Steffen Rost

The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity

We present an estimate of the performance that will be achieved during on orbit operations of the JWST Mid Infrared Instrument, MIRI. The efficiency of the main imager and spectrometer systems in detecting photons from an astronomical target are presented, based on measurements at sub-system and instrument level testing, with the end-to-end transmission budget discussed in some detail. The brightest target fluxes that can be measured without saturating the detectors are provided. The sensitivity for long duration observations of faint sources is presented in terms of the target flux required to achieve a signal to noise ratio of 10 after a 10,000 second observation. The algorithms used in the sensitivity model are presented, including the understanding gained during testing of the MIRI Flight Model and flight-like detectors.

Vibration measurements at the Large Binocular Telescope (LBT)

The Large Binocular Telescope (LBT) is an international collaboration, with partners from the United States, Italy, and Germany. The telescope uses two 8.4-meter diameter primary mirrors to produce coherent images with the combined light along with adaptive optics. The correct functioning and optimum performance of the LBT is only achieved through a complex interplay of various optical elements. Each of these elements has its individual vibration behaviour, and therefore it is necessary to characterize the LBT as a distributed vibration system. LINC-NIRVANA is a near-infrared image-plane beam combiner with advanced, multi-conjugated adaptive optics, and one of the interferometric instruments for the Large Binocular Telescope (LBT). Its spectral range goes from 1.0 μm to 2.45 μm, therefore the requirements for the maximum optical path difference (OPD) are very tight (λ/10 ~ 100 nm). 1 During two dedicated campaigns, the vibrations introduced by various actuators were measured using different kinds of sensors. The evaluation of the obtained data allows an estimation of the frequency and amplitude contributions of the individual vibration sources. Until the final state of the LBT is reached, further measurements are necessary to optimize and adapt the equipment and also the investigated elements and configurations (measurement points and directions, number of sensors, etc.).

LINC-NIRVANA piston control elements

We review the status of hardware developments related to the Linc-Nirvana optical path difference (OPD) control. The status of our telescope vibration measurements is given. We present the design concept of a feed-forward loop to damp the impact of telescope mirror vibrations on the OPD seen by Linc-Nirvana. At the focus of the article is a description of the actuator of the OPD control loop. The weight and vibration optimized construction of this actuator (aka piston mirror) and its mount has a complex dynamical behavior, which prevents classical PI feedback control from delivering fast and precise motion of the mirror surface. Therefore, an H-; optimized control strategy will be applied, custom designed for the piston mirror. The effort of realizing a custom controller on a DSP to drive the piezo is balanced by the outlook of achieving more than 5x faster servo bandwidths. The laboratory set-up to identify the system, and verify the closed loop control performance is presented. Our goal is to achieve 30 Hz closed-loop control bandwidth at a precision of 30 nm.

The LINC-NIRVANA Fringe and Flexure Tracking System

LINC-NIRVANA is the near-infrared homothetic imaging camera for the Large Binocular Telescope. Once operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. Its Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA. It is tailored to compensate low-order phase perturbations in real-time to allow for a time-stable interference pattern in the focal plane of the science camera during the integration. Two independent control loops are realized within FFTS: A cophasing loop continuously monitors and corrects for atmospheric and instrumental differential piston between the two arms of the interferometer. A second loop controls common and differential image motion resulting from changing orientations of the two optical axes of the interferometer. Such changes are caused by flexure but also by atmospheric dispersion. Both loops obtain their input signals from different quadrants of a NIR focal plane array. A piezo-driven piston mirror in front of the beam combining optics serves as actuator in the cophasing loop. Differential piston is determined by fitting a parameterized analytical model to the observed point spread function of a reference target. Tip-tilt corrections in the flexure loop are applied via the secondary mirrors. Image motion is sensed for each optical axis individually in out-of-focus images of the same reference target. In this contribution we present the principles of operation, the latest changes in the opto-mechanical design, the current status of the hardware development.

The LINC-NIRVANA fringe and flexure tracker: image analysis concept and fringe tracking performance estimate

The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in real-time. In the fringe tracking concept that we present, differential piston information is gathered in the image plane by analyzing the PSF of a reference star anywhere in the large field of view of the LBT. We have developed and tested a fast PSF analysis algorithm that allows to clearly identify differential piston even in the case of low S/N. We present performance estimates of the algorithm. Since the performance of the FFTS algorithm has a strong impact on the overall sky coverage of LINC-NIRVANA, we studied the required limiting magnitudes of the fringe tracking reference star for different scenarios. As the FFTS may not necessarily operate on the science target, but rather uses a suitable reference star at a certain angular distance to the science target, differences between piston values at the two positions add to the residual piston of the FFTS. We have dealt with the question of differential piston angular anisoplanatism and studied a possible improvement of the isopistonic patch size by the use of multi-conjugate adaptive optics (MCAO). In its final stage, LINC-NIRVANA will be equipped with such a system.

The LINC-NIRVANA Fringe and Flexure Tracker: Testing Piston Control Performance

LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA: the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera. Differential piston effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of 100Hz-200Hz. A dedicated piston mirror will then be moved in a corresponding manner by a piezo actuator. The long-term flexure tip/tilt variations will be compensated by the AO deformable mirrors. A testbed interferometer has been designed to simulate the control process of the movement of a scaled piston mirror under disturbances. Telescope vibration and atmospheric variations with arbitrary power spectra are induced into the optical path by a dedicated piezo actuator. Limiting factors of the control bandwith are the sampling frequency and delay of the detector and the resonance frequency of the piston mirror. In our setup we can test the control performance under realistic conditions by considering the real piston mirrors dynamics with an appropriate software filter and inducing a artificial delay of the PSF detector signal. Together with the expected atmospheric OPD variations and a realistic vibration spectrum we are able to quantify the piston control performance for typical observation conditions. A robust control approach is presented as result from in-system control design as provided by the testbed interferometer with simulated dynamics.

The LINC-NIRVANA Fringe and Flexure Tracker: Cryo-Ambient Mechanical Design

The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in a real-time closed loop. Being a Fizeau-Interferometer, the LBT provides a large field of view (FoV). The FFTS can make use of the large FoV and increase the sky coverage of the overall instrument if it is able to acquire the light of a suitable fringe tracking reference star within the FoV. For this purpose, the FFTS detector needs to be moved to the position of the reference star PSF in the curved focal plane and needs to precisely follow its trajectory as the field rotates. Sub-pixel (1 pixel = 18.5 micron) positioning accuracy is required over a travel range of 200mm x 300mm x 70mm. Strong are the constraints imposed by the need of a cryogenic environment for the moving detector. We present a mechanical design, in which the Detector Positioning Unit (DPU) is realized with off-the-shelf micro-positioning stages, which can be kept at ambient temperature. A moving baffle will prevent the intrusion of radiation from the ambient temperature environment into the cryogenic interior of the camera. This baffle consists of two nested disks, which synchronously follow any derotation - or repositioning trajectory of the DPU. The detector, its fanout board and a filter wheel are integrated into a housing that is mounted on top of the DPU and that protects the FFTS detector from stray light. Long and flexible copper bands allow heat transfer from the housing to the LINC-NIRVANA heat exchanger.

The LINC-NIRVANA fringe and flexure tracker: first measurements of the testbed interferometer

LINC-NIRVANA is the near-infrared Fizeau interferometric imaging camera for the Large Binocular Telescope (LBT). For an efficient interferometric operation of LINC-NIRVANA the Fringe and Flexure Tracking System (FFTS) is mandatory: It is a real-time servo system that allows to compensate atmospheric and instrumental optical pathlength differences (OPD). The thereby produced time-stable interference pattern at the position of the science detector enables long integration times at interferometric angular resolutions. As the development of the FFTS includes tests of control software and robustness of the fringe tracking concept in a realistic physical system a testbed interferometer is set up as laboratory experiment. This setup allows us to generate point-spread functions (PSF) similar to the interferometric PSF of the LBT via a monochromatic (He-Ne laser) or a polychromatic light source (halogen lamp) and to introduce well defined, fast varying phase offsets to simulate different atmospheric conditions and sources of instrumental OPD variations via dedicated actuators. Furthermore it comprises a piston mirror as actuator to counteract the measured OPD and a CCD camera in the focal plane as sensor for fringe acquisition which both are substantial devices for a fringe tracking servo loop. The goal of the setup is to test the performance and stability of different control loop algorithms and to design and optimize the control approaches. We present the design and the realization of the testbed interferometer and comment on the fringe-contrast behavior.

The LINC-NIRVANA fringe and flexure tracker: an update of the opto-mechanical system

LINC-NIRVANA (LN) is a German/Italian interferometric beam combiner camera for the Large Binocular Telescope. Due to homothetic imaging, LN will make use of an exceptionally large field-of-view. As part of LN, the Fringe-and-Flexure-Tracker system (FFTS) will provide real-time, closed-loop measurement and correction of pistonic and flexure signals induced by the atmosphere and inside the telescope-instrument system. Such compensation is essential for achieving coherent light combination over substantial time intervals (~ 10min.). The FFTS is composed of a dedicated near-infrared detector, which can be positioned by three linear stages within the curved focal plane of LN. The system is divided into a cryogenic (detector) and ambient (linear stages) temperature environment, which are isolated from each other by a moving baffle. We give an overview of the current design and implementation stage of the FFTS opto-mechanical and electronic components. We present recent important updates of the system, including the development of separated channels for the tracking of piston and flexure. Furthermore, the inclusion of dispersive elements will allow for the correction of atmospheric differential refraction, as well as the induction of artificial dispersion to better exploit the observational-conditions parameter space (air mass, brightness).

Near-infrared polarization images of the Orion proplyds

Aims. The aim is to study structure and polarization properties of the stars and planet systems in the active Orion H II-region. Methods. We performed AO-assisted high-resolution imaging polarimetry on selected Orion proplyds close to the Trapezium stars in the J, H, and K bands. Differential polarimetric images of one of the largest and brightest proplyds are interpreted using 3D radiation transfer simulations based on the Monte Carlo method. Results. Although not fully resolvable by ground-based observations, the circumstellar material can be mapped with polarimetry. We present constraints on the disk parameters of the giant proplyd 177-341. We tested whether dust models with different grain size distributions could explain the observed extent of the polarization patterns and find that simple models with larger grains will not reproduce the spectral polarization behavior. Conclusions. The technique of polarimetric differential imaging (PDI) in the NIR provides a good opportunity to study the structure of the Orion proplyds.