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1.INTRODUCTIONEUCLID mission1 has been selected by ESA in 2012 in the context of the Cosmic Vision program to study the nature of dark energy and dark matter. The launch of the mission is foreseen in 2021. The mission is designed to map the geometry of the dark Universe by investigating the distance-redshift relationship and the evolution of cosmic structures thanks to two scientific instruments: the Near Infrared Spectroscopic Photometer (NISP)2 and the Visible instrument (VIS)3. The NISP channel of Euclid is dedicated to measure the redshift of millions of galaxies and to analyze their spatial distribution in the Universe. NISP works with both photometric and spectroscopic modes by switching between broadband filters and grisms, mounted on two rotating wheels, to acquire data of the same field. The spectroscopic mode acquires dispersed images on the detector without a slit by using four different grisms mounted on a wheel. The NISP project is now entering its final phase as the integration of the complete NISP instrument is started at Laboratoire d’Astrophysique de Marseille (LAM). The grisms designed for NISP are complex optical and mechanical components that have been studied deeply during phase A and B of NISP project4 through the development of several prototypes done by LAM. One Engineering and Qualification Model (EQM) and four Flight Models (FM) grisms have been manufactured and successfully delivered to the project NISP by end of 2017 and the FM components are now integrated and tested on the wheel since Spring 2018. We present in this paper the results of the optical performance measurements of the grisms FM characterized at LAM, which will be used in NISP instrument: wavefront error, spectral transmission and grating groove profiles. We present finally the result of the alignment of the optical part on the mechanical part measured with a coordinate measurement machine. 2.EUCLID GRISM DESCRIPTION2.1Euclid NISP grisms overviewThe Euclid NISP grisms are critical parts of the NISP instrument as they are complex optical and mechanical components. In NISP, there are four grisms mounted on a wheel:
Each grism is made of two parts, as shown in Figure 1 :
2.2Euclid NISP grism optical part descriptionThe optical part of NISP grism i.e. the grism itself, combines four optical functions in one component, which are represented in Figure 2:
Table 1.Optical specifications of the grisms for NISP instrument.
2.3EUCLID NISP grism manufacturing and test strategyThe complete manufacturing process and test sequence done on the NISP FM grisms is fully described in [6]. We recall in this section the main important points concerning the manufacturing of the grisms that were established thanks to the development of prototypes and the Engineering and Qualification Model (EQM) developed at the beginning of phase D of the project. The manufacturing process of the optical part is quite complex as several manufacturers operate on the optical component one after the other. The complete manufacturing process follows the sequence below:
The manufacturing of the four grisms FM has been started in early 2016 as the manufacturing time for one component is quite long. The manufacturing of a full component (optical part, mechanical part, gluing and test) lasts 9 months per component. The four FM for NISP have been delivered to the project at the end of 2017. These components are now fully assembled and aligned onto the grism wheel. Figure 3 presents a picture of each FM delivered to NISP project after the final inspection of the component. We can see on these pictures that the “red” grisms are very similar. The name identification of each component is engraved onto the mechanical mount to distinguish the components. 3.OPTICAL PERFORMANCE OF NISP GRISMS FLIGHT MODELSWe present in this section the optical performance measured on each FM for NISP. The measurement of the optical performance is done with the test setups described in [6] according to the test plan of the grism components elaborated at the beginning of the project. 3.1Groove profile measurementThe first validation of the performance done on the grism is the verification of the groove profile with respect to the specification. We measure the groove profile of the grating with an interferential microscope Wyko NT9100. In particular, we check the groove height of each grating and the groove width. Figure 4 presents the grating groove profile measured on the four grisms FM. The measurements done on the three red gratings demonstrate the very good repeatability of the manufacturing process concerning the groove dimensions: period and height. The profile of the blue grating is provided for information but its period and height is different from the red gratings. We can see also the discretization of the groove slopes in 16 small steps due to the manufacturing process proposed by SILIOS Technologies11, which uses several photolithographic masks and etching phases. The little peaks observed at certain steps are due to mis-alignment errors between the masks but have a very few impact on the overall performance of the grating. Same profile have been obtained on the flight spares (FS) components showing a very good control of the grating manufacturing process. 3.2Transmission measurementThe transmission of the grism into the bandpass is measured with a spectrophotometer specially adapted to measure the grism on 90mm aperture6. The measurement is done at the end of the manufacturing on the final components at LAM in order 0 and 1 onto the bandpass. All components are within specifications in the bandpass as all transmission is better than 65% for each FM. In average on the four FM measurements, the mean efficiency of the FM is about 75% with a maximum transmission of 90%. This is a very good result for components that combine a grating and a filter function. This is due to the high quality of the filter but also the high transmission done by the grating itself. In order 0, the average transmission is better than 1.4% for each model, which is also fine with respect to the calibration needs for NISP requiring more than 1%. Figure 5 presents the transmission curve of the NISP FM onto the bandpass done on the final components with filter and grating manufactured. We can remark that the transmission curves are very similar for the three red grisms, which demonstrate a very good reproducibility of the filter and the grating manufacturing process. One must note that the ripples that are seen on the curves are not due to the components but to the measurement set-up. More details on the transmission measurement can be found in [12]. 3.3SFE performanceThe other main characteristics to be validated for the grism FMs are the Surface Form Error (SFE) performance, obviously linked to the transmitted Wave Front Error (WFE) performance of the component. We measure the SFE of the filter and grating surfaces thanks to a phase-shifting Fizeau interferometer working at 633nm[6]. The SFE and focus budgets have been distributed on each surface of the grism and also along the manufacturing process phases. The complete strategy for the SFE measurement of the grisms is fully described in [6,12]. The performance and compliance of the FM with the WFE specifications are provided in Table 2. We can see that all “red” grisms reach the specifications defined by the project concerning the SFE and focus term after manufacturing. Only the BGS000 grism shows some non-compliances with the specification that has been accepted by the project as the optical performance of the blue channel will not be affected too much. One can note also that the curvature introduced by the filter deposition and the gluing is larger than expected for the FM RGS000 and FM RGS180. After discussion with the project, and analysis that this deformation has a small impact on the transmitted WFE, it has been agreed to accept the components with this non-compliance. Table 2 also indicates the type of verification used to validate the specification. Only the compliance of the transmitted WFE is obtained thanks to analysis on Zemax software taking into account the measured SFE. Table 2.Summary of the WFE specifications and measurements obtained for all the grisms FM.
The validation of the SFE performance of the grisms has shown that the grating manufacturing was very good and that the equation of the grating was properly engraved by the grating manufacturer. The specification of the grating surfaces takes into account both the manufacturing of the surface itself and the grating manufacturing error i.e. the difference with a perfect grating. The error due to the grating manufacturing for each FM is lower than 15 nm RMS. Figure 6 presents an example of the theoretical interferogram of the grating function of the RGS270 component compared with the measured interferogram on the FM component in order 5. We can see a very good similarity between both interferograms. After analysis, the difference between manufacturing and theory is lower than 15 nm RMS, focus error included. This is a very good result. 4.METROLOGY OF THE COMPONENTIn addition to the mechanical and optical characterization of the component, we have done a complete metrology of each grism to measure accurately the position of the optical reference (the center of the grating, indicated by 3 crosses engraved onto the grating surface) and the mechanical structure. The goal of this measurement is to verify that the component is correctly glued onto the mechanical structure. It is important to be in the budget allocated by the grism wheel assembly to ensure a good alignment of the grisms on the wheel. The measurement of each grism is done at LAM with an accura II Coordinate Measurement Machine (CMM) equipped with a RDS XXX head from ZEISS. This head can be equipped with a mechanical contact sensor and an optical sensor (VIS camera). A picture of the measurement setup is provided in Figure 7. Results of the measurement of the three red grisms is provided in Table 3. Measurement uncertainties are estimated of +/- 15μm in Tx direction, +/-30μm in Ty and Tz, and 40” for all rotations. The metrology of the FM shows a very good accuracy of the gluing and alignment of the optical part inside the mechanical part. In particular, we have a good reproducibility of this process that allows us to provide components within specifications. Table 3.Position of the optical center of the grism with respect to the mechanical structure for the 3 red FM grisms.
5.CONCLUSIONWe have presented in this paper the overall performance of the NISP flight models for EUCLID mission. A more complete and detailed presentation can be found in [12]. The grisms manufactured, integrated and tested under the LAM responsibility have shown a very well compliance with the specifications of the project. The analyses of the tests results have shown that the grisms flight models for NISP are within specifications with an efficiency better than 70% on the spectral bandpass and a wavefront error on surfaces better than 30nm RMS. The components have also passed successfully the acceptance level vibrations and a thermal cycling at 130K. The EUCLID grisms flight models have been delivered to the NISP grism wheel in November 2017 and are now fully integrated onto the wheel. They will be installed in NISP instrument in October 2018. The grism FM will be tested and validated in NISP in early 2019 during NISP thermal test to demonstrate the full performance of the spectroscopic mode of NISP. REFERENCESG. Racca; R. Laureijs; L. Stagnaro; J.-C. Salvignol,
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