Large aperture telescope technology (LATT) is a design study for a differential lidar (DIAL) system; the main investigation being into suitable methods, technologies and materials for a 4-metre diameter active mirror that can be stowed to fit into a typical launch vehicle (e.g. ROKOT launcher with ∼ 2.1-metre diameter cargo) and can self-deploy – in terms of both leaving the space vehicle and that the mirrors unfold and self-align to the correct optical form within the tolerances specified. The primary mirror requirements are: main wavelength of 935.5 nm, RMS corrected wavefront error of λ/6, optical surface roughness better than 5 nm, areal density of less than 16 kg/m2 and 1-2 mirror shape corrections per orbit. The primary mirror consists of 7 segments - a central hexagonal mirror and 6 square mirror petals which unfold to form the 4-meter diameter aperture. The focus of the UK LATT consortium for this European Space Agency (ESA) funded project is on using lightweighted aluminium or carbon-fibre-composite materials for the mirror substrate in preference to more traditional materials such as glass and ceramics; these materials have a high strength and stiffness to weight ratio, significantly reducing risk of damage due to launch forces and subsequent deployment in orbit.
We present an overview of the design, which includes suitable actuators for wavefront correction, petal deployment mechanisms and lightweight mirror technologies. Preliminary testing results from manufactured lightweight mirror samples will also be summarised.
Large aperture telescope technology (LATT) is a design study for a differential lidar (DIAL) system; the main investigation being into suitable methods, technologies and materials for a 4-metre diameter active mirror that can be stowed to fit into a typical launch vehicle (e.g. ROKOT launcher with ∼ 2.1-metre diameter cargo) and can self-deploy – in terms of both leaving the space vehicle and that the mirrors unfold and self-align to the correct optical form within the tolerances specified. The primary mirror requirements are: main wavelength of 935.5 nm, RMS corrected wavefront error of λ/6, optical surface roughness better than 5 nm, areal density of less than 16 kg/m2 and 1-2 mirror shape corrections per orbit. The primary mirror consists of 7 segments - a central hexagonal mirror and 6 square mirror petals which unfold to form the 4-meter diameter aperture. The focus of the UK LATT consortium for this European Space Agency (ESA) funded project is on using lightweighted aluminium or carbon-fibre-composite materials for the mirror substrate in preference to more traditional materials such as glass and ceramics; these materials have a high strength and stiffness to weight ratio, significantly reducing risk of damage due to launch forces and subsequent deployment in orbit.
We present an overview of the design, which includes suitable actuators for wavefront correction, petal deployment mechanisms and lightweight mirror technologies. Preliminary testing results from manufactured lightweight mirror samples will also be summarised.
The sea and land surface temperature radiometer (SLSTR) to be flown on the European Space Agency’s (ESA) Sentinel-3 mission is a multichannel scanning radiometer that will continue the 21 year dataset of the along-track scanning radiometer (ATSR) series. As its name implies, measurements from SLSTR will be used to retrieve global sea surface temperatures to an uncertainty of <0.3 K traced to international standards. To achieve, these low uncertainties require an end-to-end instrument calibration strategy that includes prelaunch calibration at subsystem and instrument level, on-board calibration systems, and sustained postlaunch activities. The authors describe the preparations for the prelaunch calibration activities, including the spectral response, the instrument level alignment tests, and the solar and infrared radiometric calibrations. A purpose built calibration rig has been designed and built at the Rutherford Appleton Laboratory space department (RAL Space) that will accommodate the SLSTR instrument, the infrared calibration sources, and the alignment equipment. The calibration rig has been commissioned and results of these tests will be presented. Finally, the authors will present the planning for the on-orbit monitoring and calibration activities to ensure that the calibration is maintained. These activities include vicarious calibration techniques that have been developed through previous missions and the deployment of ship-borne radiometers.
The Sea and Land Surface Temperature Radiometer (SLSTR) to be flown on ESA's Sentinel-3 mission is a multichannel
scanning radiometer that will continue the 21-year datasets of the Along Track Scanning Radiometer (ATSR)
series. As its name implies, measurements from SLSTR will be used to retrieve global sea surface temperatures to an
uncertainty of <0.3K traced to international standards. To achieve these low uncertainties requires an end to end
instrument calibration strategy that includes pre-launch calibration at subsystem and instrument level, on-board
calibration systems and sustained post launch activities.
The authors describe the preparations for the pre-launch calibration activities including the spectral response, instrument
level alignment tests, solar and infrared radiometric calibration. A purpose built calibration rig has been designed and
built at RAL space that will accommodate the SLSTR instrument, infrared calibration sources and alignment equipment.
The calibration rig has been commissioned and results of these tests will be presented.
Finally the authors will present the planning for the on-orbit monitoring and calibration activities to ensure that
calibration is maintained. These activities include vicarious calibration techniques that have been developed through
previous missions, and the deployment of ship-borne radiometers.
In preparation for the Laser Interferometer Space Antenna (LISA) space mission, the prototype engineering model of the LISA-Pathfinder optical bench instrument has been built and tested. The instrument is the central part of an interferometer whose purpose is to measure the separation of two free-floating test masses in the spacecraft, with required accuracy to a noise level of 10 pm/Hz?1/2 between 3 mHz and 30 mHz. This will allow the spacecraft to achieve drag-free flight control to a similar level, as a demonstration of technology capability for detection of gravitational waves in the later LISA mission. The optical bench design, fabrication, and experimental results are described in detail, with attention to the strategies for building and alignment. These are particularly problematic in this instrument due to restrictions on the allowable materials and devices, the limited size, the tight alignment requirements for interferometry and interfaces, and the challenging environment specification for space flight. The finished optical bench was integrated to the complete optical metrology package for system-level tests, which were successful, both in meeting the metrology accuracy and in environmental testing. This verifies the feasibility of the design and build methods demonstrated here for use in the space-flight version.
This paper describes the design and development of an accurate temperature compliant lens mounting technique being
used on the camera of the UK-FMOS near infrared spectrograph for operation at the Subaru Telescope in Hawaii. A
series of fused silica lenses of up to 4.4kg, 255mm in diameter and operating at temperatures as low as 70K are
supported within flexures cut away from stainless steel outer rings. Intermediate low thermal expansion pads are
attached to these flexures and in turn bonded to the glass during the alignment process. This mounting method lends
itself to the domino chips type of assembly process which can be carried out on a rotary table to maintain accurate axial
alignment. A detailed description of the overall design progression including the methods of manufacture, alignment
process, adhesive selection, assembly methods and testing is included.
We describe the integration and test phase of the construction of the VISTA Infrared Camera, a 64 Megapixel, 1.65 degree field of view 0.9-2.4 micron camera which will soon be operating at the cassegrain focus of the 4m VISTA telescope. The camera incorporates sixteen IR detectors and six CCD detectors which are used to provide autoguiding and wavefront sensing information to the VISTA telescope control system.
The VISTA IR Camera has now completed its detailed design phase and is on schedule for delivery to ESO’s Cerro Paranal Observatory in 2006. The camera consists of 16 Raytheon VIRGO 2048x2048 HgCdTe arrays in a sparse focal plane sampling a 1.65 degree field of view. A 1.4m diameter filter wheel provides slots for 7 distinct science filters, each comprising 16 individual filter panes. The camera also provides autoguiding and curvature sensing information for the VISTA telescope, and relies on tight tolerancing to meet the demanding requirements of the f/1 telescope design. The VISTA IR camera is unusual in that it contains no cold pupil-stop, but rather relies on a series of nested cold baffles to constrain the light reaching the focal plane to the science beam. In this paper we present a complete overview of the status of the final IR Camera design, its interaction with the VISTA telescope, and a summary of the predicted performance of the system.
As detailed instrument design progresses, judgements have to be made as to what changes to allow and when models such as thermal, stray-light and mechanical structure analysis have to be re-run. Starting from a well-founded preliminary design, and using good engineering design when incorporating changes, the design detailing and re-run of the models should bring no surprises. Nevertheless there are issues for maintaining the design and model configuration to a reasonably concurrent level. Using modern modeling software packages and foresight in setting up the models the process is made efficient, but at the same time the level of detail and number of cases now needed for instrument reviews is also large in order to minimise risks.
We describe examples from the detailed instrument design of the VISTA IR Camera to illustrate these aspects and outline the design and analysis methods used.
The infrared camera for the Visible and Infrared Survey Telescope for Astronomy (VISTA) sets many technical challenges for mechanical and thermal design. The flexion between optical subsystems must be minimised to maintain alignment in various camera orientations and meet performance requirements. Thermally induced stresses, atmospheric pressure and earthquake loads place high demands on structural components, some of which must also thermally isolate the cold (~70 K) detectors and optics. The success of the design hinges on the optimisation of heat flow to minimise thermal loads on the detectors whilst holding external temperatures very close to ambient to reduce misting and convective disturbances in the field of view.
This paper describes the mechanical and thermal components of the design and discusses the analyses in detail.
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