318
Chapter 30
yield low optical throughput. The typical solution consists in using
achromatized prism doublets. Wide band single optics may necessitate a
spectral over-sampling in the 550–780 nm range to avoid saturation,
since the radiometric aperture will be defined by the lower and higher
wavelengths
Spectral and spatial co-registration accuracy (typically 0.1 to 0.2
pixels): This is another incisive parameter for the optics and FPA choice,
optical bench mechanical and thermal stability, OGSE. The preference is
for single common optics for all channels.
4.
WORKING PRINCIPLES OF IMAGING
SPECTROMETERS, EXAMPLES
Fig. 7 shows, as a typical example for a pushbroom IS, the optical
scheme of the HRIS instrument. For simplicity reasons, only the SWIR path
of the spectrometer is presented.
The main sub-units, following the optical and electronics path, are briefly
summarized below:
A pointing unit (or scan unit for whiskbroom IS) to provide a variable
swath (except large FOV instruments such as MERIS, MODIS, ROSIS,
where an along-track tilt of the swath is partly introduced to avoid Sun
glint over water bodies) or along-track pointing for BRDF measurements
(e.g., PRISM), mostly used also to access in-flight and external calibration references; critical for two-axis pointing in terms of mechanisms
A common front optics or telescope or a set of modular optics (e.g.,
MERIS), providing the TFOV, the required geometrical (EFL) and radiometric resolution via the performance compliant effective aperture,
focal length and/or or f-number. The front optics can be a lens system
for limited spectral ranges (see Fig. 4 for MERIS), or a reflective mirror
optics for wider spectral ranges and high throughput (see Fig. 7 for
HRIS), mostly combined with the need of some aspheric corrector
elements, or mixed catadioptric systems (e.g., HYDICE)
A spectral separation unit (when covering more than one spectral region), e.g., by dichroic beam splitters or in-field separation. A dedicated
example for the latter is the HRIS in-field separation concept which is
unique in providing a single pixel separation in along-track direction
Following the spectrometer entrance slit, the spectrometer optics, single
or dual, consisting of a collimating system for a collimated beam onto
the spectral disperser (grating or prism(s)), and an imager optics, in a
Littrow configuration (e.g., ROSIS), or a Non-Littrow arrangement (e.g.,
Chapter 30
yield low optical throughput. The typical solution consists in using
achromatized prism doublets. Wide band single optics may necessitate a
spectral over-sampling in the 550–780 nm range to avoid saturation,
since the radiometric aperture will be defined by the lower and higher
wavelengths
Spectral and spatial co-registration accuracy (typically 0.1 to 0.2
pixels): This is another incisive parameter for the optics and FPA choice,
optical bench mechanical and thermal stability, OGSE. The preference is
for single common optics for all channels.
4.
WORKING PRINCIPLES OF IMAGING
SPECTROMETERS, EXAMPLES
Fig. 7 shows, as a typical example for a pushbroom IS, the optical
scheme of the HRIS instrument. For simplicity reasons, only the SWIR path
of the spectrometer is presented.
The main sub-units, following the optical and electronics path, are briefly
summarized below:
A pointing unit (or scan unit for whiskbroom IS) to provide a variable
swath (except large FOV instruments such as MERIS, MODIS, ROSIS,
where an along-track tilt of the swath is partly introduced to avoid Sun
glint over water bodies) or along-track pointing for BRDF measurements
(e.g., PRISM), mostly used also to access in-flight and external calibration references; critical for two-axis pointing in terms of mechanisms
A common front optics or telescope or a set of modular optics (e.g.,
MERIS), providing the TFOV, the required geometrical (EFL) and radiometric resolution via the performance compliant effective aperture,
focal length and/or or f-number. The front optics can be a lens system
for limited spectral ranges (see Fig. 4 for MERIS), or a reflective mirror
optics for wider spectral ranges and high throughput (see Fig. 7 for
HRIS), mostly combined with the need of some aspheric corrector
elements, or mixed catadioptric systems (e.g., HYDICE)
A spectral separation unit (when covering more than one spectral region), e.g., by dichroic beam splitters or in-field separation. A dedicated
example for the latter is the HRIS in-field separation concept which is
unique in providing a single pixel separation in along-track direction
Following the spectrometer entrance slit, the spectrometer optics, single
or dual, consisting of a collimating system for a collimated beam onto
the spectral disperser (grating or prism(s)), and an imager optics, in a
Littrow configuration (e.g., ROSIS), or a Non-Littrow arrangement (e.g.,