Stephen Rolt

Modelling and optimization of novel laser multilateration schemes for high-precision applications

A computer simulation technique has been developed to investigate the performance of established and novel laser multilateration schemes. The models focus on analysing the impact of variations in system configuration, optimizing the system self-calibration process and evaluating the volumetric measurement error propagation. Careful optimization of the system configuration and self-calibration planning significantly reduces the uncertainty of system self-calibration and measurement. Comparing the simulated performances of multilateration systems with different number of measurement stations, e.g. four, five and six stations, quantifies the performance improvement obtained by increasing the number of stations. Specific recommendations for optimization of multilateration set-ups and measurement plans and for minimizing measurement uncertainty are set out in the paper. A novel, sequential multilateration set-up for high-precision calibration of small artefacts has been defined, and measurements have been made to support the analysis presented here.

Status of the KMOS multi-object near-infrared integral field spectrograph

KMOS is a multi-object near-infrared integral field spectrograph being built by a consortium of UK and German institutes. We report on the final integration and test phases of KMOS, and its performance verification, prior to commissioning on the ESO VLT later this year.

EAGLE: an MOAO fed multi-IFU working in the NIR on the E-ELT

EAGLE is an instrument for the European Extremely Large Telescope (E-ELT). EAGLE will be installed at the Gravity Invariant Focal Station of the E-ELT, covering a field of view of 50 square arcminutes. Its main scientific drivers are the physics and evolution of high-redshift galaxies, the detection and characterization of first-light objects and the physics of galaxy evolution from stellar archaeology. These key science programs, generic to all ELT projects and highly complementary to JWST, require 3D spectroscopy on a limited (~20) number of targets, full near IR coverage up to 2.4 micron and an image quality significantly sharper than the atmospheric seeing. The EAGLE design achieves these requirements with innovative, yet simple, solutions and technologies already available or under the final stages of development. EAGLE relies on Multi-Object Adaptive Optics (MOAO) which is being demonstrated in the laboratory and on sky. This paper provides a summary of the phase A study instrument design.

Management of optical interfaces in the VLT KMOS instrument

The heart of the KMOS instrument is a complex optical system with over 300 separate optical paths. The optical design is spread between 4 sub-systems which have been designed at three different institutions. In order that the end to end performance of the final design can be monitored and controlled it is necessary to specify the performance and interface requirements of each sub-system clearly. This paper describes the parameters that were necessary to control so that the sub-system designs could be carried out independently while maintaining visibility and control of the end to end performance. The method of apportioning the budgets between the sub-systems and the modeling performed to verify compliance is also described.

The integral field unit for the James Webb space telescope's near-infrared spectrograph.

The European Space Agency (ESA) is providing the Near Infrared Spectrograph (NIRSpec) developed by EADS Astrium GmbH to fly on the James Webb Space Telescope (JWST). NIRSpec covers the 0.6-5.0 µm domain. It will be primarily operated as a multi-object spectrograph, using a MEMS micro-shutter array (MSA) provided by NASA to select multiple objects from the field of view at an intermediate image plane formed by the NIRSpec fore-optics. The MSA apertures form multiple entrance slits of the spectrometer section.

The KMOS Integral Field System: fabrication, alignment, and test of 1000+ optical surfaces

The Centre for Advanced Instrumentation (CfAI) of Durham University (UK) has recently successfully completed the development of 24 Integral Field Units (IFUs) for the K-band Multi-Object Spectrometer (KMOS). KMOS is a second generation instrument for ESO’s Very Large Telescope (VLT) which is due for delivery during the summer of 2012. The KMOS IFU is based on the Advanced Image Slicer Concept developed by the CfAI and previously successfully implemented on the Gemini Near-InfraRed Spectrograph and JWST NIRSpec. Each IFU contains 14 channels which have to be accurately aligned. In addition, all 24 IFUs have to be co-aligned requiring the accurate alignment of an unprecedented grand total of 1152 optical surfaces. In this paper we describe how this has been achieved through the use of complex monolithic multi-faceted metal mirror arrays, which were fabricated in-house by means of freeform diamond machining. We will summarise the results from the metrology performed on each of the optical components and describe how these were integrated and aligned into the system. We will also summarise the results from the system level acceptance tests, which demonstrate the excellent performance of the IFUs. Each of the 24 IFUs is essentially diffraction limited across the entire field (Strehl ratios ~ 0.8) with throughput predictions (based on measurements of the surface roughness) rising from 86% at a wavelength of 1 micron to 93% at 2.5 micron. We believe that this level of performance has not previously been achieved in any image slicing IFU and showcases the potential of the current state-of-the-art technology.

DRAGON: a wide-field multipurpose real time adaptive optics test bench

DRAGON is be a new and in many ways unique visible light adaptive optics test bench. Initially, it will test new and existing concepts for CANARY, the laser guide star tomographic adaptive optics demonstrator on the WHT, then later it will be used to explore concepts for other existing and future telescopes. Natural and Laser Guide Stars (NGS and LGS) are emulated, where the LGSs exhibit the effects of passing up through turbulence and spot elongation. AO correction is performed by one high and one low order deformable mirror, allowing woofer-tweeter control, and multiple high and low order wave front sensors detect wave front error. The Durham Adaptive Optics Real-time Controller (DARC) is used to provide real-time control over various DRAGON configurations. DRAGON is currently under construction, with the turbulence simulator completed. Construction and alignment of the system is expected to be finished in the coming year, though first results from completed modules follow sooner.

Diamond machining of aspherical mirrors and mirror arrays for Integral Field Units: part II. Metrology

The application of freeform diamond machining techniques to the manufacture of precision optics for astronomical instrumentation has opened up a wide range of exciting new design possibilities. Freeform surfaces, not limited by symmetry considerations, and complex multi-faceted mirror arrays can be precisely machined. This capability has removed many constraints from the design process and has enabled the fulfilment of radical instrument designs. However, this flexibility poses a significant challenge for component characterisation. Accurate measurement of the form accuracy of surfaces lacking symmetry is particularly difficult using standard interferometric techniques. Furthermore, the accurate 3 dimensional characterisation of complex multi-faceted components presents an added challenge. The authors describe techniques that have been applied to the measurement of complex surfaces in Integral Field Units (IFUs). In particular, we present novel approaches for the measurement of the form of complex non-radially symmetric surfaces to an accuracy of a few nanometres. These measurements use a Twyman-Green Interferometer configured in a variety of non-standard arrangements. In addition, the authors consider the difficulties in the measurement of multifaceted surfaces, and the accurate determination of the geometric relations between these surfaces. The authors discuss the use of confocal gauge techniques in the characterisation of these surfaces.

Performance of the K-band multi-object spectrograph (KMOS) on the ESO VLT

KMOS is a multi-object near-infrared integral field spectrograph built by a consortium of UK and German institutes for the ESO Paranal Observatory. We report on the on-sky performance verification of KMOS measured during three commissioning runs on the ESO VLT in 2012/13 and some of the early science results.

Calibration of the KMOS Multi-Field Imaging Spectrometer

When it is delivered to the VLT KMOS, will be the first near infrared (0.8–2.5 μm) spectrograph to use multiple integral fields units. Pick-off arms provide the optical relay to image objects selected from the patrol field at the Nasmyth focus onto 24 image slicing IFUs. The output from 8 IFUs forms a single slit which feeds one of three identical spectrometers. The calibration of such an instrument presents a number of challenges. Flat field and wavelength calibration for the instrument will be provided by an internal calibration system of unique design. External light sources are fed via integrating spheres and high transmission light pipes to a calibration sphere with 24 output ports addressed by the 24 arms. Obtaining a high degree of field flatness is key to the sensitivity of NIR instruments. The KMOS calunit is predicted to deliver flatnesses of ∼0.1% over the KMOS IFU fields.