The Near-Infrared Spectrometer and Photometer (NISP) is one of the key instruments on-board the Euclid satellite, one of the major mission of ESA’s 2015 Cosmic Vision program. The instrument is located at the payload of the Euclid satellite, and it is initially planned to be operated at ∼135 K, except the detectors that are cooled to about 90 K. The NISP instrument will perform spectroscopic observations and photometric imaging in 3 near-infrared bands (Y, J, H) covering a wavelength range from 0.92 μm – 2.0 μm over a field of view (FoV) of ∼0.5 deg2.
The optical system of the Near Infrared Spectrometer and Photometer (NISP) of the EUCLID mission consists mainly of a filter and grism wheel and 4 aspherical lenses with large diameters up to 170 mm. The single lenses require a high precision positioning at the operational temperature of 150 K. An additional design driver represents the CaF2 material of a lens, which is very sensitive wrt brittleness.
The technical maturity of the combination of single features such as CaF2, large diameter (and mass), high precision and cryogenic conditions is considered as low. Therefore, a dedicated pre-development program has been launched to design and develop a first prototype of lens holder and to demonstrate the functional performance at representative operational conditions.
The 4 lenses are divided into 3x lenses for the Camera Lens Assembly (CaLA) and 1x lens for the Corrector Lens Assembly (CoLA). Each lens is glue mounted onto solid state springs, part of an adaption ring. The adaption ring shall provide protection against vibration loads, high accuracy positioning, as well as quasi load free mounting of the lens under operational conditions. To reduce thermomechanical loads on the lens, the CTE of the adaption ring is adapted to that of the lens. The glue between lens and solid state spring has to withstand high tension loads during vibration. At the operational temperature the deviating CTE between glue and lens/adaption ring introduces shear loads into the glue interface, which are critical, in particular for the fragile CaF2 lens material. For the case of NISP the shear loads are controlled with the glue pad diameter and the glue thickness.
In the context of the development activity many technology aspects such as various solid state spring designs, glue selection and glue handling have been investigated. A parametric structural model was developed to derive the specific design feature of each ring, such as spring force, number of springs, eigenfrequency, etc.
This paper presents the design of the adaption ring in conjunction with test results from functional verification. These results are presented on behalf of the EUCLID consortium.
The Near Infrared Spectrometer and Photometer (NISP) of the EUCLID satellite project encompasses high precision large lens mounts of 168 mm diameter that are operated at cryogenic temperatures down to 135K. The four lenses of the optical system are made of different materials: SUPRASIL 30001, CaF2, and S-FTM16, which are mounted in a separate lens mount design using glue connections. Each lens assembly has its individual mechanical interface to the structure, the so called lens barrel. Exhaustive structural and thermal investigations have determined lens surface deformations and lens position changes that are introduced by various environmental loads, such as thermal-, mechanical-, interface-, and gravity loads, as well as mechanical stress of the lenses due to glue shrinkage during curing. All these impacts change the lens optical behaviours under real operational conditions of the optical assembly, which are thoroughly investigated in the optical performance assessment activity. Especially, great effort has been made for the simulation of interface tolerances. Due to the complexity of all mechanical interfaces (baffle, lens mounts, housing, telescope structure, etc.) statistical simulation is conducted applying Monte Carlo method. From the result of the statistical simulation 3 representative cases are selected for the optical performance assessment, which have 95% confidence level of the lens surface deformation. In the context of the evaluation procedure the surface form error of all EUCLID lenses as well as the RMS WFE at the focal plane is assessed, and results are compared with the nominal performance of the system, as well as with interferometrically measured results achieved during the interface– and gravity release test campaign. The performance of the lens holder design in terms of glue shrinkage effects, gravity release and interface tolerances is verified by an adapted test facility including an interferometer based optical metrology system. Finally, the measured values are compared with the analytical results, which show great confidence and hence proves validation of the analytical model. The paper presents the optical performance analysis results and the measured performance of the EUCLID high precision large cryogenic lens mounts. The achievements are presented on behalf of the EUCLID consortium.
The Near Infrared Spectro-Photometer Optical assembly (NIOA) of EUCLID satellite requires high precision large lens holders with different lens materials, shapes and diameters. The aspherical lenses are glued into their separate CTE matched lens holder. The gluing of the lenses in their holder with 2K epoxy is selected as bonding process to minimize the stress in the lenses to achieve the required surface form error (SFE) performance (32nm) and lens position stability (±10μm) due to glue shrinkage. Adhesive shrinkage stress occurs during the glue curing at room temperature and operation in cryogenic temperatures, which might overstress the lens, cause performance loss, lens breakage or failure of the gluing interface.
The selection of the suitable glue and required bonding parameters, design and qualification of the gluing interface, development and verification of the gluing process was a great challenge because of the low TRL and heritage of the bonding technology. The different material combinations (CaF2 to SS316L, LF5G15 and S-FTM16 to Titanium, SUPRASIL3001 to Invar M93), large diameter (168mm) and thin edge of the lenses, cryogenic nonoperational temperature (100K) and high performance accuracy of the lenses were the main design driver of the development. The different coefficients of thermal expansion (CTE) between lens and lens holder produce large local mechanical stress. As hygroscopic crystal calcium fluoride (CaF2) is very sensitive to moisture therefore an additional surface treatment of the gluing area is necessary.
Extensive tests e.g glue handling and single lap shear tests are performed to select the suitable adhesive. Interface connection tests are performed to verify the feasibility of selected design (double pad design), injection channel, the roughness and treatment of the metal and lens interfaces, glue thickness, glue pad diameter and the gluing process. CTE and dynamic measurements of the glue, thermal cycling, damp- heat, connection shear and tension tests with all material combinations at RT and 100K are carried out to qualify the gluing interface. The gluing interface of the glued lenses in their mounts is also qualified with thermal cycling, 3D coordinate measurements before and after environmental tests, Polarimetry and vibration test of the lens assemblies.
A multi-function double pad gluing tool and lens mounting tool is designed, manufactured and verified to meet the lens positioning and alignment performance of the lens in the holder which provides the possibility to glue lenses, filters, mirrors with different diameters, shapes and thickness with ±10μm accuracy in plane, out of plane and ±10 arcsec in tip/tilt with respect to the lens holder interface. The paper presents the glue interface qualification results, the qualification/verification methods, the developed ground support equipment and the gluing process of the EUCLID high precision large cryogenic lens mounts. Test results achieved in the test campaign demonstrate the suitability of the selected adhesive, glue pad design, interface parameters and the processes for the precise gluing of the lenses in lens holders for all lenses. The qualification models of the NIOA are successfully glued and qualified. The developed process can also be used for other glass materials e.g. MaF2 and optical black coated metallic surfaces.
The Near Infrared Spectrometer and Photometer (NISP) of EUCLID requires high precision large lens holders (Ø170
mm) at cryogenic temperatures (150 K). The lenses of the optical system are glued into separate lens holders, the so
called adaption rings. For the selection and verification of a suitable adhesive extensive glue selection tests are
performed and results presented in this paper. With potential glue candidates, handling, single lap shear, connection
tension and shear tests are carried out at room temperature (RT) and 150 K (OPS). For the NISP optical system DP490 is
selected as the most suitable adhesive. The test results have shown that an even distribution of the glue in the glue gap is
of crucial importance for the functioning and performance of the bonded lens system. The different coefficients of
thermal expansion (CTE) between lens and lens holder produce large local mechanical stress and might cause lens
breakage or failure of bonding. The design of the injection channel and the gluing procedure are developed to meet the
lens performance requirements. An example is shown that after thermal cycling the remaining 0.5 mm – 1 mm thick
adhesive in the injection channel results in large local mechanical stresses, and hence, damage of the lens. For a
successful performance of the glue interface not only an optimum glue gap of 80 – 150 μm is important, also micro-cracks
of the glass at the gluing area have to be avoided. The performed glue tests with DP490 for 3 different lens/ring
material combinations show sufficient mechanical tension and shear strength for bonding of the lens system.
Titanium/LF5G15 and Invar/Fused Silica combinations have reached the strength of 30 MPa at RT and 50 GPa at 150 K.
These results are presented on behalf of the EUCLID consortium.
For the Euclid mission a Pre-Development phase is implemented to prove feasibility of individual components of the
system . The Near Infrared Spectrometer and Photometer (NISP) of EUCLID requires high precision large lens
holders (Ø170 mm) at cryogenic temperatures (150K). The four lenses of the optical system are made of different
materials: fused silica, CaF2, and LF5G15 that are mounted in a separate lens barrel design. Each lens has its separate
mechanical interface to the lens barrel, the so called adaption ring. The performance of the lens holder design is verified
by adapted test equipment and test facility including an optical metrology system. The characterization of the lens
deformation and displacement (decenter, tilt) due to mechanical loads of the holder itself as well as thermally induced
loads are driven by the required submicron precision range and the operational thermal condition. The surface
deformation of the lens and its holder is verified by interferometric measurements, while tilt and position accuracy are
measured by in-situ fibre based distance sensors. The selected distance measurement sensors have the capability to
measure in a few mm range with submicron resolution in ultra high vacuum, in vibration environments and at liquid
nitrogen temperatures and below. The calibration of the measurement system is of crucial importance: impacts such as
temperature fluctuation, surface roughness, surface reflectivity, straylight effects, etc. on the measured distance are
carefully calibrated. Inbuilt thermal expansion effects of the fibre sensors are characterized and proven with lens dummy
with quasi zero CTE. The paper presents the test results and measured performance of the high precision large cryogenic
lens holders attained by the metrology system. These results are presented on behalf of the EUCLID consortium.
For the Euclid mission a Pre-Development phase is implemented to prove feasibility of individual components of the
system. The Near Infrared Spectrometer and Photometer (NISP) of EUCLID requires high precision and large lens
holders (Ø170 mm) at cryogenic temperatures (120K - 150K). The four lenses of the optical system are made of
different materials: fused silica, CaF2, and LF5G15 that are mounted in a separate lens barrel design. Each lens has its
separate mechanical interface to the lens barrel, the so called adaption ring. The performance of the lens holder design
shall be verified by an adapted test facility including an optical metrology system. The characterization of the lens
deformation and displacement due to thermally induced loads are driven by the required micrometer precision range and
by the operational thermal condition. The surface deformation of the lens and its holder is verified by interferometric
measurements, while tilt and position accuracy are measured by fiber based distance sensors. The applied distance
measurement sensors have the capability to measure in a few mm range with submicron resolution at ultra high vacuum,
in vibration environments and at liquid nitrogen temperatures and below. The calibration of the measurement system is
of crucial importance; therefore the sensors shall be mounted on a stiff and well characterized reference structure made
of nearly zero-CTE ceramic material. The verification program is currently under development at Kayser-Threde in the
context of a contract with Max-Planck-Institute for Extraterrestrial Physics. The paper presents the vacuum chamber
design, the metrology system, the used Ground Support Equipment, and the detailed verification program.
For the Euclid mission a pre-development phase is implemented to prove feasibility of individual components of the
system. The optical system of EUCLID Near-Infrared Spectrometer & Photometer (NISP) is composed of 4 lenses,
bandpass filters and grisms. The lenses are made of different materials: the corrector lens (fused silica) directly behind
the dichroic and the lenses L1 (CaF2), L2 (LF5G15), and L3 (LF5G15) that are mounted in a separate lens barrel design.
Each lens has its separate mechanical interface to the lens barrel, the so called adaption ring.
The adaption ring shall provide the necessary elasticity caused by different CTEs of the lens and ring materials, as well
as shall allow the high position accuracy of the lenses relative to the lens barrel and the optical axis.
The design drivers for the adaption ring are high precision, cryogenic operation temperature (150 K) and the large
dimension of the lenses (150 - 170 mm). The design concept of the adaption ring is based on solid state springs, which
shall both provide sufficient protection against vibration loads at ambient temperature as well as high precision (<
±10 μm) and stability at cryogenic temperatures.
Criteria for the solid state spring design shall be low radial forces at cryogenic conditions to avoid any refractive index
and polarization variations. The design shall be compliant to the large temperature differences between assembly and
operation, the high precision and non-deformation requirements of the lenses as well as to the deviating CTEs of the
selected lens materials. The paper describes the selected development approach including justification, thermal and