of the test set-up (NISP and TS) and the laser tracker into ERIOS.

Fig. 7.

Zoom on the metrology system. Outside erios the laser tracker and the theodolite, inside ERIOS, the reflectors and mirrors.