2.1

Optical Requirements

For the requirement derivation, the intention was to use available data from the alignment tolerance from various optical space instrument developments in order to get a better understanding of the typical stroke amplitudes needed for a comparable mission. The same order of magnitude is expected for in-orbit thermo-elastic deformations in these kind of instruments.

There are various applications for deploying a deformable mirror in a space optical instrument. For example when the deformable mirror is used to compensate deformations of a large primary mirror of a telescope due to gravity release or thermos-elastic effects. In this case, the deformable mirror should be placed at a conjugate plane of the primary mirror and the size ratio between primary and deformable mirror will directly limit the effectiveness of the optical correction. The deformable mirror concept under study in this project however aims at being a versatile tool, not linked to a too specific system design. A diameter of 50mm is comparable to the accessible pupil planes of the typical programmes described above, was thus chosen for this activity.

Active tip tilt correction can be performed by an actuated mirror mount providing the advantage of not driving the actuators stroke requirements at the detriment of other conflicting requirements (correction of high-order Zernike polynomials etc.). Depending on the application, it can therefore be beneficial to put the main tip tilt function to an additional mechanism.

For this project the required tilting was derived from the ESA large telescope study, which specifies a correction of 0.5mrad. However, OHB assessment is that a 0.3mrad correction is also be sufficient. As mentioned before, assuming that large tip/tilt corrections are realized on a separate mechanism, so that this DM is rather used for fine tuning.

The following discussion applies to all Zernike terms. In order to assess what optical disturbances in a mid-class earth observation instrument can be expected and whether they can be corrected with a DM, analysis results from the mentioned programmes were taken into account. For the evaluation of the optical disturbances, it was assumed that only a rough placement of the optical components based on mechanical I/F characterizations would be done and the residual optical aberrations are then compensated by the DM. This is considered as an absolute worst case requirement, since analysis shows also that in-orbit deformations of the optical elements with the optical chain to be compensated by a DM would be much smaller. For information all four mirrors of the IRS front telescope were placed with an average accuracy of 100μm. For this investigation, M4 of the telescope was considered to be a deformable mirror. The performance is evaluated using a Monte Carlo simulation with 1000 samples and the error budgets derived for MTG. The performance obtained are comparable and validate the potential approach. We observe that the strokes specified (15μm for defocus and 5μm for the other Zernike) are very large compared to what would be required for such an application. From a comparison of the WFE values in nm rms and the calculated deformation amplitudes in the nm regime, a defocus requirement of 5μm amplitude is well suitable for a deformable mirror of that size.

In order to derive stroke requirements to compensate In-Orbit deformations of a large primary telescope mirror we used the following Korsch type telescope layout.

Figure 2.

Korsch type telescope design for conceptual study on stroke requirements.

This telescope has the advantage of providing a location in the beam path where a deformable mirror will be conjugated to the primary mirror without the need for relay optics. However, it does not give much flexibility in the size of this conjugate plane. In the system under consideration, the diameter was 400mm.

Given this conjugation strategy, the strokes required to correct the modes are identical to the modes themselves. For a primary mirror of 3m in diameter, and the deformable mirror size would need to be 400mm, given a 50mm DM (as considered in this study) the primary will be about 750mm in diameter. In order to derive a requirement from the analyses for the further development a margin of 100% is considered. This shall ensure appropriate flexibility for further potential applications. The only exception is the astigmatism Z4 and Z5. Here the driving case is the MTG case for a not finally aligned (only placement) telescope which is considered as an absolute worst case (factor of six to the large telescope study). Therefore, there is not additional margin considered for Z4 and Z5. The consolidated requirement for the different Zernike Z4 to Z10 can be found in the table below.

Table 1.

Required strokes for active correction.

The residual wave-front deviation needs to be defined as a function of the required system performance. The example of MTG-IRS can be used as described before. The value of 30nm PV for the DM in order to achieve an instrument WFE performance of ca. 80nm RMS is reasonable. The un-powered mirror deformation of the DM will contribute to a reduction in the achievable stroke. An unpowered mirror deformation of 2μm still allows for 13μm strokes which is largely sufficient for the target applications.

2.2

Structural Requirements

As a starting point for the structural design of the DM, the quasi-static design loads were defined according to ECSS-E-HB-32-26A. These are used at the beginning of the design phase as an envelope of the loads. To derive preliminary design loads for the DM, the following formula as shown below shall be used [5].

These loads were used to dimension structural parts, calculation preliminary interface forces, momentums and the primary notching. Using the formula with the mass of the current DM design (m=1.57[kg]) results in a preliminary quasi-static load of 61.14[g] in all axes.

The sine loads were not considered critical for the DM with a mass of 1.57kg, since the first Eigen-frequency was higher than the maximum frequency of sine loads (150 Hz). For the same reason, the sine test was considered as being a quasi-static load test.

To derive and consolidate the requirement for the random vibrations the ECSS - Space Engineering, Testing was used [4]10. The basis for the load calculations is the mass of the DM of 1.57 Kg. The resulting loads are derived by using the mass and the formulas for units not located on external panels given in the cited ECSS. These calculated loads are shown in the figure below. In comparison, the similar requirements on the DM from the previous study are plotted for in-plane and out-of-plane random excitation.

Figure 3.

Loads for a 1.57 kg derived from ECSS 10, in-plane and out-of-plane (DMAO IP and DMAO OOP).

These loads have compared to real loads of comparable mirrors from MTG and EnMap programmes. One fact that became obvious is that the generically derived load specification has a more flat shape in the low and high frequency domains as the real loads of MTG and EnMap. This has the consequence that more energy is introduced into the system whereas it is not of actual interest and does not bring more benefit. Therefore, we decided to steepen the ramp-up of the loads in the low frequency region and the ramp-down in the high frequency domain. The finally defined ramp-up is set with 4dB/oct and down with 5dB/oct. An illustration is given in the next curves.

Figure 4.

Loads for a 1.57 kg derived from ECSS [4]; Loads from former project for in-plane and out-of-plane (DMAO IP and DMAO OOP), Defined load level for requirement of this qualification campaign (DMAO OHB prop.).

In general, the generically derived loads from ECSS and the real curves of OHB experiences (MTG and EnMap) are mostly in line. Only the minor modification of a steeper ramp up and down was necessary. This resulted to the requirement on random loads given in the next table for the DM to be sustained without any damage or performance degradation.

Table 2.

Specified random vibration level for DM testing.

Shock loads play an important role for optical instrument since damage or rather failure of an optical component could result in the loss of the entire mission. Considering the critical aspects of shock loads and considering the risk of a total failure of the DM, it was considered necessary to verify the capability of the DM design to sustain shock loads by test.

To specify shock levels for the DM a few assumptions have been taken into account. As shock source a clamp band excitation (far-field environment) has been considered. Single point sources, like frangibolts or pyrotechnically actuated mechanisms were not taken into account since these sources and their locations are very specific to one particular platform and payload concept. The load path from the source (clamp band adapter) has been assumed to be larger than 1m. Reference shock response spectrum of EnMap and MTG IRS have both been used as basis [8][7]. The loads are applicable inside the optical instrument (MTG or EnMap). A damping factor of Q=10 was applied for these applications.

By considering these assumptions as well as real experience from these projects, finally a shock requirement was derived. This derived levels can be found in Figure 5 and has a maximum shock load at 300g above 1000 Hz.

Figure 5.

Specified and Tested Shock Levels.

Vibration tests (sinusoidal vibration and random vibration) have been successfully conducted in in-plane and out-of-plane directions with respect to the mirror surface. Sinusoidal vibration was performed with up to 20 g and random vibration with up to 17.8 gRMS.

2.3

Thermal Requirements

Considering the wide spectrum of potential applications of a DM also a wide range of possible operational temperatures is possible. For example if a DM is used in a science mission like EUCLID which is located at a Lagrange-Point, it would need to be operated at a very cold temperatures between roughly 120K and 140K. If the DM would be used for a typical earth observation mission like MTG in a Geo-Stationary Orbit, operational temperatures are strongly influenced by the sun. Such operational temperature ranges of the optical components are typical between -15°C and +60°C. The temperature limits are dominated by radiative exchange or the incoming sun light on the mirror (sun intrusion) [7]. Important to mention is that from OHB experience, it is not likely that a very wide temperature range (100K to 300K) for operational temperatures will occur within one mission only. The temperature set point will either be around a low temperature as for the example in EUCLID or to a higher temperature as for MTG [9][7]. However the alignment of instruments will be performed at room temperature what is to be considered for the definition of the temperature range and also the non-operational temperatures need to be considered for survival. It might be considered to split the requirement in two applications low and higher temperature. However the thermal qualification of the DM of the previous GSTP showed that there was no problem from the large temperature range. The resulting thermal requirement was defined at that time such, that the DM shall fulfil the functional and performance requirements for the huge temperature range from 100K to 333K [3].

For various projects sun intrusion is a valid case and very stringent to the optical components as well. For example for the ADM Aeolus ALADIN instrument, a SiC mirror of 25mm diameter was heated up to a temperature of 80°C in non-operational mode [6]. For the MTG mission sun intrusion plays also an important role. Here a mirror with about the same size of the DM is exposed to sun illumination also within an operational mode. As a result, the mirror is heated up to +60°C. Of course the actual temperature of a mirror under sun illumination depends on the mirror size, reflectivity for the sun spectrum and most relevant on the thermal capacity and thermal conductivity of the material of substrate and mount.

For the MTG FCI Instrument, the sun intrusion is about 100W/cm². However, the optical component is next to an intermediate focus in MTG, whereas for most of the applications of the DM, it will be place at a conjugated plane. This means that the intensity for the DM would be about a factor of 7 higher as for the primary mirror of the same telescope. Including margin, a factor of ten should be considered. The Solar constant is roughly 1350 W/m² what corresponds to 0.135W/cm². Taking into account the factor derived from the examples, the solar intrusion which should be sustained by the DM is 1.35W/cm². A verification of the DM was done by analysis using COMSOL tool. It confirmed that the DM will sustain a sun intrusion of up to 1.5W/cm² c.

2.4

Dynamic Bandwidth

The requirements on the dynamic bandwidth for DM actuation is quite difficult to define, since the needed frequency range strongly depends on the dedicated application. Hence, we split the regimes the DM can be used in the three categories: Static Phenomena, Low Frequency Phenomena and High Frequency Phenomena.

Static Phenomena are occurring only once in the lifetime of the mission. Compensation with a DM could be used for the following effects: manufacturing error correction, alignment, gravity release, compensation of CME-effects, alignment deterioration caused by slippage due to launch loads etc. Since these events occur only once, there is no specific requirement on the bandwidth.

Low-frequency Phenomena are disturbances with frequent occurrences like thermal drifts. This effects can easily be compensated with a DM. In order to compensate these kind of effects the correction chain shall have a slightly higher bandwidth as the effect itself. For usual thermal events the temporal constant is greater than 10 seconds, what would correspond to a frequency of 0.1 Hz. Considering an application where the DM might be used as a scanning or refocus mechanism the frequency could be considered up to 10Hz.

High-frequency phenomena are potential application of a DM with an associated correction chain could also be used to compensate for micro-vibrations occurring in the instrument and other disturbing the optical Point Spread Function (PSF). Micro-vibrations are defined for a bandwidth of up to 500Hz. Compensating these vibrations would require an even higher bandwidth of the DM and correction chain. However, the compensation of high frequency phenomena like micro-vibrations is considered to be in the domain of “Adaptive Optics” which is beyond of the scope of this Active Optics activity. Furthermore, the optical aberrations caused by the micro-vibrations would be needed to be better understood before defining requirements for their compensation by a DM.

The maximum achieved closed loop actuation frequency of 19 Hz is compliant with the required frequency of 10 Hz. The mirror was operated with an open loop frequency of 100 Hz which is compliant to the required specification for that activity. Nevertheless it is important to mention that for the conducted project the limiting factor of the achieved frequency is the driver electronics. This is a minor point, so that higher frequencies are easily achieved with the same technology.

2.5

Radiation

The radiation levels seen during a mission depend strongly on the instrument architecture and the mission and the orbit itself. Therefore, it is not straight forward to define a generic requirement for radiation for the DM. In the previous study the DM was tested by exposure to γ-irradiation and p+-irradiation, yielded no visible degradation of the deformable mirror. The unpowered surface deformation as well as the stroke of the influence functions remained the same before and after irradiation. Therefore, it can be considered that the technology is not sensitive to radiation.