Jahresbericht 08 - PMOD/WRC

14
Instrument Development
Monitor to Determine the Integrated Transmittance of Windows (MITRA)
and the Cryogenic Solar Absolute Radiometer (CSAR)
André Fehlmann, Wolfgang Finsterle, and Werner Schmutz in collaboration with Ulrich Straumann, University of Zürich,
Peter Blattner, METAS, Rainer Winkler, Eric Usadi, David Gibbs, and Nigel Fox, NPL, England
We report on the nearly finished CSAR design phase
and the prototype status of MITRA.
The PMOD/WRC, METAS and NPL project (Fehlmann,
2008) to build a cryogenic solar absolute radiometer is progressing
well. During the last year the CSAR design was
further refined. Hardware components such as the cryogenic
cooler have been selected and ordered. Currently,
detailed drawings of the instrument are being finalized at
NPL in London. In April 2009 METAS (Bern, Switzerland)
will then start to produce the CSAR parts which we then
plan to assemble in July 2009. Figure 1 shows a drawing
of the CSAR instrument.
Figure 1. The CSAR instrument without vacuum tank. The dark gray part
on the left is the cryogenic cooler which allows the instrument to be cooled
to 20 Kelvin.
Development of MITRA
The monitor to determine the integrated transmittance of
windows has already reached prototype status (Figure 2).
MITRA has two identical cavities which are mounted on a
common heat sink via two thermal resistors. Thermopiles
form these resistances and simultaneously measure the
temperature differences between the cavities and the heatsink.
Since these temperature differences are proportional
to the solar irradiance and both cavities see the same sun,
we end up with a constant ratio of the two signals produced
by the thermopiles.
Characterization of MITRA
Experiments with MITRA in the laboratory revealed a nonconstant
ratio of the thermopile signals with changing irradiance.
This is due to thermal relaxation time constants that
are not equal for both cavities. We managed to fine-tune
the thermal resistances and thermal capacities of the instrument
to obtain a constant signal at all times.
Once MITRA has proven itself in ground-based solar measurements
we will start to measure the transmittance of our
Sapphire and Quartz windows. These windows have been
characterized at METAS in the 300-1000 nm wavelength
range and will be characterized at PMOD/WRC in the 250-
500 nm and 1000-5000 nm ranges.
As soon as the final CSAR design is fixed, we will start to
develop the definitive MITRA instrument. In doing so, special
attention will be payed so that both instruments have
the same optical setup, i.e. baffles, apertures and geometries.
Thus, the problem of stray light, which is to be characterized,
will be similar for both instruments and will not
unintentionally introduce a systematic effect.
Figure 2. The dual cavity monitor MITRA prototype. Not shown is
the instrument housing which acts as a wind-shield and thus makes
it insensitive to ambient temperature.
References: Fehlmann A., Finsterle W., Schmutz W., Straumann U.,
Blattner P., Winkler R., Gibbs D., Fox N., 2008,
Cryogenic Solar Absolute Radiometer (CSAR),
PMOD/WRC Annual Report 2007.