324
Chapter 30
optics, FPA, electronics—to an airborne demonstrator. In the course of this
program, it turned out that the high-resolution TMA (Three Mirror
Anastigmat) telescope, in its final assembling stage at Carl Zeiss, does not
make sense for aircraft missions since it would yield much too high spatial
resolution, and at least in SWIR this would be sacrificed by a too small
swath width. Thus, our co-author E. Schmidt designed a new medium FoV
optics schematically shown in Fig. 8, to fit to the same spectrometer optics.
As can be seen, the new telescope is comparatively small, but in order to get
the telescope focus via a 200 mm back focal length into the existing
spectrometer entrance, a massy Offner relay optics had to be introduced.
Here, the wide spectral range was a design driver in view of the necessity to
design all-reflective optics.
A rather peculiar and interesting IS represents the airborne HYDICE, see
Fig. 6. It utilizes a single hybrid InSb detector array for the total spectral
range from 400 to 2450 nm, with two different dotation (passivation) zones.
This advanced array is currently not available outside the USA. A butting to
about 1000 spatial pixels is currently not planned.
The PRISM instrument for the ESA Earth Explorer Programme, as
sketched in Fig. 5, is peculiar with respect to its ambitious calibration
scheme, induced by the rather tight ESA mission and performance
requirements in terms of radiometric accuracy. The in-flight calibration
devices are accessed by a pointing mirror rotated in the optical axis for
across-track targets access, which also allows to point the optical entrance
beam to cold space for the TIR channels, the Sun (seen through an aperture
plate with a set of tiny holes, illuminating some 50 pixels each, and also
occasionally to the moon (its value for on-board calibration is subject of an
on-going debate among the experts). The calibration subsystems include:
the mentioned aperture plate with a pattern of thin holes for absolute
calibration of the VNIR and SWIR channels, used against the Sun over
the equator, also providing a section for dark current measurements
a protected rotatable diffuser against Sun for relative calibration of the
total FoV in relation to the apertures
a cavity IR blackbody for the two TIR channels, floating temperature,
but precisely measured
optional internal laser sources for calibration of the wavelength location
on the arrays.
In earlier phases of the PRISM studies, a two-axis pointing unit had been
included for along-track pointing as well as cross-track to enable
multidirectional reflectance function (MDRF) measurements. In current
concepts, the along-track pointing is left to the platform attitude control. The
MDRF measurements have to be paid by long passes of more than 2500 km
(angle dependent) without other target areas access, thus, it is primarily
Chapter 30
optics, FPA, electronics—to an airborne demonstrator. In the course of this
program, it turned out that the high-resolution TMA (Three Mirror
Anastigmat) telescope, in its final assembling stage at Carl Zeiss, does not
make sense for aircraft missions since it would yield much too high spatial
resolution, and at least in SWIR this would be sacrificed by a too small
swath width. Thus, our co-author E. Schmidt designed a new medium FoV
optics schematically shown in Fig. 8, to fit to the same spectrometer optics.
As can be seen, the new telescope is comparatively small, but in order to get
the telescope focus via a 200 mm back focal length into the existing
spectrometer entrance, a massy Offner relay optics had to be introduced.
Here, the wide spectral range was a design driver in view of the necessity to
design all-reflective optics.
A rather peculiar and interesting IS represents the airborne HYDICE, see
Fig. 6. It utilizes a single hybrid InSb detector array for the total spectral
range from 400 to 2450 nm, with two different dotation (passivation) zones.
This advanced array is currently not available outside the USA. A butting to
about 1000 spatial pixels is currently not planned.
The PRISM instrument for the ESA Earth Explorer Programme, as
sketched in Fig. 5, is peculiar with respect to its ambitious calibration
scheme, induced by the rather tight ESA mission and performance
requirements in terms of radiometric accuracy. The in-flight calibration
devices are accessed by a pointing mirror rotated in the optical axis for
across-track targets access, which also allows to point the optical entrance
beam to cold space for the TIR channels, the Sun (seen through an aperture
plate with a set of tiny holes, illuminating some 50 pixels each, and also
occasionally to the moon (its value for on-board calibration is subject of an
on-going debate among the experts). The calibration subsystems include:
the mentioned aperture plate with a pattern of thin holes for absolute
calibration of the VNIR and SWIR channels, used against the Sun over
the equator, also providing a section for dark current measurements
a protected rotatable diffuser against Sun for relative calibration of the
total FoV in relation to the apertures
a cavity IR blackbody for the two TIR channels, floating temperature,
but precisely measured
optional internal laser sources for calibration of the wavelength location
on the arrays.
In earlier phases of the PRISM studies, a two-axis pointing unit had been
included for along-track pointing as well as cross-track to enable
multidirectional reflectance function (MDRF) measurements. In current
concepts, the along-track pointing is left to the platform attitude control. The
MDRF measurements have to be paid by long passes of more than 2500 km
(angle dependent) without other target areas access, thus, it is primarily