Stray-light characterisation of the GLORIA-Lite limb sounder

GLORIA-Lite is a newly developed lightweight imaging Fourier transform spectrometer (IFTS)
developed by IMKASF at KIT and FZ Jülich. It measures thermal infrared emissions of atmospheric
components in limb geometry to retrieve temperature and trace gas concentration profiles with
high spatial resolution. The instrument features a compact and versatile design by using a double
pendulum Michelson interferometer and a monolithic core unit that incorporates the beam splitter
and retroreflectors that were aligned together in the manufacturing process. This leads to athermal
... mehr
behaviour of the interferometer as key feature. With a mass of 28kg and a space demand of 20l
GLORIA-Lite is significantly smaller and simpler as its predecessor GLORIA. The current version
of GLORIA-Lite acts as basis for a new generation of compact IFTS instruments suited for a wide range of atmospheric research applications, including future operation on research aircraft,
long-duration balloons and small satellite platforms.
Stray light by definition is unwanted light within an optical system. The presence of stray light reduces measurement accuracy and affects the final vertical profiles by bringing additional radiation
from other atmospheric regions to the detector. Thus, stray light characterisation is an important part of the further instrument development. The methods used comprise both a analysis of the data gained during the first balloon campaign and extensive laboratory measurements. During the TRANSAT campaign, GLORIA-Lite performed its maiden flight on a stratospheric balloon at altitudes up to 40km. During the flight, it obtained atmospheric measurements and demonstrated functionality under campaign conditions. The experimental setup used an uncollimated blackbody with chopper modulation, allowing precise variation of azimuth and elevation angles of the incoming radiation. Data processing was done by a digital lock-in algorithm using Fourier analysis to separate the modulated signal from background radiation. The results gained from the flight data provided an estimate for the amount of stray light present in the optical system of 5nW/cm2 srcm−1 and showed stray light artifacts caused by the sun. The laboratory measurements could reproduce the effects seen in the flight. The measurement results
identified the main stray light effects and their causes. Most of the stray light originates from ghost reflections caused by the wedged beam splitter, double pass reflections within the interferometer
and ghost-like reflections from radiation incident at elevations more than 6◦ above the line of sight (LOS). Other spot-like artifacts from radiation incident at large horizontal angles are assigned to
reflections within the interferometer housing. Effects of aperture diffraction were also assessed but this turned out to play a minor role. The point source transmittance (PST) measurement enabled a
first instrument model including stray light over ±10◦ in azimuth and elevation, showing that most stray light originates within ±4◦ of the LOS. Forward simulations with atmospheric radiance data supported the stray light magnitude derived from flight data. Design optimization recommendations include reducing reflectivity within the rotating cylinder of the pointing optic module, coating or baffling reflective surfaces in the interferometer housing, minimizing the pass-throughs between the modules, and adding a stray light baffle at the pointing optic module (POM) entrance. These measures follow a “coat or block” strategy to mitigate stray light without major redesigns.

GLORIA-Lite is a newly developed lightweight imaging Fourier transform spectrometer (IFTS)
developed by IMKASF at KIT and FZ Jülich. It measures thermal infrared emissions of atmospheric
components in limb geometry to retrieve temperature and trace gas concentration profiles with
high spatial resolution. The instrument features a compact and versatile design by using a double
pendulum Michelson interferometer and a monolithic core unit that incorporates the beam splitter
and retroreflectors that were aligned together in the manufacturing process. This leads to athermal
... mehr
behaviour of the interferometer as key feature. With a mass of 28kg and a space demand of 20l
GLORIA-Lite is significantly smaller and simpler as its predecessor GLORIA. The current version
of GLORIA-Lite acts as basis for a new generation of compact IFTS instruments suited for a
wide range of atmospheric research applications, including future operation on research aircraft,
long-duration balloons and small satellite platforms.
Stray light by definition is unwanted light within an optical system. The presence of stray light
reduces measurement accuracy and affects the final vertical profiles by bringing additional radiation
from other atmospheric regions to the detector. Thus, stray light characterisation is an important
part of the further instrument development.
The methods used comprise both a analysis of the data gained during the first balloon campaign and
extensive laboratory measurements. During the TRANSAT campaign, GLORIA-Lite performed
its maiden flight on a stratospheric balloon at altitudes up to 40km. During the flight, it obtained
atmospheric measurements and demonstrated functionality under campaign conditions. The experimental
setup used an uncollimated blackbody with chopper modulation, allowing precise variation
of azimuth and elevation angles of the incoming radiation. Data processing was done by a digital
lock-in algorithm using Fourier analysis to separate the modulated signal from background radiation.
The results gained from the flight data provided an estimate for the amount of stray light present in
the optical system of 5nW/cm2 srcm−1 and showed stray light artifacts caused by the sun. The
laboratory measurements could reproduce the effects seen in the flight. The measurement results
identified the main stray light effects and their causes. Most of the stray light originates from ghost
reflections caused by the wedged beam splitter, double pass reflections within the interferometer
and ghost-like reflections from radiation incident at elevations more than 6◦ above the line of sight
(LOS). Other spot-like artifacts from radiation incident at large horizontal angles are assigned to
reflections within the interferometer housing. Effects of aperture diffraction were also assessed but
this turned out to play a minor role. The point source transmittance (PST) measurement enabled a
first instrument model including stray light over ±10◦ in azimuth and elevation, showing that most
stray light originates within ±4◦ of the LOS. Forward simulations with atmospheric radiance data
supported the stray light magnitude derived from flight data.
Design optimization recommendations include reducing reflectivity within the rotating cylinder
of the pointing optic module, coating or baffling reflective surfaces in the interferometer housing,
minimizing the pass-throughs between the modules, and adding a stray light baffle at the pointing
optic module (POM) entrance. These measures follow a “coat or block” strategy to mitigate stray
light without major redesigns.