Urban Bitenc

Experience with wavefront sensor and deformable mirror interfaces for wide-field adaptive optics systems

Recent advances in adaptive optics (AO) have led to the implementation of wide field-of-view AO systems. A number of wide-field AO systems are also planned for the forthcoming Extremely Large Telescopes. Such systems have multiple wavefront sensors of different types, and usually multiple deformable mirrors (DMs). Here, we report on our experience integrating cameras and DMs with the real-time control systems of two wide-field AO systems. These are CANARY, which has been operating on-sky since 2010, and DRAGON, which is a laboratory AO real-time demonstrator instrument. We detail the issues and difficulties that arose, along with the solutions we developed. We also provide recommendations for consideration when developing future wide-field AO systems.

Assessing the stability of an ALPAO deformable mirror for feed-forward operation

A deformable mirror (DM) is a mirror whose surface can be deformed in order to correct for optical aberrations. If a DM is used in a feed-forward operation (i.e. without feed-back, also known as open-loop) it is, among other requirements, crucial that a set of actuator commands repeatedly results in the same surface shape. We have tested an ALPAO DM against this criterion, by repeatedly applying a set of actuator commands over hours and monitoring the DM shape with an interferometer. We found that if the surface shape was held to shape A for several hours, then changed to a second shape, ℬ, the DM surface will drift from this new shape over the course of several hours. During this period the root-mean-square (RMS) of the deviation from shape ℬ can exceed 30% of the RMS of the difference between shapes A and ℬ. This can correspond to a surface deviation with RMS of several hundred nanometers, and would severely impact the resulting performance of an AO system using such a DM in a feed-forward operation. We have developed a model to correct for the time-varying surface shape in software by continuously adapting the actuator commands over the stabilization period. Application of the stabilisation procedure allows the surface to remain stable to within 4 nm RMS after a period of 6 minutes. We also provide a suggestion on how to improve the repeatability of surface response to different sets of actuator commands, which can be affected by the surface drift.

CANARY phase B: on-sky open-loop tomographic LGS AO results

CANARY is an on-sky Laser Guide Star (LGS) tomographic AO demonstrator that has been in operation at the 4.2m William Herschel Telescope (WHT) in La Palma since 2010. In 2013, CANARY was upgraded from its initial configuration that used three off-axis Natural Guide Stars (NGS) through the inclusion of four off-axis Rayleigh LGS and associated wavefront sensing system. Here we present the system and analysis of the on-sky results obtained at the WHT between May and September 2014. Finally we present results from the final ‘Phase C’ CANARY system that aims to recreate the tomographic configuration to emulate the expected tomographic AO configuration of both the AOF at the VLT and E-ELT.

Green FLASH: energy efficient real-time control for AO

The main goal of Green Flash is to design and build a prototype for a Real-Time Controller (RTC) targeting the European Extremely Large Telescope (E-ELT) Adaptive Optics (AO) instrumentation. The E-ELT is a 39m diameter telescope to see first light in the early 2020s. To build this critical component of the telescope operations, the astronomical community is facing technical challenges, emerging from the combination of high data transfer bandwidth, low latency and high throughput requirements, similar to the identified critical barriers on the road to Exascale. With Green Flash, we will propose technical solutions, assess these enabling technologies through prototyping and assemble a full scale demonstrator to be validated with a simulator and tested on sky. With this R&D program we aim at feeding the E-ELT AO systems preliminary design studies, led by the selected first-light instruments consortia, with technological validations supporting the designs of their RTC modules. Our strategy is based on a strong interaction between academic and industrial partners. Components specifications and system requirements are derived from the AO application. Industrial partners lead the development of enabling technologies aiming at innovative tailored solutions with potential wide application range. The academic partners provide the missing links in the ecosystem, targeting their application with mainstream solutions. This increases both the value and market opportunities of the developed products. A prototype harboring all the features is used to assess the performance. It also provides the proof of concept for a resilient modular solution to equip a large scale European scientific facility, while containing the development cost by providing opportunities for return on investment.

Software compensation method for achieving high stability of Alpao deformable mirrors

Deformable mirrors (DMs) are used in adaptive optics for correcting optical aberrations: the DM surface can be deformed to compensate for them. Recently we reported the results on investigation of the stability of Alpao DMs, i.e. how accurately a DM surface shape can be maintained over minutes and hours without any optical feed-back. We observed a creep behavior of the DM surface and we presented a proof-of-concept software compensation for it, showing that very high stability is achievable. In this paper we develop a generalized creep compensation method that covers a wide range of DM use-cases and compensates for 90% - 95% of the creep observed. Furthermore, we report an observation of a DM shape dependence on the magnitude of the DM steering commands over the last few minutes. This effect is likely due to the warming up of the structure supporting the DM surface. Similarly as for creep, we have developed a compensation in software which corrects for about 90% of this effect. Both compensation mechanisms are based solely on pre-calibration input and do not receive any optical feedback about the actual DM surface shape. With the application of these two compensation mechanisms, the Alpao DM exhibits excellent stability and is well suited for feed-forward operation, where high reliability of the DM surface is crucial for operation in the absence of an optical feedback.

Novel algorithm implementations in DARC: the Durham AO real-time controller

The Durham AO Real-time Controller has been used on-sky with the CANARY AO demonstrator instrument since 2010, and is also used to provide control for several AO test-benches, including DRAGON. Over this period, many new real-time algorithms have been developed, implemented and demonstrated, leading to performance improvements for CANARY. Additionally, the computational performance of this real-time system has continued to improve. Here, we provide details about recent updates and changes made to DARC, and the relevance of these updates, including new algorithms, to forthcoming AO systems. We present the computational performance of DARC when used on different hardware platforms, including hardware accelerators, and determine the relevance and potential for ELT scale systems. Recent updates to DARC have included algorithms to handle elongated laser guide star images, including correlation wavefront sensing, with options to automatically update references during AO loop operation. Additionally, sub-aperture masking options have been developed to increase signal to noise ratio when operating with non-symmetrical wavefront sensor images. The development of end-user tools has progressed with new options for configuration and control of the system. New wavefront sensor camera models and DM models have been integrated with the system, increasing the number of possible hardware configurations available, and a fully open-source AO system is now a reality, including drivers necessary for commercial cameras and DMs. The computational performance of DARC makes it suitable for ELT scale systems when implemented on suitable hardware. We present tests made on different hardware platforms, along with the strategies taken to optimise DARC for these systems.

Recent improvements of high density magnetic deformable mirrors: faster, larger and stronger

The performances of high resolution magnetic deformable mirrors have been recently improved: the mechanical bandwidth has been increased to 2 kHz, and a fast stroboscopic Shack-Harman wavefront was used to measure a settling time as low as 400μs. Recent improvements in the substrate-thinning processes made possible the availability of large, high-quality membranes compatible with deformable mirrors. Prototype testing and simulations show that devices with up to 60x60 actuators are now possible. For open-loop operations, a novel feed-forward algorithm was developed to compensate for residual creeping and improve the DM stability to below10nm RMS over 6 hours.

Real-time control for the high order, wide field DRAGON AO test bench

DRAGON is a high order, wide field AO test-bench at Durham. A key feature of DRAGON is the ability to be operated at real-time rates, i.e. frame rates of up to 1kHz, with low latency to maintain AO performance. Here, we will present the real-time control architecture for DRAGON, which includes two deformable mirrors, eight wavefront sensors and thousands of Shack-Hartmann sub-apertures. A novel approach has been taken to allow access to the wavefront sensor pixel stream, reducing latency and peak computational load, and this technique can be implemented for other similar wavefront sensor cameras with no hardware costs. We report on experience with an ELT-suitable wavefront sensor camera. DRAGON will form the basis for investigations into hardware acceleration architectures for AO real-time control, and recent work on GPU and many-core systems (including the Xeon Phi) will be reported. Additionally, the modular structure of DRAGON, its remote control capabilities, distribution of AO telemetry data, and the software concepts and architecture will be reported. Techniques used in DRAGON for pixel processing, slope calculation and wavefront reconstruction will be presented. This will include methods to handle changes in CN2 profile and sodium layer profile, both of which can be modelled in DRAGON. DRAGON software simulation techniques linking hardware-in-the-loop computer models to the DRAGON real-time system and control software will also be discussed. This tool allows testing of the DRAGON system without requiring physical hardware and serves as a test-bed for ELT integration and verification techniques.

Wide field of view wavefront sensor for active optics correction chain for future space telescopes

The continuous strive for increased sensitiv ity and higher resolution of space based telescopes can only be satisfied with larger primary mirrors. There are quite a few challenges in launching large mirrors in space such as surviving the stress created from the launch acceleration, deployment, thermoelastic deformations, the gravity release etc. Major constraint to space based application is weight which drives the development of thin, extremely lightweight mirrors. Such mirrors are prone for stress based deformations and need active optics correction chain (AOCC) in order to be operated at their full potential. An AOCC for large monolithic mirrors consists of three key active optics components: corrective element (e.g. deformable mirror or DM), wavefront sensor (WFS) and correction algorithm. In order to assess the feasibility of such a system we have developed an AOCC test stand in a collaboration with the European Space Agency (ESA) a nd Netherlands Organisation for Applied Scientific Research (TNO). With this development we aim to measure the performance and the long-term reliability of an AOCC in controlled laboratory conditions. Our design consists of two separate parts, one where the expected aberrations are generated and another where they are measured and corrected. Two deformable mirrors of 37.5 mm and 116 mm are used, the smallest mirror to generate aberrations and the largest to correct them. For wavefront sensing we are using two different wavefront sensors, an 11x11 Shack-Hartmann as well as phase diversity based at the science sensor. We are able to emulate the conditions for both, astronomy related, and Earth observations. Here, we present the design of the system, including the test stand and the correction algorithms, the performance expected from simulations, and the results from the latest lab tests.

Suitability of GPUs for real-time control of large astronomical adaptive optics instruments

Adaptive optics (AO) is a technique for correcting aberrations introduced when light propagates through a medium, for example, the light from stars propagating through the turbulent atmosphere. The components of an AO instrument are: (1) a camera to record the aberrations, (2) a corrective mechanism to correct them, (3) a real-time controller (RTC) that processes the camera images and steers the corrective mechanism on milliseconds timescales. We have accelerated the image processing for the AO RTC with the use of graphics processing units (GPUs). It is crucial that the image is processed before the atmospheric turbulence has changed, i.e., in one or two milliseconds. The main task is to transfer the images to the GPU memory with a minimum delay. The key result of this paper is a demonstration that this can be done fast enough using commercial frame grabbers and standard CUDA tools. Our benchmarking image consists of