TY - JOUR
T1 - In-flight calibration of the HIFI diplexers
AU - Mueller, Michael
AU - Jellema, Willem
AU - Delforge, Bertrand
AU - Teyssier, David
AU - Eggens, Martin
AU - Huisman, Robert
AU - Keizer, Geert
PY - 2014/7/1
Y1 - 2014/7/1
N2 - HIFI is a heterodyne spectrometer aboard the Herschel Space Observatory,
providing high-spectral-resolution capabilities. Of its seven frequency
bands, four (bands 3, 4, 6, and 7) employ Martin-Puplett diplexers to
combine sky signal and local oscillator at the two linear polarizations
H and V, prior to feeding them into the mixers (receivers). The optical
path difference in each of these 8 diplexers must be tuned to the
observed frequency. The required actuator currents were determined in
flight before the start of routine science observations. We here report
on regular (roughly quarterly) engineering test observations to validate
the repeatability of the HIFI diplexers during the routine phase of
Herschel operations. We find the optical path difference to be stable to
within 0.4 % of the relevant wavelength, typically at the sub-micron
level. We conclude that the repeatability and precision of the diplexer
tuning mechanism are so high that science data are in no way negatively
affected. With the diplexer calibration established and validated, this
line of reasoning can be reversed, and the diplexers can be used as
relative spectrometers to measure the local-oscillator frequency, i.e.,
to check the spectral purity of the local oscillator across the diplexer
bands. This was done from before launch out to the last months of
cryogenic operations in space.
AB - HIFI is a heterodyne spectrometer aboard the Herschel Space Observatory,
providing high-spectral-resolution capabilities. Of its seven frequency
bands, four (bands 3, 4, 6, and 7) employ Martin-Puplett diplexers to
combine sky signal and local oscillator at the two linear polarizations
H and V, prior to feeding them into the mixers (receivers). The optical
path difference in each of these 8 diplexers must be tuned to the
observed frequency. The required actuator currents were determined in
flight before the start of routine science observations. We here report
on regular (roughly quarterly) engineering test observations to validate
the repeatability of the HIFI diplexers during the routine phase of
Herschel operations. We find the optical path difference to be stable to
within 0.4 % of the relevant wavelength, typically at the sub-micron
level. We conclude that the repeatability and precision of the diplexer
tuning mechanism are so high that science data are in no way negatively
affected. With the diplexer calibration established and validated, this
line of reasoning can be reversed, and the diplexers can be used as
relative spectrometers to measure the local-oscillator frequency, i.e.,
to check the spectral purity of the local oscillator across the diplexer
bands. This was done from before launch out to the last months of
cryogenic operations in space.
KW - Herschel space observatory
KW - HIFI
KW - Heterodyne
KW - Far-infrared
KW - Instrumentation
KW - Calibration
UR - http://adsabs.harvard.edu/abs/2014ExA....37..369M
U2 - 10.1007/s10686-013-9370-1
DO - 10.1007/s10686-013-9370-1
M3 - Article
VL - 37
SP - 369
EP - 379
JO - Experimental Astronomy
JF - Experimental Astronomy
SN - 0922-6435
IS - 2
ER -