ERMIS is a new high resolution spectroradiometer to measure the total column ozone concentration in the UV/VIS spectral region. The key feature of the spectroradiometer is the combination of a Czerny-Turner design with a rotating grating and a CCD detector, having a resolution of 0.25 nm over the 295 – 660 nm spectral range.
A new high resolution spectroradiometer (ERMIS, Figure 1) has been developed in the framework of the EMRP joint research project, ATMOZ (“Traceability for Total Column Ozone”). The primary objective of ERMIS is the retrieval of the total column ozone amount using measurements of direct solar irradiance in combination with a retrieval model based on the Beer-Lambert law and a DOAS technique.
ERMIS was designed with the objective of allowing not only the retrieval of total column ozone, but to be versatile enough to also retrieve other trace gas concentrations such as nitrogen dioxide (NO2), oxygen dimer (O4) and water vapour (H2O) in the 400 – 670 nm visible range. Therefore, a nominal resolution of < 0.3 nm, defined as the full-width-at-half-maximum (FWHM) was chosen. To achieve the required resolution, a Czerny-Turner design was selected with a rotating grating. To reduce the aberrations in the system, a parabolic mirror is used as a collimating mirror, while a custom-made spherical mirror is used as the focusing mirror. The high resolution and large oversampling of the detector is obtained by using a diffraction grating with 1800 lines.mm-1, dispersing a wavelength range of ~95 to 65 nm across the detector (Hamamatsu-S101411108) depending on the grating position.
Figure 1. The ERMIS spectroradiometer.
In order to minimise the noise level, the detector is cooled down to -20°C using a three-stage cooling system. The spectroradiometer is filled with N2 and pressurised to 1.5 bar to avoid condensation on the detector and deterioration of the optical components. The solar radiation is collected through a telescope with a 1° field-of-view which images the radiation onto an optical fibre. The radiation exiting the optical fibre is fed through two filter-wheels. The first one contains neutral density filters to attenuate the radiation by up to a factor of 103, while the second filter-wheel contains several bandpass filters to select the optimal wavelength range depending on the selected measurement type. The radiation entering the monochromator is focussed onto the entrance slit of 60 mm width and 1 mm height. The parabolic mirror, the grating and the spherical mirror are carefully aligned to achieve the theoretical spectral resolution given by the ZEMAX simulation of the ERMIS monochromator. A further degree of freedom during the alignment process is the ability to rotate the detector around its vertical axis, which results in a nearly constant bandpass across its wavelength range.
ERMIS was characterised with the PMOD/WRC ATLAS tunable laser system. The wavelength range in the default grating position is 294 – 390 nm with FWHM = 0.23 nm ± 0.01 nm (2s) which remains constant over the 295 – 660 nm wavelength range. The over-sampling is 6 – 12 pixels within the FWHM of the slit function. The presence of dispersed light reflection of about 10-3 is evident which will be accounted for by applying the measured stray-light correction. ERMIS will be in operation in 2017 and will participate in the RBCC-E XII (Huelva, Spain) campaign where the ability to retrieve ozone will be validated. A prototype version of the system participated in the ATMOZ campaign at Izana, Spain in September 2016 where the ozone agreement was within ±5 DU against the RBCC-E triad of reference instruments.