The prelaunch specifications for nadir sensors of the Ozone Mapping and Profiler Suite (OMPS) were designed to ensure that measurements from them could be used to retrieve total column ozone and nadir ozone profile information both for operational use and for use in long-term ozone data records. In this paper, we will show results from our extensive analysis of the performance of the nadir mapper (NM) and nadir profiler (NP) sensors during the first year and a half of OMPS nadir operations. In most cases, we determined that both sensors meet or exceed their prelaunch specifications. Normalized radiance (radiance divided by irradiance) measurements have been determined to be well within their 2% specification for both sensors. In the case of stray light, the NM sensor is within its 2% specification for all but the shortest wavelengths, while the NP sensor is within its 2% specification for all but the longest wavelengths. Artifacts that negatively impacted the sensor calibration due to diffuser features were reduced to less than 1% through changes made in the solar calibration sequence. Preliminary analysis of the disagreement between measurements made by the NM and NP sensors in the region where their wavelengths overlap indicates that it is due to shifts in the shared dichroic filter after launch and that it can be corrected. In general, our analysis indicates that both the NM and NP sensors are performing well, that they are stable, and that any deviations from nominal performance can be well characterized and corrected.

/pdf/postlaunch-performance-of-the-suomi-national-polar-orbiting-26eajach1r.pdf

Postlaunch Performance of the Suomi National Polar-Orbiting Partnership Ozone Mapping and Profiler Suite (OMPS) Nadir Sensors

Refractive index variations in the earth's atmosphere cause wavefront aberrations and limit thereby the resolution in ground-based telescopes With Adaptive Optics (AO) the temporally and spatially varying wavefront distortions can be corrected in real time Most implementations in a ground based telescope include a WaveFront Sensor, a Deformable Mirror and a real time wavefront control system The largest optical telescopes built today have a ~ 1 Om primary mirror Telescopes with more collecting area and higher resolution are desired ELTs are currently designed with apertures up to 42m For these telescopes serious challenges for all parts of the AO system exist This thesis addresses the challenges for the DM An 8m class telescope on a representative astronomical site is the starting point The atmosphere is characterized by the spatial and temporal spectra of Kolmogorov turbulence and the frozen flow assumption The wavefront fitting error, caused by a limited number of actuators and the temporal error, caused by a limited control bandwidth, are the most important for the DM design It is shown that ~5000 actuators and 200Hz closed loop bandwidth form a balanced choice between the errors and correct an 8m wavefront in the visible to nearly diffraction limited An actuator stroke of ~56J!m and ~036J!m inter actuator stroke is thereby needed Together with the nm's resolution, low power dissipation, no hysteresis and drift, these form the main DM requirements The design, realization and tests of a new DM that meets these requirements and is extendable and scalable in mechanics, electronics and control to suit further Extremely Large Telescopes (ELTs) is presented In the DM a few layers are distinguished: a continuous mirror facesheet, the actuator grid and the base frame In the underlying layer - the actuator grid - low voltage electromagnetic push-pull actuators are located Identical actuator modules, each with 61 actuators, hexagonally arranged on a 6mm pitch can be placed adjacent to form large grids The base frame provides a stable and stiff reference A thin facesheet is needed for low actuator forces and power dissipation, whereby its lower limit is set by the facesheets inter actuator deflection determined by gravity or wind pressure For both scaling laws for force and dissipation are derived Minimum power dissipation is achieved when beryllium is used for the mirror facesheet Pyrex facesheets with 100J!m thickness are chosen as a good practical, alternative in the prototype development Struts (001 x 8mm) connect the facesheet to the actuators and ensure a smooth surface over the imposed heights and allow relative lateral movement of the facesheet and the actuator grid Measurements show 3nm RMS surface unflattness from the glued attachment The stiffness of the actuators form the out-of-plane constraints for the mirror facesheet and determine the mirrors first resonance frequency and is chosen such that the resonance frequency is high enough to allow the high control bandwidth but not higher that needed to avoid excessive power dissipation and fix points in the surface in case of failure The electromagnetic variable reluctance actuators designed, are efficient, have low moving mass and have suitable stiffness Other advantages are the low costs, low driving voltages and negligible hysteresis and drift The actuators consist of a closed magnetic circuit in which a PM provides static magnetic force on a ferromagnetic core that is suspended in a membrane This attraction force is increased of decreased by a current through a coil The actuators are free from mechanical hysteresis, friction and play and therefore have a high positioning resolution with high reproducibility The actuator modules are build in layers to reduces the number of parts and the complexity of assembly and to improve the uniformity in properties Dedicated communication and driver electronics are designed FPGA implemented PWM based voltage drivers are chosen because of their high efficiency and capability to be implemented in large numbers with only a few electronic components A multidrop LVDS based serial communication is chosen for its low power consumption, high bandwidth and consequently low latency, low communication overhead and extensive possibilities for customization A flat-cable connects up to 32 electronics modules to a custom communications bridge, which translates the ethernet packages from the control PC into LVDS Two DMs prototypes were successfully assembled: a 050mm DM with 61 actuators and a 0l50mm DM with 427 actuators In the second prototype modularity is shown by the assembly of seven identical grids on a common base The dynamic performance of each actuator is measured, including its dedicated driver and communication All actuators were found to be functional, indicating that the manufacturing and assembly process is reliable A nonlinear mathematical model of the actuator was derived describing both its static and dynamic behavior based on equations from the magnetic, mechanic and electric domains The actuator model was linearized, leading to expressions for the actuator transfer function and properties such as motor constant, coil inductance, actuator stiffness and resonance frequency From frequency response function measurements these properties showed slight deviations from the values derived from the model, but the statistical spread for the properties was small, stressing the reliability of the manufacturing and assembly process The mean actuator stiffness and resonance frequency were 047kN/m and 18kHz respectively, which is close to their design values of 500N/m and 19kHz The time domain response of an actuator to a 4Hz sine voltage was used to determine hysteresis and semi-static nonlinear response of the actuator This showed the first to be negligible and the second to remain below 5% for ±10J!m stroke Measurements showed that in the expected operating range, the total power dissipation is dominated by indirect losses in FPGAs The static DM performance is validated using interferometric measurements The measured influence matrix is used to shape the mirror facesheet into the first 28 Zernike modes, which includes the piston term that represents the best flat mirror The total RMS error is ~25nm for all modes The dynamic behavior of the DM is validated by measurements A laser vibrometer is used to measure the displacement of the mirror facesheet, while the actuators are driven by zero-mean, bandlimited, white noise voltage sequence Using the MOESP system identification algorithm, high-order black-box models are identified with VAF values around 95% The first resonance frequency identified is 725Hz, and lower than the 974Hz expected from the analytical model This is attributed to the variations in actuator properties, such as actuator stiffness The power dissipation in each actuator of the 050mm mirror to correct a typical Von Karmann turbulence spectrum is ~ 15m W

Adaptive deformable mirror : based on electromagnetic actuators

Model-Based Feedforward Control of Large Deformable Mirrors

Optical sparse-aperture systems can be designed to obtain a high resolution when imaging an astronomical object. These systems are the arrays synthesized by several small filled imaging systems. A principal experiment is set up for an optical sparse-aperture system by which imaging of an extended complex object is achieved. Image restoration of the direct output by the sparse-aperture system is performed by measuring the point-spread functions (PSF) and by using a Weiner filter. The correlation coefficient is proposed as a criterion to determine the optimal parameter and to evaluate the performance of the algorithms. The results show that among the three kinds of configuration models, the Tri-Arm array configuration can produce the highest resolution and largest correlation coefficient value after the restoration. Accordingly, the experimental data, demonstrate that an optical sparse-aperture system can achieve almost the same resolution and image quality as an equivalent filled system.

Experimental study on imaging and image restoration of optical sparse aperture systems

Adaptive optics (AO) is a technique to actively correct the wavefront distortions introduced in a light beam as it propagates through a turbulent medium. Nowadays, it is commonly applied in ground-based telescopes to counteract the devastating effect of atmospheric turbulence. This thesis focuses on the control aspects of AO. Whereas most AO systems use a simple control law derived from physical insights, this thesis addresses the AO control problem from a control engineering perspective. The main objective is to show that the performance of the current generation of AO systems can be improved by applying advanced control strategies that account for the AO system dynamics and the spatio-temporal correlation in the wavefront. To this end a data-driven optimal control is proposed. It consists of a dedicated subspace-identification algorithm to identify an atmospheric disturbance model from open-loop wavefront sensor data, followed by an optimal control design. By an efficient implementation, this approach can be used to design a full multi-variable controller for AO systems with up to a few hundred sensors and actuators, without assuming any form of decoupling. The data-driven optimal control strategy is thoroughly tested both in simulations and on an AO laboratory setup. The experiments show, that compared to the standard approach, optimal control is able to improve the wavefront suppression performance, especially under low light level conditions and rough turbulence. The gain in performance is explained the improved ability to anticipate future distortions when accounting for the spatio-temporal correlation in the wavefront.

/pdf/data-driven-optimal-control-for-adaptive-optics-e3e8tal4vh.pdf

Data-Driven Optimal Control for Adaptive Optics

Earth observation satellites are commonly equipped with an on-board diffuser Their use will be pointed out and the errors associated with their use will be explained The origin of the errors, the speckles formed by scattering on the diffusers, will be dealt with in detail and solutions to minimize these errors will be presented The use of diffusers, and the types of diffusers will be discussed in combination with their ability to minimise the contrast of the spectral features

Speckles and their effects in spectrometers due to on-board diffusers

With the future growing size of telescopes, new, high-resolution, affordable wavefront corrector technology with low power dissipation is needed. A new adaptive deformable mirror concept is presented, to meet such requirements. The adaptive mirror consists of a thin (30-50 μm), highly reflective, deformable membrane. An actuator grid with thousands of actuators is designed which push and pull at the membrane’s surface, free from pinning and piston effects. The membrane and the actuator grid are supported by an optimized light and stiff honeycomb sandwich structure. This mechanically stable and thermally insensitive support structure provides a stiff reference plane for the actuators. The design is extendable up to several hundreds of mm's. Low-voltage electro-magnetic actuators have been designed. These highly linear actuators can provide a stroke of 15 micrometers. The design allows for a stroke difference between adjacent actuators larger than 1 micron. The actuator grid has a layer-based design; these layers extend over a large numbers of actuators. The current actuator design allows for actuator pitches of 3 mm or more. Actuation is free from play, friction and mechanical hysteresis and therefore has a high positioning resolution and is highly repeatable. The lowest mechanical resonance frequency is in the range of kHz so a high control bandwidth can be achieved. The power dissipation in the actuator grid is in the order of milliwatts per actuator. Because of this low power dissipation active cooling is not required. A first prototype is currently being developed. Prototypes will be developed with increasing number of actuators.

Large adaptive deformable membrane mirror with high actuator density

A test-bench simulating the GAIA metrology system has been developed by TNO-TPD. The lines-of-sight of the two telescopes in the GAIA instrument are separated by an angle (called `basic angle') of about 106 degrees. The basic angle stability should be within 10 microarcsecond rms over the satellite revolution period of 3 hours, or should be at least known with this accuracy. The basic angle is monitored by a high-precision interferometric laser metrology system.

GAIA testbench: monitoring the basic angle with micro-arcsecond accuracy

A key technique for optical aperture synthesis instruments is the cophasing of the beams from the individual apertures. A cophasing system includes a number of delay lines for the adjustment of the optical path lengths. The design of a high-resolution delay line is based on the mission requirements of a space-borne interferometer. A cat's-eye type of retroreflector is the selected option for the optics. Three delay lines, including actuators and control electronics, have been built and have been tested in a dedicated setup, which includes a star simulator and a cophasing interferometer. The test results support the feasibility of the presented delay line concept.

Free-beam delay line for a multi-aperture optical space interferometer stabilized on a guide star

The Delft Testbed Interferometer (DTI) will be presented. The basics of homothetic mapping will be explained together with the method of fulfilling the requirements as chosen in the DTI setup. The optical layout incorporates a novel tracking concept enabling the use of homothetic mapping in real telescope systems for observations on the sky. The requirements for homothetic mapping and the choices made in the DTI setup will be discussed. Finally the first results and the planned experiments will be presented.

/pdf/homothetic-mapping-as-means-to-obtain-a-wide-field-of-view-25opta59id.pdf

Bart Snijders

Papers

Speckles and their effects in spectrometers due to on-board diffusers

Large adaptive deformable membrane mirror with high actuator density

GAIA testbench: monitoring the basic angle with micro-arcsecond accuracy

Free-beam delay line for a multi-aperture optical space interferometer stabilized on a guide star

Homothetic mapping as means to obtain a wide field of view: the Delft Testbed Interferometer