Key components in optical spectrometers are the gratings. Their influence on the overall infield straylight of the spectrometer depends not only on the technology used for grating fabrication but also on the potential existence of ghost images caused by irregularities of the grating constant. For the straylight analysis of spectrometer no general Bidirectional Reflectance Distribution Function (BRDF) model of gratings exist, as it does for optically smooth surfaces. These models are needed for the determination of spectrometer straylight background and for the calculation of spectrometer out of band rejection performances.
Within the frame of the Fluorescence Earth Explorer mission (FLEX), gratings manufactured using different technologies have been investigated in terms of straylight background and imaging performance in the used diffraction order. The gratings which have been investigated cover a lithographically written grating, a volume Bragg grating, two holographic gratings and an off-the-shelf ruled grating. In this paper we present a survey of the measured bidirectional reflectance/transmittance distribution function and the determination of an equivalent surface micro-roughness of the gratings, describing the scattering of the grating around the diffraction order. This is specifically needed for the straylight modeling of the spectrometer.
In the frame of recent studies and missions, ESA has been performing various pre-developments of optical gratings for instruments operating at wavelengths from the UV up to the SWIR. The instrument requirements of Sentinel-4, Sentinel-5, CarbonSat and FLEX are driving the need for advanced designs and technologies leading to gratings with high efficiency, high spectral resolution, low stray light and low polarization sensitivities. Typical ESA instruments (e.g. Sciamachy, GOME, MERIS, OLCI, NIRSpec) were and are based on ruled gratings or gratings manufactured with one holographic photoresist mask layer which is transferred to an optical substrate (e.g. glass, glass ceramic) with dry etching methods and subsequently either coated with a reflective coating or used as a mold for replication. These manufacturing methods lead to blazed grating profiles with a metallic reflective surface. The vast majority of spectrometers on ground are still based on such gratings. In general, gratings based on grooved metallic surfaces tend for instance to polarize the incoming light significantly and are therefore not always suitable for ESA’s needs of today. Gratings made for space therefore evolved to many other designs and concepts which will be reported in this paper.
With the development of new spectrometer concepts, it is required to adapt the calibration facilities to characterize correctly their performances. These spectro-imaging performances are mainly Modulation Transfer Function, spectral response, resolution and registration; polarization, straylight and radiometric calibration.
The challenge of this calibration development is to achieve better performance than the item under test using mostly standard items. Because only the subsystem spectrometer needs to be calibrated, the calibration facility needs to simulate the geometrical “behaviours” of the imaging system.
A trade-off study indicates that no commercial devices are able to fulfil completely all the requirements so that it was necessary to opt for an in home telecentric achromatic design. The proposed concept is based on an Offner design. This allows mainly to use simple spherical mirrors and to cover the spectral range. The spectral range is covered with a monochromator. Because of the large number of parameters to record the calibration facility is fully automatized.
The performances of the calibration system have been verified by analysis and experimentally. Results achieved recently on a free-form grating Offner spectrometer demonstrate the capacities of this new calibration facility.
In this paper, a full calibration facility is described, developed specifically for a new free-form spectro-imager.
The Meteosat Third Generation (MTG) Programme is being realised through the well-established and successful Cooperation between EUMETSAT and ESA. It will ensure the future continuity of MSG with the capabilities to enhance nowcasting, global and regional numerical weather prediction, climate and atmospheric chemistry monitoring data from Geostationary Orbit. This will be achieved through a series of 6 satellites named MTG-I and MTG-S to bring to the meteorological community continuous high spatial, spectral and temporal resolution observations and geophysical parameters of the Earth based on sensors from the geo-stationary orbit. In particular, the imagery mission MTG-I will bring an improved continuation of the MSG satellites series with the Flexible Combined Imager (FCI) a broad spectral range (from UV to LWIR) with better spatial and spectral resolutions. The FCI will be able to take high spatial resolution pictures of the Earth within 8 VNIR and 8 IR channels. As one of the mission of this instrument is to provide a quantitative analysis of atmosphere compounds, the absolute observed radiance needs to be known with a specified accuracy for VNIR as low as to 5% at k=3 over its full dynamic. While the FCI is regularly recalibrated every 6 month at equinoxes, it is however requiring initial ground calibration for the beginning of its mission. The Multi Optical Test Assembly (MOTA) is one of the Optical Ground Support Equipment (OGSE) dedicated to various missions necessary for the integration of the FCI . This equipment, provided by Bertin Technologies, will be delivered to TAS-F by the end of 2016. One of its mission, is the on-ground absolute calibration of VNIR channels. In order to handle this, the MOTA will be placed in front of the FCI under representative vacuum conditions and will be able to project a perfectly known, calibrated radiance level within the full dynamic of FCI instrument. The main difficulty is the very demanding calibration level with respect to primary standards down to 3% (k=3) coupled with constraining environment (vacuum), large dynamic (up to factor 100), high spectral resolution of 3 nm. Another main difficulty is to adapt the specific MOTA etendue (300 mm pupil, 9 mrad field) to available primary standards. Each of these constraints were addressed by specific tool design and production, a fine optimization of the calibration procedure with a large involvement of metrology laboratories. This paper introduces the missions of MTG satellites and particularly of the FCI instrument. The requirements regarding the absolute calibration over the different spectrometric channels and the global strategy to fulfill them are described. The MOTA architecture and calibration strategy are then discussed and final expected results are presented, showing state of the art performances.
Only a small set of radiation hardened optical glasses are currently offered in the market, thus drastically limiting the optical design choices available to the engineers at the early phases of an instrument development. Furthermore, availability of those glasses cannot be easily guaranteed for the long term horizon of future space instrument developments. Radiation tests on conventional glasses on the other hand have shown significant sensitivity to high radiation levels but such levels are not necessarily representative of typical low Earth (LEO) orbits. We have conducted irradiation campaigns on several different types of conventional, non-radiation hard glasses, selected from the wider pool of the Schott “new” arsenic and lead free series (N-*) and characterized their spectral transmission properties before and after ionizing dose deposition. We report our first findings here.
ASSIST, The Adaptive Secondary Setup and Instrument STimulator, is being developed to provide a testing facility for
the ESO Adaptive Optics Facility (AOF). It will allow the off-telescope testing of three elements of the VLT AOF; the
Deformable Secondary Mirror (DSM) and the AO systems for MUSE and HAWK-I (GALACSI and GRAAL). The core
of ASSIST consists of a 2-mirror setup (AM1-AM2) allowing the on-axis test of the DSM in interferometric mode.
However, during the initial stages of ASSIST integration, DSM would not be present. This makes the task of aligning
AM1-AM2 to within an accuracy of 0.05mm/1 arcmin rather challenging. A novel technique known as Shack-Hartmann
method has been developed and tested in the lab for this purpose. A Shack Hartmann wavefront sensor will be used to
measure the mis-alignment between AM1-AM2 by recording the coma and astigmatism in the presence of large
spherical aberration introduced because of tilt/decenter of AM2 with respect to AM1. Thereafter, 20 optical components
including lenses, flat mirrors and beam-splitter cubes divided into five sub-assemblies should be aligned to AM1-AM2-
DSM axis which ultimately passes through the mechanical axis of large AMOS rotator.
The testing and verification of ESO Very Large Telescope Adaptive Optics Facility (VLT-AOF) requires new and
innovative techniques to deal with the absence of an intermediate focus on the telescope. ASSIST, The Adaptive
Secondary Setup and Instrument STimulator, was developed to provide a testing facility for the ESO AOF and will allow
off-telescope testing of three elements of the VLT Adaptive Optics Facility; the Deformable Secondary Mirror (DSM)
and the AO systems for MUSE and HAWK-I (GALACSI and GRAAL). ASSIST will provide a full testing environment
which includes an interferometric testing mode for the DSM, an
on-axis testing mode with a single wavefront sensor and
full operation testing modes for both the AO systems. Both natural as well as laser guide stars will be simulated under
various asterisms and a realistic turbulent atmosphere will be provided for varying atmospheric conditions. ASSIST
passed its final design review and is now being manufactured, integrated and tested and will be operational in mid 2011,
in time for first testing with the DSM.
The ESO Adaptive Optics Facility (AOF) consists in an evolution of one of the ESO VLT unit telescopes to a laser
driven adaptive telescope with a deformable mirror in its optical train, in this case the secondary 1.1m mirror, and four
Laser Guide Stars (LGSs). This evolution implements many challenging technologies like the Deformable Secondary
Mirror (DSM) including a thin shell mirror (1.1 m diameter and 2mm thin), the high power Na lasers (20W), the low
Read-Out Noise (RON) WaveFront Sensor (WFS) camera (< 1e-) and SPARTA the new generation of Real Time
Computers (RTC) for adaptive control. It also faces many problematic similar to any Extremely Large Telescope (ELT)
and as such, will validate many technologies and solutions needed for the European ELT (E-ELT) 42m telescope. The
AOF will offer a very large (7 arcmin) Field Of View (FOV) GLAO correction in J, H and K bands (GRAAL+Hawk-I),
a visible integral field spectrograph with a 1 arcmin GLAO corrected FOV (GALACSI-MUSE WFM) and finally a
LTAO 7.5" FOV (GALACSI-MUSE NFM). Most systems of the AOF have completed final design and are in
manufacturing phase. Specific activities are linked to the modification of the 8m telescope in order to accommodate the
new DSM and the 4 LGS Units assembled on its Center-Piece. A one year test period in Europe is planned to test and
validate all modes and their performance followed by a commissioning phase in Paranal scheduled for 2014.
MICADO is the adaptive optics imaging camera for the E-ELT. It has been designed and optimised to be mounted
to the LGS-MCAO system MAORY, and will provide diffraction limited imaging over a wide (~1 arcmin) field
of view. For initial operations, it can also be used with its own simpler AO module that provides on-axis
diffraction limited performance using natural guide stars. We discuss the instrument's key capabilities and
expected performance, and show how the science drivers have shaped its design. We outline the technical
concept, from the opto-mechanical design to operations and data processing. We describe the AO module,
summarise the instrument performance, and indicate some possible future developments.
ASSIST: The Adaptive Secondary Setup and Instrument STimulator is the test setup for the verification and calibration
of three elements of the VLT Adaptive Optics Facility.; the Deformable Secondary Mirror (DSM) the AO system for
MUSE and HAWK-I (GALACSI and GRAAL). In the DSM testing mode the DSM will be tested using both
interferometry and fast wave front sensing. In full AO mode, ASSIST will allow testing of the AO systems under
realistic atmospheric conditions and optically equivalent to the conditions on the telescope. ASSIST is nearing its final
design review and in this paper we present the current optical and mechanical design of ASSIST. In this paper we
highlight some of the specific aspects of ASSIST that we are developing for ASSIST.
ASSIST - The Adaptive Secondary Setup and Instrument STimulator is a test setup to verify the operation of three
elements of the VLT Adaptive Optics Facility, namely the Deformable Secondary Mirror (DSM) and the two AO
systems using this DSM, the AO system for the visible light integral field spectrograph MUSE (GALACSI) and the AO
system for the IR wide field imager HAWK-I (GRAAL). To support the testing of these elements, ASSIST will provide
both an interferometry setup for testing the DSM as well as a full atmospheric turbulence simulator and star simulator to
mimic the conditions at the telescope. To test the instruments using the DSM, the output beam is matched the output
beam of the VLT telescope, including the correct exit-pupil and high-quality imaging and a similar hardware interface is
provided. Since one of the modes to be verified is nearly diffraction limited, also the thermal and vibrational stability
are very important, with strong constraints on both the mechanical as well as the optical design.