A.

Key Design drivers

The opto-mechanical design of NI-OA is driven by some key requirements: the optical design requires different lens material for the four lenses, Suprasil, CaF2 and S-FTM16 (Fig. 1). The operational temperature of 134K and survival temperature requirement of 100K leads to an opto-mechanical design with a dedicated material for each lens mount to achieve the best possible match of the thermal expansion coefficient (CTE) between lens and mounting structure to minimize thermo-elastic stresses at operational temperature. Additionally, the deep operating temperature requires to come up with a mounting and alignment concept, which allows to mount and align the unit at room temperature with a “warm geometry”, which pre-compensates for the shrinkage and component movements during cool down and delivers a perfectly aligned system at 134K operational temperature. The optical tolerancing is also a major design driver for the stability of the optomechanical mount and the understanding of its thermal behavior. The lens vertices have to reach their desired position at cold temperature within a tolerance of +/-10µm laterally and +/- 15µm along the optical axis. In addition to these key drivers an extremely limited mass budget asks for extreme light-weighting of the optomechanical mounts of the lenses.

B.

NI-OA CaLA Opto-mechanical design

The NI-OA opto-mechanical design is shown in Fig. 2. All four lenses are mounted in their dedicated adaption ring. The CaF2 lens L1 of CaLA has a stainless steel adaption ring to match the CTE of CaF2. Both S-FTM16 lenses of CaLA, L2 and L3 are mounted in Titanium adaption rings. The separate Suprasil lens of CoLA requires an adaption ring made from INVAR M93 to minimize the CTE mismatch. The three lenses of CaLA are mounted on one central ring, the so-called lens-barrel, which is made from Titanium as well. For both Titanium adaption rings of L2 and L3 no CTE mismatch is present and they can be bolted to the lens-barrel directly. For the stainless steel adaption ring three Titanium bipods serve as interface (I/F) and decouple the CTE mismatch between adaption ring and lens-barrel.

Fig. 2

3D CAD view of the Near Infrared Spectro-Photometer Optical Assembly (NI-OA) consisting of the CaLA triplet and CoLA (left), cross–section (right)

As the CTE match between lens material and mount material is not sufficient to fully avoid severe stresses in the glass during cool down, additional flexure blades are part of each adaption ring design. Twelve flexure blades provide additional flexibility and reduce the stresses in the glass to a tolerable minimum.

To mount the individual lenses to the adaption rings an innovative gluing concept is applied. Instead of directly gluing the lens to the adaption ring flexure blades, additional metal pads are introduced between lens edge and the twelve flexure blades of the adaption rings. This has advantages with regard to the local stresses at the glue-to-glass interface. It allows to incorporate measurement features on the metal plates to monitor the lens position with optical sensors or a coordinate measurement machine (CMM) and these metal pads also allow to compensate fabrication tolerances of the lens outer diameter and the adaption ring inner diameter. By tailoring the thickness of the metal pads, the glue gap between lens and metal and metal and metal can be optimized very precisely. An extensive glue qualification campaign has been completed successfully, where all glass-to-metal and metal-to-metal glue interface combinations of NI-OA were tested under mission representative environmental conditions [1].

To be able to achieve highly reliable structural analysis for the NI-OA units, CTE measurements were performed for all glass, metal and glue materials systematically down to operational temperature. The reliability of the structural model is essential for calculating the mechanical loads on the lenses to predict their surface error distortion during cool down. Mechanical loadcases were analyzed and the resulting surface error assessed with ZEMAX. These analytical results were compared to actual surface error measurements of the lenses, which were performed with an interferometer. The comparison of the analytical and measurement results gives a high confidence in the representability of the structural model [2]. Additionally, the realistic CTE values are required to predict the warm geometry of CaLA which needs to pre-compensate the shrinkage of lenses, glue and all metal opto-mechanics.

The big bipods on the CaLA unit in Fig. 2 are used to mount the CaLA unit on the SIC panel of the NISP filter wheel. These bipods have to compensate for the CTE mismatch between SIC and Titanium and for any mechanical interface tolerances. Additionally, they have to provide sufficiently high stiffness to keep CaLA within its alignment budget while it is integrated vertically, tested horizontally and ultimately operates in orbit without gravity. Analysis and measurement on the CMM show a minor movement of the whole CaLA unit of up to 4µm between vertical and horizontal gravity loadcase.

Axial positioning of the individual lenses within the CaLA unit is achieved by classical shimming, as can be seen in detail in Fig. 3. Axial shims between the adaption rings and the lens-barrel define the precise position of the lens vertices along the optical axis. As CaLA is integrated at room temperature, the axial shims also have to pre-compensate the shrinkage during cool down. Additionally these axial shims are used to compensate any tip tilt of the lenses relative to their mount. The lens gluing procedure controls the lens position quite precisely. However a minor tip/tilt of a few µm of the lens relative to the adaption ring mounting interfaces might remain. This lens tilt within its adaption ring can be precisely measured after gluing with the CMM. The lens design itself features a planar chamfer outside of the active aperture and perpendicular to its optical axis for this particular purpose. It is essential for the alignment concept to eliminate any tip/tilt of the lens, which is explained in section IV.

Fig. 3

CaLA opto-mechanical design

The axial shimming must not introduce mechanical stresses into the adaption rings, as the stress may propagate into the lens and distort its wavefront. Various concepts to minimize interface stresses have been assessed. Wedged shims could be applied to compensate for minor tip/tilt tolerance between adaption ring and lens-barrel, spherical washers could be used alternatively to adapt to these minor tip/tilt between the I/Fs. As a third alternative option, the I/F of the adaption ring can feature little flexure joints, which allow the I/F to rotate by a few µm and adapt to the I/F of the axial shim. The concept using the spherical washers was extensively studied to investigate the manufacturability, accuracy of shimming and repeatability for the desired washer material Titanium. The wedged shim option was not further considered for measurement feasibility of I/F tilts and for fabrication complexity. The I/F flexure option was analyzed structurally and bread-boarded to check manufacturability. This option finally turned out to be the most effective way to minimize I/F stresses, while μm positioning accuracy and reproducibility are maintained.

The lateral positioning of the three lenses relative to each other is not predefined by the opto-mechanical mount but is achieved during mounting and alignment. Additional alignment features can be mounted on the lens-barrel to allow relative positioning of the adaption rings with a high accuracy of a few μm.

Fig. 4 shows these alignment features for the adaption ring of L3. For L2 and L1 similar alignment features can be mounted on the lens-barrel. The principle is simple: a mechanical stop is mounted on the lens-barrel (“Stop post”) and a precision gauge block is placed between this mechanical stop and the adaption ring. To achieve a constant load on the gauge block, which is necessary for good positioning reproducibility, a second “preload post” is mounted on the opposite side of the adaption ring interface, which contains a spring loaded ball screw to generate a well-defined preload. With three of these gauge blocks, oriented in 120° configuration, the position of the lens relative to the lens-barrel is precisely defined. As long as the adaption ring is not fully bolted down on the lens-barrel and still can be moved laterally, an exchange of all three gauge blocks allows a well-defined, precise movement of the lens center. Once the lens is aligned, the mounting bolts have to be tightened with full load and the adaption ring is fixed to the lens-barrel. The gauge blocks and all alignment features of the ring can be disassembled without any impact on the aligned lens position.

Fig. 4

Alignment features mounted on the lens-barrel for precise positioning of the lens with gauge blocks

Positioning of the adaption ring with the help of precision gauge blocks was investigated with a dedicated alignment breadboard and showed a positional accuracy and reproducibility of one μm.

Due to mass and volume restrictions, the alignment features had to be designed so they can be mounted for alignment sequentially for each adaption ring and can be removed after successful alignment.

A.

Cryogenic measurement concept

To verify the correct alignment of the CaLA optical subsystem at the operational temperature of 134K, an innovative approach is required. A wavefront measurement through the cryostat window will not provide sufficiently accurate results as the desired data would be distorted by a cryostat window within the measurement path and allocation of alignment errors to individual lenses would be extremely challenging. Therefore, an alternative verification concept was selected, which allows to monitor precisely all positional changes of the lenses and opto-mechanics during cool down to cryogenic operational temperature. The cryogenic geometry of the system can be derived by subtraction of the cool down movements from the warm geometry. This positional measurement, which has to achieve the same accuracy as a 3D measurement with a CMM, has to take place within the cryostat at operational temperature. OHB selected a fiberoptic distance measurement sensor system, which was developed for cryogenic applications [3] and can detect movements of a flat reflective metal or glass surface in a distance range between a few cm up to more than 15cm with a sub-μm resolution.

The fiberoptic distance sensors require small, planar, polished measurement surfaces to achieve sufficient signal-to-noise ratio for their interferometric signal. For this reason, all adaption rings and the lens-barrel feature little axial and radial measurement surfaces (see Fig. 3 “optical measurement I/F” marked in orange). Each lens assembly has three radial measurement surfaces. For monitoring the axial movement of the individual lenses, the metal gluing pads between lens and adaption ring are used (compare Fig. 2. right). To allow simultaneous measurement of all three lenses, the orientation of these gluing pads for each lens assembly is slightly rotated relative to the other lenses. Thus, twelve axial sensors can monitor the tip/tilt behavior and the axial displacement of all three CaLA lenses simultaneously. To deliver a stable and precise distance measurement, these sensors have to be mounted on a highly stable sensor mount within the cryostat. The following Fig. 8 shows how this cryogenic measurement concept is realized for verification of the NI-OA alignment.

Fig. 8

Cryogenic test setup CAD view (left), actual test hardware during preparation for calibration run (right)

B.

Cryogenic Measurement Setup

The setup to achieve these positional measurements under cryogenic conditions consists of a so-called sensor mount, which holds the individual fiber-optic sensors. Each adaption ring and the lens-barrel have a set of radial sensors. To monitor the axial movement of all lenses, twelve distance sensors are aligned axially and use the measurement surfaces of the metal glue pads. These measurement pads of the individual lenses have a slightly different rotational orientation for each lens to allow simultaneous measurement of all three lenses.

As the sensor mount has to provide maximum thermal stability during the full thermal range of the test, it is fabricated from ZERODUR. Interfaces for the fiber-optic sensors are made from INVAR to minimize stresses on the sensor mount and to minimize the thermal movement of the sensors during cool down. Nevertheless the required measurement accuracy of +/- 2μm is so high, that even the ZERODUR sensor mount with its INVAR sensor interfaces is not sufficiently stable. All movements of the sensor mount within the cryostat therefore have to be determined in a dedicated calibration run, to be able to subtract them from the actual CaLA measurements. Fig. 8 shows the sensor mount with all its fiber optic sensors attached before it is mounted on the cold plate of the cryostat. Instead of the CaLA unit, a massive CaLA dummy, made from titanium, is placed on the cryostat cold plate (see Fig. 8 right). This dummy features all measurement surfaces of the actual CaLA unit. As it is made from one material and has no mounting interfaces, shims or bipods, the shrinkage of this Titanium dummy can be precisely predicted. For the calibration test, all expected positional changes of each measurement surface can be predicted. Any additional small movements are caused by minor changes of the sensor mount geometry and are stored as test setup response function, which is later on used to correct the CaLA measurement data.

C.

Verification Results for individual lens assemblies and the CaLA calibration

This whole concept was already demonstrated for each individual lens of CaLA with a dedicated smaller setup. Sensor type, ZERODUR sensor mount, data evaluation and data compensation with the calibration data of the setup was performed identically. These cryogenic tests on the individual lenses demonstrated that this concept allows to derive a lens movement of a few μm with a high reproducibility within +/- 2μm. The Lens assembly shows some settling effect during the first thermal cycle but then shows a highly reproducible behaviour for a number of full thermal cycles.

Fig. 9

Cryogenic geometry test thermal cycling (left), results for positional change of one of the CaLA lenses relative to its adaption ring (right)

The calibration test for CaLA with a CaLA dummy, as shown in Fig. 8, achieves a reproducibility of all sensor readings during a sequence of thermal cycles below 1μm, which means that the actual CaLA measurement can be corrected for the setup response function with this accuracy.

A.

Conclusion and current status

Within the Euclid NI-OA activity the following achievements have been reached:

Recently, the EQM lenses have been glued into their adaption rings and these assemblies and the lens-barrel are characterized with the CMM. This data now allows to derive all axial shims for the CaLA EQM. The preparation of the alignment tower is under way at MPE. All GSE for integration and alignment of the CaLA unit are ready. The calibration run for the cryogenic measurements is completed and the response function of this test setup is known, so it can be used for compensation of the actual measurement of the cryogenic CaLA geometry, once the CaLA EQM alignment is completed.

B.

Outlook

The Integration of the EQM of CaLA has started and will be under way at the time of this conference. During this activity it will become clear if the stability of the interferometric alignment tower and the alignment features of the CaLA unit allow a straightforward positioning of all lenses to the desired alignment accuracy of a few μm, or if a few iterations for each lens are required. Once the unit is integrated and aligned at room temperature, its geometry and all lens assembly positions will be fully characterized with the CMM to document the room temperature geometry. Then the cryogenic cycling with the position monitoring within the cryostat will determine if all lenses reach their defined cold position within their alignment tolerance or if the combination of all settling effects during cool down leads to lens positions which are outside of the specified tolerances. If that would be the case, CaLA has to be disassembled again. With the help of the CMM data of the aligned system and the measurement data of the cryogenic deformation, cold shims can be derived which allow to pre-compensate for any misalignment during cool down. CaLA is then reintegrated with these cold shims, just based on the mechanical alignment features and known gauge blocks and without the help of the CGH. If this cold shimming really becomes necessary or if the movement of the individual lenses is sufficiently small, as the completed tests on individual lens-assemblies indicate, will become clear within 2016. An overall optical performance test of both CaLA and CoLA in NISP configuration will be performed at MPE before these units are to be delivered to Laboratoire d’Astrophysique de Marseille (LAM) where they will be integrated in the NISP instrument.