Wavefront Sensing within the VISTA Infrared Camera Paul Clark*a, Paul Berry a, Richard G. Binghamb, Nirmal Bissonautha, Martin Caldwellc, Nigel A. Dipper a, Colin N. Dunlopa, David M. Henryd, Peter Lukea, Richard M. Myersa, David J. Robertsona a University of Durham, Astronomical Instrumentation Group, UK; b Optical Design Service, UK; c Rutherford Appleton Laboratory, Space Science and Technology Department, UK; d UK Astronomy Technology Centre, Royal Observatory Edinburgh, UK ABSTRACT VISTA is a 4-metre survey telescope currently being constructed on the NTT peak of ESO’s Cerro Paranal Observatory. The telescope will be equipped with a dedicated infrared camera providing images of a 1.65 degree field of view. The telescope and camera are of an innovative f/3.26 design with no intermediate focus and no cold stop. The mosaic of 16 IR detectors is located directly at Cassegrain focus and a novel baffle arrangement is used to suppress stray light within the cryostat. The pointing and alignment of the telescope and camera is monitored by wavefront sensing elements within the camera cryostat itself. This paper describes the optical, mechanical, electronic and thermal design of the combined curvature sensor and auto-guider units positioned at the periphery of the camera field of view. Centroid and image aberration data is provided to the telescope control system allowing real time correction of pointing and alignment of the actively positioned M2 unit. Also described are the custom optics, mounted in the camera filter wheel, which are used to perform near on-axis high order curvature sensing. Analysis of the corresponding defocused images allows calibration tables of M1 actuator positions to be constructed for varying telescope declination and temperature. Keywords: VISTA, infrared camera, curvature sensing, auto-guiding, deep depletion, CCD 1 INTRODUCTION 1,2,3 The Visible and Infrared Survey Telescope for Astronomy (VISTA) is a 4 metre wide-field telescope that is purpose-designed for deep astronomical imaging surveys in the near-infrared and visible. The telescope design caters for interchangeable instruments mounted at the Cassegrain focus with the assumption that each instrument will provide its own purpose-specific wide-field correcting optics. VISTA will be located on NTT Peak in Chile and will be operated by ESO as part of the Cerro Paranal Observatory. 4 The IR Camera will be the first instrument operating on VISTA, providing a 1.65 degree field of view imaged by a mosaic of 16 Raytheon VIRGO 2048×2048 HgCdTe detectors. VISTA will be operated initially as a single-instrument telescope, and so the IR Camera is designed for continuous operation between scheduled yearly downtimes for essential maintenance. The telescope and camera are of an innovative f/3.26 design with no intermediate focus or cold stop. The mosaic of IR detectors is located directly at Cassegrain focus and a novel cold baffle arrangement is used to suppress stray light within the cryostat. The pointing and alignment of the telescope is continuously monitored by combined auto-guider and curvature sensor units located within the camera cryostat above the filter wheel, fed via pick-off mirrors with light from the periphery of the field of view. The auto-guider and curvature sensors units are non-deployable but have a sufficiently large field of view such that there is a 99% probability of suitable guide and reference stars being available for any desired * firstname.lastname@example.org; phone +44 191 3343562; fax +44 191 3343609; http://aig-www.dur.ac.uk observation. The use of cryogenic positioning mechanisms has thus been avoided removing possible doubt about the reliability of such mechanisms over the 25-year life of the instrument. To avoid the use of additional infrared detectors, the auto-guider and curvature sensor units utilize custom deep depletion, frame transfer wired, CCDs, a variant of the E2V CCD42-40, providing high QE and low fringing at wavelengths centered on 800nm. The CCDs will be supplied without a metal-layer storage mask allowing the same detector to be used for both full-frame curvature sensing and frame-transfer auto-guiding with a separate shield positioned over the storage half of the detector surface. Novel beam splitting optics are also included in the normally-unused sections of the camera filter wheel allowing the science detectors themselves to be used for off-line high order curvature sensing analysis of the telescope alignment. Window Cold Baffle LN2 Tank Lens Barrel Cold Head Filter Wheel Auto-guider / Curvature Sensor Unit (1/2) Focal Plane Array Figure 1: Cross section view of the IR Camera 2 OVERVIEW The key components of the Wavefront Sensor system, within the IR Camera cryostat, are: • Two identical combined Low Order Curvature Sensor (LOCS) / Auto-guider (AG) Units, subsequently referred to as LOCS/AG Units, positioned above the Filter Wheel, on opposite sides of the field of view, each containing: o A pickoff mirror, to divert light into the unit o A filter to limit the wavelengths used by the unit to 720-920nm (I-band) and also attenuate any science band wavelengths reflected back out into the IR Camera o A cube beamsplitter to divide light between the pair of curvature sensor CCDs and reflect light to the auto-guider CCD o Two 2Kx2K curvature sensor CCDs o One 2Kx1K frame-transfer auto-guider CCD o A PCB containing CCD buffer and protection circuitry o A mechanical assembly o CCD heating resistors o Temperature sensing thermistors • Two flexible circuit wiring harnesses to connect the LOCS/AG units to hermetic connectors on a cryostat port • The beam-splitting optic components of the High Order Curvature Sensor (HOCS), housed in an intermediate sections of the filter wheel, placing pre- and post-focus images of a single star simultaneously on one of the science detectors External to the Camera cryostat are: • Four ESO Technical CCD Controllers (mounted on the camera) • 24V Power Supply (mounted on the camera) • Fibre optic cables • A split-backplane VME Rack containing four LCU processor cards (Motorola MVME) with PMC fibre interface cards and ESO TIM cards (elsewhere in the telescope enclosure) • A guide workstation (in the VLT control room) • A wealth of purpose-written, ESO compliant software Figure 2 shows the location of the two LOCS/AG units within the camera cryostat. LN2 Tank (Outline) Cryostat Port With Hermetic Connectors LOCS/AG Unit (+Y) Flexi Harness (+Y) Filter Tray LOCS/AG Unit (-Y) Filter Wheel Figure 2: Location of the two LOCS/AG Units 3 OPTICAL DESIGN The optical design of the LOCS/AG units is constrained by a number of performance requirements, derived from the overall technical specification for the camera itself, plus additional constraints imposed by the design of the adjacent parts of the cryostat. Some of the key design features are: • The use of pairs of 2Kx2K 13.5µm CCDs for curvature sensing, each sensor having an unvignetted area on sky of ~60 arcmin2 allowing a 99% probability of a suitable reference star being available for any desired observation in the region of the galactic pole at full moon with a 30s exposure time, providing a signal to noise ratio of 150. • The use of a total auto-guider detector area of 2Kx2K 13.5µm pixels allowing a 99% probability of a suitable guide star being available for any desired observation in the region of the galactic pole at full moon at a rate of 10Hz (90ms exposures), providing a signal to noise ratio of 25. • The pick-off mirrors used to divert light into the LOCS/AG units must not vignette the science detectors. • The clearance between the science detectors and the underside of the LOCS/AG units must be at least 45mm to provide an adequate accommodation volume for the filter wheel. • The light passed to the CCDs is to be filtered to I-Band, being as close as possible to the science wavelengths but still allowing a QE of 80% to be achieved through the use of deep depletion detectors, with the rejected science wavelengths being absorbed and not reflected back out into the camera cryostat where they would cause ghosting. • Two curvature sensor units must be provided, positioned at diametrically opposite positions on either edge of the field of view, to allow the asymmetric astigmatism introduced by tilt of M2 around its coma-free point to be detected. To retain a symmetrical design the 2Kx2K auto-guider footprint is divided into two, one 2Kx1K auto-guider CCD being included in each of the two units adjacent to the curvature sensor CCDs. Several possible optical configurations were considered during the design phase. A cube beamsplitter-based design was finally selected due to its ability to actually improve some of the optical aberrations incurred by the sensors, which are positioned off-axis in a system optimized for longer wavelengths. The use of a plate beamsplitter was ruled out as major additional aberrations would have been introduced by the converging beam passing through the tilted plate. The chosen cube beamsplitter design has been validated successfully in a cryogenic qualification test, where a large BK7 cube beamsplitter was repeatedly submerged into liquid nitrogen without damage. Figure 3 shows a cross-section of the optical design of a single LOCS/AG unit (with the curvature sensor CCDs shown at focus rather than their normal ±1mm defocus positions). Figure 4 illustrates the position of the LOCS/AG CCD footprints and predicted throughput. CCD 920nm short pass filter coating AR coating Principal CCD Pick-off mirror Cube beamsplitter RG9 Filter (AR coated) Figure 3: Cross-section of the LOCS/AG Optical Path Edge of nearest WFS Throughput science detector 1 0.9 0.8 0.7 Transmittance 0.6 RG9 Transmittance 920nm Short Pass Coating 0.5 CCD QE 0.4 Total 0.3 AG LOCS 0.2 0.1 0 700 725 750 775 800 825 850 875 900 925 950 975 1000 1025 1050 1075 1100 Wavelength (nm) Figure 4: a) Position of the LOCS/AG CCD Footprints b) Predicted LOCS/AG Throughput The use of an RG9 filter, with a 920nm short-pass filter coating on the inner surface, allows the light passed through to the CCDs to be constrained to I-Band with the majority of the rejected science wavelengths being absorbed in the double-pass through filter rather than reflected back out into the cryostat. 4 MECHANICAL & THERMAL DESIGN The LOCS/AG unit mechanical parts have been designed such that they can be manufactured using precision machining techniques, removing the need for subsequent adjustment or shimming of the optical components or detectors. CCD mounting is based on a novel metal to metal (Invar to aluminium) minimum-area contact allowing alignment to be maintained and providing CCD cooling without the need for additional thermal insulation. Should a CCD fail in one of the LOCS/AG units, the complete unit can be removed from the bottom of the camera cryostat, through an empty position in the camera filter wheel and replaced with a spare without the need for manual realignment. Figure 5 shows the position of the optical components and detectors within each LOCS/AG unit. Pick-off mirror Curvature Sensor CCD Curvature Auto-guider Auto-guider Sensor CCD CCD CCD Pick-off mirror Principal Curvature Filter Cube Beamsplitter Cube Beamsplitter Sensor CCD Figure 5: LOCS/AG Optics and Detectors a) Side View b) External View Figure 6 shows transparent and external views of the assembled LOCS/AG unit. CCD Buffer & Protection PCB Cover External Mirror Retaining Spring Connections Figure 6: LOCS/AG Unit a) Transparent Side View b) External View Figure 7 shows the thermal design of the unit. AG TCCD Auto Guider CCD Mount Controller ~170K Curvature Sensor CCD (1 of 2) Mount ~170K Mount plus Strap LOCS (if required) TCCD Controller Curvature Sensor CCD Mount (2 of 2) ~170K LOCS/AG Assembly: ~80K WFS Plate: 80K Figure 7: LOCS/AG Unit Thermal Design 5 ELECTRONICS DESIGN Since VISTA will be operated by ESO as part of the Cerro Paranal Observatory, Next Generation ESO Technical CCD Controllers are used to operate the CCDs. These controllers are semi-custom SDSU Gen-III controllers which operate from a single 24VDC supply and interface to Motorola MVME processor cards via PMC fibre interface modules. Figure 8 shows the system electronics design. Figure 8: LOCS/AG System Electronics Design 6 SOFTWARE DESIGN 5,6 The wavefront sensing software, as with the rest of the IR Camera software, will be based on the VLT software . The latter is modular in design and has been used with slight modifications on several ESO telescopes. The VLT software provides the entire infrastructure for developing VLT standard compliant code (message passing, process control, global data sharing, error logging, basic image processing libraries, etc.). The VLT software also provides generic modules which can be easily customised into telescope and instrument specific applications. Within the wavefront sensing work package, an image analysis library capable of processing curvature wavefront sensing images has been developed to fulfill VISTA’s particular requirements for aberration measurement7. This will be used in modules specific to both the low order and high order curvature sensors. The two respective modules will provide an interface between the image analysis library and the other VISTA processes. The interface code will reuse the VLT infrastructure software. Re-use of standard VLT auto-guiding software functions will ensure that only a small amount of code needs to be written to meet the needs of VISTA, more specifically the customisation of a module that implements a virtual probe with VISTA-specific functionality. Purpose-written interface software, supplied by ESO, translates standard TCCD commands into SDSU commands allowing backward compatibility with the previous version of the controller and associated software. 7 HIGH ORDER CURVATURE SENSING VISTA has a requirement to provide low order curvature sensing measurements, during observations, to allow the alignment of M2 to be maintained in real time. High order, higher accuracy curvature sensing is also required to allow calibration look-up tables of M1 actuator positions to be produced for varying telescope declination and temperature. High order measurement is performed off-line using a novel dual beamsplitter optical element housed in one of the unused sectors of the filter wheel. This optical element produces pre- and post-focal images from a single star simultaneously on one of the science detectors. This is possible since the element is considerably thicker than a standard science filter. Figure 9 shows the optical element itself plus a ray-trace of the pre- and post-focal images. Partially Reflective Science Band Filter Coating AR Coating Coatings Main Images Figure 9 HOCS Optical Element and Ray Trace ACKNOWLEDGEMENTS VISTA is funded by a grant from the UK Joint Infrastructure Fund, supported by the Office of Science and Technology and the Higher Education Funding Council for England, to Queen Mary University of London on behalf of the 18 University members of the VISTA Consortium of: Queen Mary University of London; Queen's University of Belfast; University of Birmingham; University of Cambridge; Cardiff University; University of Central Lancashire; University of Durham; University of Edinburgh; University of Hertfordshire; Keele University; Leicester University; Liverpool John Moores University; University of Nottingham; University of Oxford; University of St Andrews; University of Southampton; University of Sussex; and University College London. REFERENCES 1. J. P. Emerson et al., "The Visible and Infra Red Survey Telescope for Astronomy: Overview", Proc SPIE 4836, p.35-42 (2002). 2. A. McPherson et al., "The VISTA project: a review of its progress and lessons learned developing the current programme", Proc SPIE 5489-46 (2004). 3. E. Atad-Ettedgui et al., "Optical design concept of the 4m Visible and Infra Red Survey Telescope for Astronomy", Proc SPIE 4842, p.95-105 (2002). 4. G. Dalton et al., “The VISTA IR Camera”, Proc SPIE 5492-34 (2004). 5. M. Stewart et al., "Applying VLT Software to a new telescope: methods and observations", Proc SPIE 5496-29 (2004). 6. S. M. Beard et al., "The VISTA IR Camera Software Design", Proc SPIE 5496-11 (2004). 7. N. Bissonauth et al., "Image analysis algorithms for critically sampled curvature wavefront sensor images in the presence of large intrinsic aberrations", Proc SPIE 5496-95 (2004).