SPICA (SPace Infrared telescope for Cosmology and Astrophysics, see Figure 1) is a proposed next generation space infrared observatory. The mission will study formation of planets, solar system processes, and the origin of the universe. SPICA is an international project, led by the Japanese space agency JAXA, with contributions from Europe. SPICA has been selected as a candidate ESA M-class Cosmic Vision mission. SPICA will have a single 3.5 m mirror operating at 4.5 Kelvin. The wavelength range will cover 5 to 210 =m. The SAFARI (SpicA FAR infrared Instrument) imaging FTS (Fourier Transform Spectrometer) is one of five anticipated focal plane instruments. It will operate in the wavelength range of 35 to 210 =m. The proposed European contributions to SPICA will consist of the telescope subsystem, the ground segment and SAFARI. SAFARI will be financed through national contributions. The Rutherford Appleton Laboratory (RAL) in the UK has been leading the Phase A study for this European SAFARI spectrometer. Currently SRON in the Netherlands has taken over this leading role from RAL. SAFARI contains an Optical Delay Line (ODL) scan mechanism: FTSM. TNO has been participating in a SPICA SAFARI study, led by SRON, to support the RAL SPICA SAFARI Phase A investigations. The objective of this study was to increase the overall SAFARI TRL and improve the chances for SPICA in the ESA Cosmic Vision down selection. TNO participates in the optical design trade off between the SAFARI instrument and the FTS scan mechanism and leads the preliminary design of the FTS scan mechanism. Micromega- Dynamics contributed to the magnetic bearing design. SRON is developing control electronics for the FTSM. TNO has previously been developing a cryogenic optical delay line mechanism breadboard with magnetic bearings together with Micromega-Dynamics and SRON for the Darwin mission. That heritage was the basis for the development of the SAFARI FTSM.
Tropomi, successfully launched in October 2017, and Sentinel-5, with launch due in 2021, are two pushbroom spectrometers measuring Earth’s radiance from a Low Earth Orbit (LEO) at a high spectral resolution. Both instruments have strongly overlapping spectral channels (UV-VIS, NIR, SWIR-3). While the Tropomi spectrometers are designed with standard slits, all Sentinel-5 channels make use of complex slit assemblies called “slit homogenizers” that aim at mitigating the slit heterogeneous illumination that results from the along track spatial non-uniformity of the observed scenes, and is known to distort the instrument spectral response function (ISRF). The similarity between the two missions will allow, in a few years from now, to evaluate the performance gain resulting from these devices. If their expected success is confirmed, slit homogenizers may become standard components of future space missions. This paper aims at providing a comprehensive, yet as simple and accessible as possible, overview of the slit homogenizer performance. The Sentinel-5 slit homogenizers, based on two parallel mirrors will be discussed, and a new and promising family of slit homogenizer designs will be presented for the first time. The new designs offer several advantages in terms of performance and manufacturability.
MIRI (the Mid InfraRed Instrument) is one of the focal plane instruments of the James Webb Space Telescope. The instrument comprises a camera and a spectrometer module. The instrument plays the following key roles in the JWST science program. - Discovery of the “first light”. - Assembly of galaxies: history of star formation, growth of black holes, production of heavy elements. - Formation of stars and planetary systems. - Evolution of planetary systems and conditions for life. The MIRI spectrometer covers the spectral range from 5 – 28.3 μm with a spectral resolution better than 2000. The spectral module is an imaging spectrometer with a field of view ≥ 3 arcsec. The spectrometer consists of 4 spectral channels that share 2 detectors of 1024 × 1024 pixels each. In a single measurement one third of the wavelength range of the channels is imaged onto the detectors. By changing gratings the whole spectral range is covered in 3 measurements. In this paper the optical design of the MIRI spectrometer is described.
Designing a novel optical system is a nested iterative process. The optimization loop, from a starting point to final system is already mostly automated. However this loop is part of a wider loop which is not. This wider loop starts with an optical specification and ends with a manufacturability assessment. When designing a new spectrometer with emphasis on weight and cost, numerous iterations between the optical- and mechanical designer are inevitable. The optical designer must then be able to reliably produce optical designs based on new input gained from multidisciplinary studies. This paper presents a procedure that can automatically generate new starting points based on any kind of input or new constraint that might arise. These starting points can then be handed over to a generic optimization routine to make the design tasks extremely efficient. The optical designer job is then not to design optical systems, but to meta-design a procedure that produces optical systems paving the way for system level optimization. We present here this procedure and its application to the design of TROPOLITE a lightweight push broom imaging spectrometer.
TNO, together with its partners, have designed a cryogenic scanning mechanism for use in the SAFARI1 Fourier
Transform Spectrometer (FTS) on board of the SPICA mission. SPICA is one of the M-class missions competing to be
launched in ESA's Cosmic Vision Programme2 in 2022. JAXA3 leads the development of the SPICA satellite and SRON
is the prime investigator of the Safari instrument.
The FTS scanning mechanism (FTSM) has to meet a 35 mm stroke requirement with an Optical Path Difference
resolution of less then 15 nm and must fit in a small volume. It consists of two back-to-back roof-top mirrors mounted on
a small carriage, which is moved using a magnetic bearing linear guiding system in combination with a magnetic linear
motor serving as the OPD actuator. The FTSM will be used at cryogenic temperatures of 4 Kelvin inducing challenging
requirements on the thermal power dissipation and heat leak.
The magnetic bearing enables movements over a scanning stroke of 35.5 mm in a small volume. It supports the optics in
a free-floating way with no friction, or other non-linearities, with sub-nanometer accuracy. This solution is based on the
design of the breadboard ODL (Optical Delay Line) developed for the ESA Darwin mission4 and the MABE mechanism
developed by Micromega Dynamics.
During the last couple of years the initial design of the SAFARI instrument, as described in an earlier SPIE 2010 paper5,
was adapted by the SAFARI team in an evolutionary way to meet the changing requirements of the SPICA payload
module. This presentation will focus on the evolution of the FTSM to meet these changing requirements. This work is
supported by the Netherlands Space Office (NSO).
The Safari instrument on the Japanese SPICA mission is a zodiacal background limited imaging spectrometer offering a
photometric imaging (R ≈ 2), and a low (R = 100) and medium spectral resolution (R = 2000 at 100 μm) spectroscopy
mode in three photometric bands covering the 34-210 μm wavelength range. The instrument utilizes Nyquist sampled
filled arrays of very sensitive TES detectors providing a 2’x2’ instantaneous field of view. The all-reflective optical
system of Safari is highly modular and consists of an input optics module containing the entrance shutter, a calibration
source and a pair of filter wheels, followed by an interferometer and finally the camera bay optics accommodating the
focal-plane arrays. The optical design is largely driven and constrained by volume inviting for a compact three-dimensional
arrangement of the interferometer and camera bay optics without compromising the optical performance
requirements associated with a diffraction- and background-limited spectroscopic imaging instrument. Central to the
optics we present a flexible and compact non-polarizing Mach-Zehnder interferometer layout, with dual input and output
ports, employing a novel FTS scan mechanism based on magnetic bearings and a linear motor. In this paper we discuss
the conceptual design of the focal-plane optics and describe how we implement the optical instrument functions, define
the photometric bands, deal with straylight control, diffraction and thermal emission in the long-wavelength limit and
interface to the large-format FPA arrays at one end and the SPICA telescope assembly at the other end.
TNO, together with its partners Micromega and SRON, have designed a cryogenic scanning mechanism for use in the
SAFARI Fourier Transform Spectrometer (FTS) on board of the SPICA mission.
The optics of the FTS scanning mechanism (FTSM) consists of two back-to-back cat's-eyes. The optics are mounted on a
central "back-bone" tube which houses all the important mechatronic parts: the magnetic bearing linear guiding system, a
magnetic linear motor serving as the OPD actuator, internal metrology with nanometer resolution, and a launch lock.
A magnetic bearing is employed to enable a large scanning stroke in a small volume. It supports the optics in a free-floating
way with no friction, or other non-linearities, enabling sub-nanometer accuracy within a single stage with a
stroke of -4 mm to +31.5 mm.
Because the FTSM will be used at cryogenic temperatures of 4 Kelvin, the main structure and optics are all constructed
from 6061 Aluminum. The overall outside dimensions of the FTSM are: 393 x 130 x 125 mm, and the mass is 2.2 kg.
This paper presents the TNO share of the development of the HIFI Alignment Camera System (HACS), covering the opto-mechanical and thermal design. The HACS is an Optical Ground Support Equipment (OGSE) that is specifically developed to verify proper alignment of different modules of the HIFI instrument during on-ground thermal (vacuum) testing of the ESA Herschell spacecraft.
An original optical design for two-step recording of holographic diffraction gratings is presented. It combines the merits of the two previously developed methods. On the one hand, due to the two-step recording one can use wide possibilities for the aberrations compensation. On the other hand, one can record both the grating objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously, one must use spatially coherent light for at least one of two recording steps. Our method does not require the use of additional optics. Both surfaces of the grating objective blank have a spherical form. The blank of the final grating can be plane concave, which is usual for diffraction gratings. Since the blaze angle of the grating recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations taking into consideration the polarization are also provided. The transmission of the system is optimal for the s-polarized light, additional increasing of the transmission can be achieved by use of an immersion grating objective.
The original optical design for two steps recording of the holographic diffraction gratings is presented. It gives the combination of the merits of two previously developed methods in one. From one hand, due to recording in two steps, we can use wide possibilities of the aberrations compensation. From another hand, we can record both the gratings - objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously per limit one of the two steps required using of the spatially coherent light. Method does not require using of the additional optics. Both surfaces of the grating - objective blank have spherical form. The blank of the final grating can be plane - concave, which is usual for diffraction gratings. Since the blaze angle of the grating, recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations with taking into consideration the polarization also have been provided. The transmission of the system is optimal for the S polarized light, the addition increasing of the transmission can be achieved by using of the immersion grating - objective.
Measuring the wind speed from a satellite is not new. However measuring with great precision is by far not trivial. Various methods are available for that. A common method is to use the Doppler effect. A UV-laser on board of the satellite is used to "fire" to the earth atmosphere. Some photons will be reflected back to the satellite. Because of the speed of the particles in the air the photons will experience a small Doppler shift. Wind speeds of 1 m/s are hereby
equivalent to a wave length shift of 1 femtometer.
The paper presents the patented method of how to measure these small wavelength shifts without running into trouble concerning the mechanical design. It will understood that such instrument will be very sensitive to thermal variations (a challenging requirement was that a temperature change of 0.2° in 7 seconds was specified at the interface surfaces). The optical system makes use of a modified Michelson interferometer while the mechanical system automatically
compensates for thermal expansion effects. Originally the idea was to make a complete Zerodur structure to eliminate the thermal effects. However it appeared to be possible to use a titanium structure with certain elements made from invar and aluminium. No need to say that this reduced risk and cost of the instrument drastically.
In this paper we detail the optical design of HIFI, one of three instruments to be flown on the Herschel Space Observatory and also outline the general approach taken in analysing the quasi-optical performance of the instrument. Gaussian Beam Mode Analysis theory is the principle technique chosen for the quasi-optical analysis, supplemented by the commercially available optical analysis package GLAD. These quasi-optical techniques are used for validation of the optical design and predicting the performance of the HIFI system. The HIFI system was checked for issues such as truncation, mechanical misalignment and aberrational losses, along with aperture efficiency and LO coupling efficiency. The quasi-optical analytical methods described could be applied generally to long wavelength optical instruments.
The Ozone Monitoring Instrument (OMI) is a UV/VIS spectrograph (270-500 nm) in the line of GOME3 and SCIAMACHY4. It employs two-dimensional CCD detectors for simultaneous registration of numerous spectra from ground pixels in the swath perpendicular to the flight direction. The OMI field of view is 13 x 2600 km2 per two seconds nominal exposure time providing (almost) daily global coverage in combination with small ground pixel sizes (nominally 13 x 24 km2, minimum 13 x 12 km2). The small ground pixels will allow retrieval of tropospheric constituents. The OMI contains various new and innovative design elements such as a polarisation scrambler and programmable CCD read-out modes. This paper discusses the overall design of the OMI together with the instrumental capabilities.
In this paper, a comparison of two different solutions for the illuminator is presented. The systems are intended to comply with the illumination requirements, but have different advantages and drawbacks. The examples represent solutions based on conical reflection and on a fly's-eye integrator. A comparison is given and the potentials of the different solutions are outlined.
The Ozone Monitoring Instrument (OMI) is a Dutch-Finnish contribution to NASA's EOS-Chemistry satellite, which is due to be launched in December 2002. The aim of OMI is to contribute to climate monitoring and atmospheric chemistry research by providing daily global measurements of the total ozone column, ozone profile, NO2 column, other trace gases like SO2 and BrO2, aerosols, cloud fraction, cloud to pressure, and surface UV irradiance.
To gain insight in the dynamics and the long-term behavior of constituents (e.g., ozone, CFCs) of the Earth atmosphere, satellite-based instruments allowing global monitoring provide unsurpassed information. In order to study atmospheric trends over a very long period of time, these instruments generally have to be calibrated normalized to the sun very accurately. Furthermore, the accuracy has to be maintained during the lifetime of the instruments. This contribution addresses the issue of radiometric calibration of earth-observation instruments, both on ground and in flight. Some of the related issues will be illustrated by focusing on two specific instruments (to be) calibrated at TPD: the global ozone monitoring experiment (GOME), which was launched on ERS-2 in April 1995, and the scanning imaging absorption spectrometer for atmospheric cartography (SCIAMACHY), to be launched on Envisat around the turn of the century. We distinguish between sun-normalized calibration and absolute radiometric calibration. In both cases instrument sensitivity to polarization is a complicating factor. Other factors to be dealt with are, e.g., the etalon effect and the influence of humidity during on-ground calibration. These all require a sophisticated calibration approach and well-adapted radiometric calibration equipment. After on-ground calibration the instruments are susceptible to possible changes or degradation. Therefore, GOME and SCIAMACHY both contain a well-calibrated on-board diffuser providing an accurate reflectance standard. However, this diffuser itself is degrading with time due to contamination and radiation effects. An in-flight monitoring concept is therefore mandatory. The addressed calibration aspects are elucidated using the example of SCIAMACHY.
SCIAMACHY (SCanning Imaging Absorbtion spectroMeter for Atmospheric CHartographY) is a spectrometer in the UV, VIS and near IR region, with which the concentration of atmospheric trace gases both in the troposphere and in the stratosphere can be determined. It has been selected for ENVISAT of which the launch is scheduled in 1999. SCIAMACHY is a common German-Dutch project. The optical configuration consists of 2 scan mirrors (a nadir/elevation mirror and an azimuth mirror), a telescope and a complex spectrometer, with which a spectral range from 0.24 - 2.4 micrometers is covered in 8 channels with array detectors. Axillary optics consist of two on-board calibration light sources, a sun/moon follower and a polarization measurement device (PMD). The sun/moon follower enables SCIAMACHY to track the sun or moon during occultation and sun/moon calibration measurements. The PMD is a simple seven channel spectrometer with basically sensitivity for one polarization direction only. The PMD is used to correct for the difference in sensitivity for different polarization states of the regular spectrometer channels. The optical system is described in detail elsewhere. The operational success of an instrument like SCIAMACHY depends strongly on the accuracy of the calibration. Proper calibration enables comparison of measurements with those of other instruments, that measure atmospheric trace gases. Besides it gives information about degradation of the spectrometer during its lifetime, so correction of the measurements can be performed to make the real trends in detection of ozone (and other gases) visible. For on-ground calibration of the diffuser and of the spectral channels at room temperature a special facility has been developed at our institute. In the following chapters some essential aspects of the calibration of SCIAMACHY and of the calibration facility are described.
Hamilton's method of characteristic functions can be used to calculate the effects of small system perturbations on image formation. In this paper we apply perturbation theory to the calculation of centering tolerances. We show how the aberrations caused by component shifts and tilts can be expressed as sums over surface contributions. We made up a computer program (called NODES) to calculate Eikonal coefficients of optical systems. With this program lens design can be optimized with regard to specific aberrations. Decentering aberrations can be included in the merit function. We give a few examples of optimization including centering tolerances.
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