1.1

Scope and purpose

Increasingly demanding space-based applications require a large primary mirror diameter, on which depends the optical instrument resolution. However, mass and volume of a monolithic primary mirror would dramatically increase along with its size, which, in addition to cost and technical issues, is also limited by the launch vehicle fairing available volume. Thus, to reach large primary mirror diameters, development of instrument that will include a deployable, segmented primary mirror and an active correction loop to optimize its optical performance will be of utmost interest. Such technologies will allow addressing highly demanding space-based applications such as high-contrast imaging for Earth-like exoplanets direct detection or high resolution Earth observation from the geostationary orbit.

This study aims at defining a technological roadmap of such an active optics correction loop, which will include sensing devices and active correction components. This technological roadmap will be supported by the design of two active correction loop which will be implemented in two preliminary instrument designs dedicated to two different missions:

For both cases, the active optics design has been performed through trade-off analysis of correction strategies and technologies, which will be based on the requested performances evaluation at system level. Such an evaluation will allow identifying the active optics correction chain technical challenges and proposing a development roadmap.

1.2

Earth Observation case

The way to proceed has been to systematically analyze the potential combinations of active optics components, in order to identify the ones that can be studied. The systematic approach is beneficial to ensure that every interesting case is not forgotten and that any case which would not be intuitively considered interesting is also evaluated. The definition of an interesting case is discussed later in this paper, where we propose the criteria chosen to select or dismiss configurations.

1.3

Science case

Futur fundamental Science missions like LUVOIR (Large UV/Optica/Infrared Surveryor) shall be with a large international sharing funding and technological developments such as a complete Primary Mirror system. They shall be within a Science Objectives approach and disruptive from the JWST technologies already close to their asymptotic limit and too far from the LUVOIR-A Ultra Stability needs (10pm / 10mn stability).

2.1

Summary of requirements

The active optics correction loop will here be used to guarantee the MTF requirements in PAN and multispectral channels by:

Figure 1:

Functional diagram of the active optics loop strategy for the EO mission case.

This active optics loop for the EO mission case relies on the following elements:

2.3

Optical Design Trade-Offs

To limit the optical computation we selected the following criteria:

Finally 2 solutions were identified as promising:

Figure 2:

Optical combination n°3

Figure 3:

Optical combination n°8

2.4

Performance Assesment

We have simulated in CodeV the wavefront at the exit pupil level, after correction:

Figure 4:

Simulation with CodeV(WFE, FTM, spot diagram) before correction by the DM

Thus, with Matlab we have simulated the WFE_overall obtained, after correction with the deformable mirror, for different numbers of actuators on a perfect DM. The purpose of these simulations is to estimate approximately the minimum number of actuators necessary to correct the WFEoverall.

Figure 5:

Matlab simulates the best shape of the DM to correct the WFE_overall(for different numbers of NLA actuators)

Hereafter a table which resume the versatility of each optical architecture studied above. Each column represents the maximum authorized value of the parameter considered to hold the final specification MTF shall be > 5 points at Nyquist. This translates the margin to relax the parameter considered for each optical architecture.

In conclusion we can say that:

Below is presented the first study of the FEM under NASTRAN of the telescope GeoHR, that is to say:

Figure 6:

Mechanical models – front cavity in Si3N4 – Left,18 petals; Right, 6 petals telescope

The size of the individual Si3N4 structures, MFD and MLA under the petal are the same than on the 18 petals model.

Figure 7:

Details of the primary mirror interfaces with Si3N4 structure

Initial temperature at 20°C, on nodes of the whole FE model of the telescope.

Displacements

Displacement calculated:

Table 1:

Left Maximum residual gap between 2 petals; Right Maximum displacement in the plane for 1 petal

• ○ WFE

The petals nodes are defined in local frames whose X and Y axis projected on the interface plane of the telescope are collinear to each other. The WFE is calculated for each of these frames in the largest inscribed circle in the hexagon.

Figure 8:

Left WFE M1 18 petals 1,7 nm RMS PTF; Right WFE M1 6 petals 2nm RMS PTF

Thanks to Zerodur® used for the segmented primary mirrors, the thermal impact on the WFE is insignificant.

2.5

Trade-offs

2.6

Design Description

The configuration chosen by the TAS team is that (named N°8) with a segmented hexapod(MLA) on the first pupil and a segmented DM(NLA) on the second pupil and with a deployable front cavity. Main advantages:

The architecture for a GeoHR satellite has been retained to be compliant with the main requirement asking a MTF > 5 pts at Nyquist frequency.

Figure 9:

The Optical Architecture retained

This architecture is summarized hereafter it consists in:

3.1

The Astrophysical Context

To be exhaustive, the science objectives of a Space Observatory[1] with diameter of collecting area > 12m are:

Exploring the full diversity of exoplanets / Discovering and characterizing exoplanets in the habitable zones of Sun-like stars and searching for biosignatures in their atmospheres / Remote sensing of the planets, moons, and minor bodies of the solar system / Exploring the building blocks of galaxies both in the local universe and their emergence in the distant past, and elucidating the nature of dark matter / Understanding how galaxies form and evolve from active to passive, both by studying their stars and their gaseous fuel across all temperatures and phases / Following the history of stars in the local volume out to tens of megaparsecs to understand how they form and how they depend on their environment / Observing the birth of planets and understanding how the diversity of planetary systems arises.

The most demanding one is the search and characterisation of exoplanets by direct imaging to allows their atmosphere characterisation [1].

Since about the early 80ths active optics techniques have became essential in large Earth-based telescopes (E-ELT, VLT, GMT, etc.) mainly for Atmophere correction (eXtrme Adaptive Optics). In addition coronagraphy technics allowed suppressing the stellar light. Static residual supperssion and Post-precessing are finishing isolating the flux from the exoplanet:

Figure 10:

Exemple of results after Extreme adaptative Optics and Coronagraph with SPHERE on VLT.

If the contrast limitation is around 10^-6 on ground the expectation in Space is about 10^-10 using Apodized Pupil Lyot Coronagraphs (APLC) type with two internal deformable mirrors (DMs) that correct static optical field error to create a “dark hole” and can also correct some wavefront error drift on the order of 0,002 Hz (10 minute update rate) depending on the target star brightness:

Figure 11:

APLC type coronagraph

A coronagraph dark hole is a region of the detector with low background signal that can support high contrast imaging. The dark hole for an APLC is typically annular and centered on the host star under observation, as in Figure 12. The inner radius of the dark hole is called the “inner working angle” and sets the limit for how close a planet can be to its host star and still be detected. The outer radius of the dark hole is called the “outer working angle” and is limited by the spatial resolution of the DMs (approximately the number of actuators across the telescope pupil divided by 2). The phrase “digging the dark hole” refers to the process by which the deformable mirrors are adjusted to correct for static errors in the telescope to create the annular region. This report is focused on stability – or “maintaining the dark hole”

Figure 12:

Left: The simulated PSF of a bright star with a segmented telescope and no coronagraph. Center: The coronagraph creates an annular region called the “dark hole” where exoplanets can be detected. Right: PSFs reveal the ability of the coronagraph to create very low contrast (~10^-11) in the dark hole From[2].

In the last 5 years at least scientists have set a few important points:

Figure 13:

Static per-segment piston tolerances for the LUVOIR-A 15 m telescope with the baseline coronagraph design for local piston aberrations and a target contrast of ctarget = 10^-10. [11]

Figure 14:

Expected disturbances and notional control systems in the temporal domain. The green bar represents the notional bandwidths of the frequency “bins”. The “quasi static” region is of highest concern since the coronagraph is most sensitive to errors in this frequency regime and there are fewer active/passive controls [13]

Figure 15:

Time evolution of wavefront error for “Set and Forget” on the left and ULTRA appication on this drift calculation on the right.

3.2

Proposed Science case architecture and solutions

A space observatory LUVOIR type for such mission as exoplanet charactisation shall be focused on:

Figure 16:

PM Active WFE Segment and Ultra Stable Picometric Piston/Tip/Tilt tripod Actuator

Principle:

Working directly on a picometer displacement & high stability is directly facing the physic laws, for reminder atomic radius ofHydrogen (H1) = 53pm. The use of a huge Sunshield in L2 will guaranty a very high level of thermal stability (~ mK), allowing to use the intrinsic thermal material stability. Smart structural design and characterisation will make it fit to requirements by adjusting temperature compatible with acceptable drift and time correction.

Advantages:

Primary Mirror Deployment: A Two Parts Option

Looking forward simplification and efficiency the PM shall be Symetrical to ease optical and postprocessing correction, no-gap articulations for simplified on-ground end-stop calibration and in-orbit motorization control.

Table 2:

TAS 2 and 3 parts Mirror Design scaled to Launcher options

Hereafter some 3D view of the TAS 2 parts design in stowed and deployed configuration:

Figure 17:

TAS 2 parts Telescope in stowed (left) and deployed (right) configuration

For the size of alf the PM diameter we can fit all the instrument and minize number of deployment and associated risks. The M2 is first deployed with the 3 tape spring used also as Secondary Mirror pointing mechanism. The central baffle is deployed after the M2. Finaly the two parts of the M1 are deployed and locked:

• • Deployment of M2

The TAS solution is based on the 3 Tape Springs Deployment technologies with the following advantages:

• - Stowed configuration simplified, the M2 will be directly stowed on the central structure

• - Smooth and controlled deployment with less risks of blocking and seizure than articulated beams

• - Active Piston/Tip/Tilt correction when deployed (Accuracy 100nm/Stroke 100μm/Stability 100nm RMS/s)

• • Supporting Structure:

Future structure will be a compromise of “smart” structure as “auxetic” shape to counterbalance thermoelastic and mechanical deformation during motion of the complete observatory and adjustment capabilities of the active segments (WFE and Piston/Tip/Tilt).

Figure 18:

View of the structural frame / Picometric Tripod / Supporting Structure arrangement

CAD model thermo-mechanical sensitivity and WFE impact first results:

Table 3:

Thermo-mechanical impact on TAS design options

Table 4:

WFE calculation and expected for optimized TAS design

No issues on both needed corrections and impact of thermal stability on the primary mirror WFE.

4.1

On GEO-HR Case

Such an instrument will have on board correction of micrometric level mirrors (hexapod allowing an overall correction in piston and tip / tilt) but also at the nanometric level (optimization of the mirror surface).

There are four technological products essential to the realization in the near future of the first high resolution instrument for monitoring the earth in real time and subjected to two dedicated unit validation:

Figure 19:

Essentials Building Blocks & System Validation

Units Validation

• ○ Metrology System at actuator level & at “equipment” level (for instance M1, deformable mirror…)

• ○ Digital Simulator and Optical System Bench

Figure 20:

Necessary Developments Roadmap