Space missions, as EChO, or ground based experiments, as SPHERE, have been proposed to measure the atmospheric transmission, reflection and emission spectra. In particular, EChO is foreseen to probe exoplanetary atmospheres over a wavelength range from 0.4 to 16 micron by measuring the combined spectra of the star, its transmission through the planet atmosphere and the emission of the planet. The planet atmosphere characteristics and possible biosignatures will be inferred by studying such composite spectrum in order to identify the emission/absorption lines/bands from atmospheric molecules such as water (H2O), carbon monoxide (CO), methane (CH4), ammonia (NH3) etc. The interpretation of the future EChO observations depends upon the understanding of how the planet atmosphere affects the stellar spectrum and how this last affects the planet emission/absorption. In particular, it is important to know in detail the optical characteristics of gases in the typical physical conditions of the planetary atmospheres and how those characteristics could be affected by radiation induced phenomena such as photochemical and biological one. Insights in this direction can be achieved from laboratory studies of simulated planetary atmosphere of different pressure and temperature conditions under the effects of radiation sources, used as proxies of different bands of the stellar emission.
The Exoplanet Characterisation Observatory (EChO) mission was one of the proposed candidates for the European Space Agency’s third medium mission within the Cosmic Vision Framework. EChO was designed to observe the spectra from transiting exoplanets in the 0.55-11 micron band with a goal of covering from 0.4 to 16 microns. The mission and its associated scientific instrument has now undergone a rigorous technical evaluation phase and we report here on the outcome of that study phase, update the design status and review the expected performance of the integrated payload and satellite.
The Visible and Near Infrared (VNIR) is one of the modules of EChO, the Exoplanets Characterization Observatory
proposed to ESA for an M-class mission. EChO is aimed to observe planets while transiting by their suns. Then the
instrument has be designed to assure a high efficiency over the whole spectral range. In fact, it has to be able to observe
stars with an apparent magnitude Mv= 9÷12 and able to see contrasts of 10-4÷10-5 in order to reveal the characteristics of
the atmospheres of the exoplanets under investigation.
VNIR was originally designed for covering the spectral range from 0.4 to 1.0 μm  but now the design has been
reviewed and its spectral range has been extended up to 2.5 μm. It is a spectrometer in a cross-dispersed configuration
that, then, uses the combination of a diffraction grating and a prism to spread the light in different wavelengths and in a
useful number of orders of diffraction. Its resolving power is about 330 over the entire spectral range and its field of view
is approximately 2 arcsec.
The spectrometer is functionally split into two channels respectively working in the 0.4-1.0 μm and 1.0-2.5 μm
spectral ranges. Such a solution is imposed by the fact the light at low wavelengths has to be shared with the EChO Fine
Guiding System (FGS) devoted to the pointing of the stars under observation. The instrument works at 45K and its
weight is 6 kg.
The Exoplanet Characterisation Observatory (EChO) is a space mission dedicated to undertaking spectroscopy of
transiting exoplanets over the widest wavelength range possible. It is based around a highly stable space platform with a
1.2 m class telescope. The mission is currently being studied by ESA in the context of a medium class mission within
the Cosmic Vision programme for launch post 2020. The payload suite is required to provide simultaneous coverage
from the visible to the mid-infrared and must be highly stable and effectively operate as a single instrument. In this
paper we describe the integrated spectrometer payload design for EChO which will cover the 0.4 to 16 micron
wavelength band. The instrumentation is subdivided into 5 channels (Visible/Near Infrared, Short Wave InfraRed, 2 x Mid Wave InfraRed; Long Wave InfraRed) with a common set of optics spectrally dividing the input beam via dichroics.
We discuss the significant design issues for the payload and the detailed technical trade-offs that we are undertaking to
produce a payload for EChO that can be built within the mission and programme constraints and yet which will meet the
exacting scientific performance required to undertake transit spectroscopy.
The Visible and Near Infrared (VNIR) spectrometer of the EChO will cover the spectral range between 0.55 and 2.50 μm. It has to be designed to assure a high efficiency over whole spectral range. It has to be able to observe stars with an apparent magnitude Mv= 9÷12 and able to see contrasts of the order of 10-4÷10-5 in order to measure characteristics of the exoplanets under investigation. VNIR would be a spectrometer in a cross-dispersed configuration by using a combination of a diffraction grating and a prism to spread the light in different wavelengths and in a useful number of orders of diffraction. It will use a Mercury Cadmium Telluride detector to satisfy the requirements of low thermal noise and the EChO system to operate at the working temperature of 40-45K. The instrument will be interfaced to the telescope optics by optical fibers to assure an easier coupling and an easier colocation of the instrument inside the EChO optical bench. The preliminary design of the instrument foresees a resolving power of about 330 with an entrance aperture of 2 arcsec.
This paper is based on the analysis and the interpretation of groundbased lidar and balloonborne laser backscatter sonde measurements of Polar Stratospheric Clouds. A classification of the clouds will be made on the basis of the measured parameters. This classification is also reproduced by different temperatures and thermal histories of the sampled air masses. A Mie aerosol model is used to identify the physical meaning of the variation in the optical parameters for liquid particles. This model is also used for the retrieval of the size distribution of the aerosol. It will be compared with a different model, based on the color index computation and with particle counter measurements.
VNIR is an imaging spectrometer working in the 350 divided by 1050 nm spectral range. It simultaneously acquires multiple images of the same region, each in a different narrow spectral band. The resulting images provide a spectrum for each point in the scene. The VNIR spectrometer together with an infrared channel form the instrument OMEGA (Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite') that will be used from a Russian orbiter to map the geochemical and mineralogical distribution of materials of the surface of Mars. The main purpose of VNIR is to extend the mapping capability towards short wavelengths, where a number of natural materials have their signatures, and to determine the location on the surface of spectral features mapped in the IR range. The instrument is composed of a lens objective and a concave holographic grating mirror based spectrometer. It can acquire images of 384 X N spatial pixels (N being the number of swaths, as the push broom technique is used) of 0.4 mrad each and 5 nm spectral resolution over 144 channels.
PFS is a two-channel Michelson interferometer operating in the infrared wavelengths between 1.25 and 45 micrometers . The instrument is mainly devoted to the study of the Martian atmosphere. The principal goals are the measurement of the atmospheric temperature and pressure, atmospheric constituents, aerosol and clouds, ground pressure for surface topography, and optical and thermophysical properties of the Martian soil. PFS will fly on the Mars 94 spacecraft which should be launched in 1994 and reach the planet in 1995. Essentially it consists of two different interferometers located in the same box which is divided in two parts. An edge filter placed on the PFS entrance is used to separate the spectral range into two parts. The reason for that is the different optical materials which have to be used in each spectral range. The optical layout of the experiment is very compact. Cubic mirrors are mounted on an L-structure pivoted on a stepping motor. The stepping motor moves the mechanics and permits the optical path difference between the arms to be varied. Each interferometer operates in a different spectral range between 1.25 - 4.8 micrometers (8000 - 2083 cm-1) and 6 - 45 micrometers (1666 - 220 cm-1), respectively. The spectral resolution is 2 cm-1. The entrance aperture area is 30 cm2 per channel and the field of view (FOV) 2 and 4 degs. Every measurement lasts about 4 s and the respective resolving power is 4166 and 1041. The time and, therefore, the relative optical path difference for the measurement of every point of the interferogram is given by a monochromatic reference channel at 1.2 micrometers which uses a laser diode as a source. The interferograms are double sided and have 16384 and 4096 points, respectively, corresponding to spectra of 6250 and 1823 points.