Precision engineering for astronomy and gravity science

The impact of Aeolus wind retrievals on ECMWF global weather forecasts

The European Space Agency launches in 2009 two flagship missions in the domain of submillimeter space astronomy. The Herschel Space Observatory is a common user facility featuring a 3.5 m aperture Cassegrain telescope passively cooled to 80 K. The Planck survey mission includes a 1.5 m unobscured aperture off-axis aplanatic telescope passively cooled to 40 K. Herschel will make pointed target observations of astrophysical objects and phenomena in the frequency range 448 GHz to 5.3 THz (or 672 to 55 mum wavelength). Planck, on the other hand, will map the entire sky by strip scanning at a spin rate of one revolution per minute covering a frequency bandwidth of 30-857 GHz (or wavelength from 10 to 0.35 mm). Its spin axis is pointed antisunward and can be oriented within a 10deg cone around that direction. The telescope line of sight is fixed at an angle of 85deg to the spacecraft spin-axis. This paper describes briefly the specific telescopes developed for each mission; their design characteristics, the development process for each, their achieved performances (from on ground testing) and their expected performances in flight.

The Herschel and Planck Space Telescopes

Athena, the largest space-based x-ray telescope to be flown by the European Space Agency, uses a new modular technology to assemble its 2.5 m diameter lens. The lens will consist of several hundreds of smaller x-ray lenslets, called mirror modules, which each consist of up to 76 stacked mirror pairs. Those mirror modules are arranged in circles in a large optics structure and will focus x-ray photons with an energy of 0.5 to 10 keV at a distance of 12 m onto the detectors of Athena. The point-spread function (PSF) of the optic shall achieve a half-energy width (HEW) of 5” at an energy of 1 keV, with an effective area of about 1.4 m2, corresponding to several hundred m2 of super-polished mirrors with a roughness of about 0.3 nm and a thickness of down to 110 µm. This paper will present the status of the technology and of the mass production capabilities, show latest performance results and discuss the next steps in the development.

Silicon pore optics x-ray mirror development for the Athena telescope

The Athena mission, under study and preparation by ESA as its second Large-class science mission, requires the largest X-ray optics ever flown, building on a novel optics technology based on mono crystalline silicon. Referred to as Silicon Pore Optics technology (SPO), the optics is highly modular and benefits from technology spin-in from the semiconductor industry. The telescope aperture of about 2.5 meters is populated by around 700 mirror modules, accurately co-aligned to produce a common focus. The development of the SPO technology is a joint effort by European industrial and research entities, working together to address the challenges to demonstrate the imaging performance, robustness and efficient series production of the Athena optics. A technology development plan was established and is being regularly updated to reflect the latest developments, and is fully funded by the ESA technology development programmes. An industrial consortium was formed to ensure coherence of the individual technology development activities. The SPO technology uses precision machined mirror plates produced using the latest generation top quality 12 inch silicon wafers, which are assembled into rugged stacks. The surfaces of the mirror plates and the integral support structure is such, that no glue is required to join the individual mirror plates. Once accurately aligned with respect to each other, the surfaces of the mirror plates merge in a physical bonding process. The resultant SPO mirror modules are therefore very accurate and stable and can sustain the harsh conditions encountered during launch and are able to tolerate the space environment expected during operations. The accommodation of the Athena telescope is also innovative, relying on a hexapod mechanism to align the optics to the selected detector instruments located in the focal plane. System studies are complemented by dedicated technology development activities to demonstrate the capabilities before the adoption of the Athena mission.

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The Athena x-ray optics development and accommodation

Herschel space observatory of ESA has a parabolic M1mirror of silicon carbide (SiC) with large diameter of 35 m, fast focal ratio of f/05 and extreme light-weighting to 25 kg/m 2 , which make the polishing of the mirror a very challenging task Use of very high surface pressure is necessary to polish efficiently this hard material, which increases the risk of quilting effects to the shape of the only 25 mm thick front face of the mirror In this paper we present descriptions of the testing and polishing methods used during Herschel M1 lapping and polishing to the specified surface shape accuracy < 15 μm rms and micro roughness <30 nm rms

Polishing and testing of the 3.5 m SiC M1 mirror of the Herschel space observatory of ESA

The optics of ATHENA (Advanced Telescope for High-ENergy Astrophysics) – the next high-energy astrophysical mission of the European Space Agency – consists of 678 Silicon Pore Optics mirror modules integrated and co-aligned onto a common supporting structure. The integration process, already proved, exploits an optical bench to capture the focal plane image of each mirror module when illuminated by an ultra-violet plane wave at 218 nm. Each mirror module focuses the collimated beam onto a CCD camera placed at the 12 m focal position of the ATHENA telescope and the acquired point spread function is processed in real time to calculate the centroid position and intensity parameters. This information is used to guide the robot-assisted alignment sequence of the mirror modules. To implement the above process for the entire ATHENA optics, a dedicated vertical optical bench is being designed. The facility consists of a paraboloid mirror that collects the light from an ultraviolet point source and generates a single reference plane wave large enough to illuminate the 2.6 m aperture of the X-ray telescope; at 12 m from the ATHENA optics (focal plane position) a tower will support the CCD camera, where the light from each mirror module is focused. The facility must also allow an alignment accuracy of 1 arcsec for the integration of two mirror modules per day in any arbitrary integration sequence, including the option of removing, re-aligning, or replacing any mirror module. The detailed design of the optical bench and the status of the construction activities are presented.

Integration facility for the ATHENA X-Ray Telescope

Several hundreds of Silicon Pore Optics (SPO) mirror modules will be integrated and co-aligned onto the ATHENA (Advanced Telescope for High-ENergy Astrophysics) Mirror Assembly Module (MAM). The selected integration process, developed by Media Lario, exploits a full size optical bench to capture the focal plane image of each mirror module when illuminated by an UV plane wavefront at 218 nm. Each mirror module, handled by a manipulator, focuses the collimated beam onto a CCD camera placed at the 12 m focal position of the ATHENA telescope. The image is processed in real time to calculate the centroid position and overlap it to the centroid of the already integrated Mirror modules. Media Lario has designed the ATHENA Assembly Integration and Testing facility to realize the integration process for the flight telescope and has started its construction. The facility consists of a vertical optical bench installed inside a tower with controlled cleanroom conditions. The MAM axis is aligned along gravity and supported on actuators to compensate for gravity deformations. A robot device above the MAM is used for aligning the SPO Mirror modules. The 2.6 m paraboloid mirror that collects the light emitted by a UV source is in final polishing. The alignment system, the cell support and the metrology system for the UV collimator have been qualified and accepted for installation. Details about the optical bench and the status of the facility construction will be presented.

Facility for alignment, assembly, and integration of the SPO mirror modules onto the ATHENA telescope

Polishing and testing methods used in the manufacture of the 3.4 m primary mirror of the Iranian National Observatory (INO) telescope are described and the test results of the finished mirror are presented. Mirror lapping and polishing was performed using several rectangular non-rotating tools arranged in a linear array across the mirror radius. Each tool is equipped with two computer controlled force actuators for regulating the surface pressure and removal efficiency during the lapping and polishing operations. The same tool system was used from the lapping phase to the end of the final polishing. The principal optical test method was the interferometric Hartmann test with the aid of a two component null lens in the mirror center of curvature. Mirror measurements were made also with pentaprism test to verify its correct conic constant. The mirror was finished to extremely good surface accuracy and smoothness.

Polishing and testing of the 3.4 m diameter f/1.5 primary mirror of the INO telescope

The Aladin instrument on the ADM-Aeolus satellite of ESA has a parabolic M1 mirror of silicon carbide (SiC) with diameter of 1.5 m, fast focal ratio of f/0.9 and light-weighting to 25 kg/m 2 . The lidar instrument is operated at the near ultraviolet wavelength of 355 nm, which requires high optical quality and good smoothness of the polishing finish. In this paper we present descriptions of the testing and polishing methods used during ALADIN M1 lapping and polishing to the specified wave front shape accuracy <150 nm rms and surface micro roughness < 5 nm rms.

Mikko Pasanen

Papers

Polishing and testing of the 3.5 m SiC M1 mirror of the Herschel space observatory of ESA

Integration facility for the ATHENA X-Ray Telescope

Facility for alignment, assembly, and integration of the SPO mirror modules onto the ATHENA telescope

Polishing and testing of the 3.4 m diameter f/1.5 primary mirror of the INO telescope

Polishing and testing of the 1.5 m SiC M1 mirror of the ALADIN instrument on the ADM-Aeolus satellite of ESA