A.

Optical Sub-System

The optical system consists of a ø135 mm, f=880mm, F/6.5 off-axis 4× reflective telescope with a FoV of 0.878° in the plane of symmetry and 1.336° in the across-direction (Fig. 2.). The initial baseline considered a Three-Mirror-Anastigmat (TMA) with an intermediate image and an additional flat folding mirror. It turned out that such TMA would result with a Tertiary Mirror almost as big as the Primary Mirror and would never fit into the available volume. To find configurations with reduced element sizes, iterations were made of a system with 3 ideal lenses, keeping the Focal Length and overall length constrained and taking the element sizes as performance merits.

Fig. 2.

CaSSIS optical design

The only optical performance metric involved was the Petzval correction (the sum of the element powers to be zero). Two basic configurations emerged from this iteration:

The latter configuration would eventually result in a Korsch-like telescope with a minimum size Tertiary, yet the mirror would be too large.

Configuration i) was selected, which required 4 mirrors with optical power. In the transition from the ideal lenses to a design with real mirrors, the first element was substituted by a Cassegrain Telescope (M1, M2), yielding the intermediate image (a). The second element became the ocular mirror M3, yielding nearly collimated space (b). The third element became M4 and delivers the final image. Mirrors M3 and M4 are about the same size and have less than half the size compared to a single Tertiary Mirror of a TMA. This yields also a mass saving and likely also a cost saving. The increased DoF were further used to incorporate an existing Primary Mirror (from another telescope built in a small series at OEI Opto AG), thus eliminating the longest lead item. The severest drawback has been the loss of the flat folding mirror. The off-axis clearance had now to be achieved solely with tilts and decentres of the aspheric mirrors, which costs imaging performance. The most critical – and performance limiting – feature proved to be the clearance (c) between M3 and the beam underneath from M4 to the image.

The first order design had foreseen an accessible Exit Pupil for sake of Straylight suppression. This Pupil still exists (d), but there was no hope to achieve enough clearance. The remaining options for Straylight baffling were the placement of a Field Stop (e) at the intermediate image (a) and two intermediate walls (f), (g) which subdivide the telescope in an upper compartment with M1 and M2 and a lower compartment with M3 and M4. The optical design has further taken care to prevent direct sky view from the detector through the field stop into object space, line (h).

The overall imaging performance achieves an MTF of >0.3 at the detector Nyquist Frequency of 50 lines/mm (44 lines/mrad in object space) @λ=632nm over almost the whole Field of View, thus making optimal use of the detector (i.e. the MTF should also not be much more than 0.3 at the Nyquist Frequency to avoid aliasing effects [4])

The 4 aspheric mirrors are made of ZERODUR® Expansion Class 0. All four mirrors carry a protected Silver coating, by a process previously qualified by OEI Opto AG.

B.

Structural Sub-System

The key concept of the CaSSIS system is that it uses a rotation mechanism to produce the stereo pair [5]. The telescope in this case is not nadir-pointing but at an angle of 10° with respect to nadir. After acquisition of the first image, the telescope is rotated through 180° so that it points 10° stern.

A complexity introduced by the rotation is the need to ensure proper routing of the cables needed to drive the electronics. The instrument structure was then based around a T-like structure with the optics and focal plane assembly on one side of a main support element, the focal plane electronics in the centre, and the cable management system on the other side of the support. The interface between the Telescope Assembly and the Rotation Drive incorporates three Titanium bipods (Fig. 1.). The feet are then bolted to the Rotation Drive Mechanism with six Titanium M8 bolts. The bipods have been designed based on heritage to provide a highly stiff 3-points interface to the Telescope Assembly. They have been optimised several times during the development phase. VETRONITE® washers have been implemented at the Bipod interface to thermally isolate the Telescope Assembly from the Rotation Mechanism.

CFRP material has been used for the Telescope structure. The structure was reinforced throughout the development phase to increase its stiffness and its ability to withstand the random vibration loads.

The tube structure, which has a wall thickness of 4.8mm, was manufactured as one piece. Inserts were then added to the tube allowing it to be ready for receiving walls and other metallic structural components. The walls which are also 4.8mm thick are made in sheets from the same quasi-isotropic near zero CTE CFRP type, where they were then cut-out and machined to their required dimensions.

Optimising the mass of the Telescope Assembly has been a major effort throughout the development phase. One of the main reasons is that the strengthening of the structure following loads analysis has consumed a significant amount of the design contingency. The CFRP structure has been deeply light weighted, as can been seen in Fig. 3.

Fig. 3.

CaSSIS CRFP structure

The structure has internal baffles. They are made of Aluminium and located adjacent to the Field Stop allowing them blocking unwanted optical paths from the entrance aperture to the FPA sensor, and radiatively and conductively transferring heat to the Telescope Assembly from a series of electrical heaters.

INVAR® mounts are attached to the CFRP tube, keeping the mirrors in place throughout the environmental changes and allowing testing in 1g gravity without significant degradation to the image.

C.

Thermal and Electrical Sub-System

The Telescope Assembly thermal sub-system has to provide the ability to the Instrument to regulate the temperature of the Telescope Assembly during Survival and Operational phases. In addition it has also to provide the temperature of the Telescope Assembly to the S/C. For the survival phase 7W are needed by the Thermal sub-system. For the operational case 14.5W are needed to keep the Telescope Assembly at the operational temperature of 21±5°C. The thermal sub-system design is supported by the analysis (operational cold case in Fig. 4.), and has satisfied the above requirements by implementing heaters, thermostats and thermistors.

Fig. 4.

CaSSIS Telescope operational Cold Case

A.

Schedule, phasing and risk mitigation activities

The scope of the project was to perform the full design, development, manufacturing, integration and testing of the Telescope Assembly, which consists of major sub-units at the technological edge. The project, which is currently in its operation phase E, was structured in sequential phases (B, C and D) with quality gates and major critical reviews in between.

The schedule control was the key driver from the very beginning, in order to comply with the overall schedule of the ExoMars launch. The launch was only possible in a limited time window (January-March 2016), due to the selected trajectory from Earth to Mars, with the next suitable window available only 2 years later.

Due to the very late final decision about the adoption of CaSSIS (Oct 2013), flying without this Instrument was an extreme but yet possible worst case considered by the ExoMars program. Therefore and in order to get as close as possible to the requested delivery time, a Proto-Flight model (PFM) development approach with specific risk mitigation activities has been agreed among the involved parties since the very beginning:

The main objective of the phase B was the feasibility study for the design, assembly, integration and programmatic aspects. The phase B has been concluded with the Preliminary Design Review in November 2013. At the PDR the procurement of the Long Lead Items was kicked-off, together with the phase C. The phase C had the objective to consolidate the design, and it has been concluded with the Critical Design Review in March 2014. The implementation phase was split in two, with the phase D1 having the objective to perform the assembly and integration of the PFM, and the phase D2 with the objective to test and deliver the PFM. An overview of the major milestones and of the overall planning is schematically provided in the table Tab.1. below.

Tab. 1.

CaSSIS Telescope phasing

B.

Building-blocks and model philosophy

OEI Opto AG has extensive heritage in developing highly customized optical systems, with dedicated feasibility, development and design phases to finally deliver small series or even single units.

This was also the case of CaSSIS, with on top design freedom limitation dictated by the schedule impact. The scopes of the design were clear, and the objectives were inter-dependent (mass, power, volume, interface requirement and operational environmental conditions). In CaSSIS, a longer trade-off design iteration for a potentially improved design, could have meant missing the flight opportunity on board of ExoMars.

Having in mind the goal to comply with the challenging schedule, technological building blocks have been used as starting point. The technology philosophy of the CaSSIS Telescope Assembly has been to utilize as much as possible heritage from similar projects. This was realised through extensively re-use of technology used in other similar projects.

Without this solution the on-time manufacturing of CaSSIS couldn’t be possible, from any other company worldwide (up to our knowledge) due to the lead time of the critical components. Here an example of the used building blocks:

Of course the environment for the ExoMars mission has different parameters compared to the other missions, resultingly there has been an effort from all the involved parties to minimise the impact of the environmental differences between the missions, by optimizing the Survival and Operating conditions to maintain the foreseen hardware within an already qualified environment. At the project start the Technology Readiness Level [6] of the overall system equipment was 5, as all of the technologies and processes were qualified at component level for the Environmental conditions. Due to the high specificity of the application and the singularity of the requirements, fine tuning, delta-qualifications and technological innovations was also necessary. Examples are the combination of thermal control with low thermal conductive materials, and the qualification against the aero-breaking requirement.

C.

Synergy of testing campaign

Due to the time scales imposed on the project, critical bread-boarding in parallel with a basic Development Model followed by a PFM approach was the only viable model philosophy to be taken forwards. The components for a Flight Spare have been as well procured which could be assembled, integrated and tested in the event of a major issue occurring.

Based on the above presented considerations, the development and verification approach included a combination of acceptance at component/sample level (e.g. mirrors, EEE, CFRP material, etc.) and at assembly level. The adopted Acceptance Test Flow is showed in Fig. 6.

Fig. 5.

CaSSIS Telescope Assembly Models Flow

Fig. 6.

CaSSIS Telescope Assembly Test Flow

A pragmatic decision was taken, about deviating from the requirement to Shock test the PFM unit. It has been possible to prove that the CaSSIS shock level was covered by similarity, from previous qualification programs for the more critical component (Primary Mirror). This is another good example of how CaSSIS benefited from the use of building blocks.

The CaSSIS Telescope was delivered to the University of Bern for completing the acceptance. The University of Bern was indeed responsible for the mounting of the detector to the telescope, and hence the Thermal Vacuum test has been performed in synergy with the complete CaSSIS Instrument. The optimum focus position was found in the laboratory using interferometric techniques. The mounting of the focal plane was however slightly shifted to pre-compensate the shrinkage of the CFRP structure by moisture release [2]. This has been based on the prediction from the well mastered CFRP modelling at OEI Opto AG, and this turned out to be the optimum testing sequence.