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Optical Throughput Monitoring

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The OBM contains a neutral density filter (NDF) for which needs to be corrected for those solar measurements where the NDF is in the beam. This is done by dividing the recorded signal with the throughput of the NDF.

4.7 Optical Throughput Monitoring

Contrary to system monitoring, optical performance monitoring aims at assuring an as complete as possible description of the optical performance in order to correct for degradation effects throughout the instrument’s lifetime. Therefore, monitoring serves as a general prerequisite for continuous high data product quality. Monitoring related to the optical performance of SCIAMACHY is to a large degree linked to the instrument calibration & characterisation status. It establishes in-flight information which permits proper application of on-ground calibrations and modelling of the in-orbit environment.
Optical component degradation monitoring is one of the main long-term monitoring activities to be performed over the mission’s lifetime (Noël et al. 2003). It applies regular trend analyses to measurement data obtained with the internal WLS and of observations of the unobscured sun above the atmosphere. In order to monitor the different SCIAMACHY light paths, solar measurements are taken in various viewing geometries: in limb/occultation geometry (via ASM and ESM mirrors), in nadir geometry (via the ESM mirror through the sub-solar port), and via the so-called ‘calibration light path’ involving the ASM mirror and the ESM diffuser. Particularly the WLS produces a rather stable output over time – except for some degradation in channel 1 – which makes it well suited for throughput monitoring. (fig. 4-6)

 

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fig. 4-6:

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Schematic view of SCIAMACHY light paths used in performance monitoring. (Graphics: IUP-IFE, University of Bremen)
 

Because of the status of the operational data processing in the initial part of the mission, the monitoring of the optical performance of the SCIAMACHY instrument had to be based on the analysis of Level 0 data – which had been corrected for dead/bad pixels, dark current (fixed value from August 2002), scan angle dependencies, quantum efficiency changes and the seasonally varying distance to the sun – and not the fully calibrated level 1b spectra as originally envisaged. Due to this approach additional calibration corrections, e.g. stray light correction, were not applied. Therefore, optical throughput variations smaller than about 1% require careful investigation. However, once the light path monitoring can be based on fully calibrated data, it will yield so called m-factors to describe how the individual light paths degrade. These m-factors will be fed into operational data processing to ensure that the measured signals are fully matched to the performance of SCIAMACHY.

Moreover, from the combination of the results for the different light paths it will be possible to derive information about the degradation of individual optical components (mirrors and diffusers, see fig. 4-6). The degradation of the ASM mirror, for example, may be determined from the ratio of the limb to the nadir light path degradation. To determine the ESM mirror degradation it is necessary to combine the limb light path results with dedicated measurements involving the extra mirror which is located inside the instrument, only rarely used and thus assumed not to degrade. Finally, the degradation of the ESM diffuser can be computed from the combination of the nadir, limb, and calibration light path. A comparison of the limb and nadir light path monitoring results indicates that the major degrading element in the SCIAMACHY optical train seems to be the ESM mirror which shows around 50% degradation over 7 years in the UV (channel average).


An example for the wavelength dependence of the instrument degradation is depicted in fig. 4-7. It displays for channel 2 the relative variation of the nadir throughput – light enters the spectrometer via the ESM mirror only – as a function of time and wavelength, based on internal WLS measurements. The available measurement data have been interpolated to a daily grid. Times of reduced instrument performance (like switch-offs or decontamination periods) as well as dead/bad pixels have been masked out (gray bars). All measured signals are referenced to August 2nd, 2002 at about orbit 2200. The degradation is clearly wavelength dependent: In channel 2 (fig. 4-7) degradation can be identified which peaks at spectral regions of high polarisation sensitivity and in the overlap regions between the channels. However, the channel 4 monitoring data presented in Fig. 4-8 show the excellent absolute radiometric stability of SCIAMACHY. The degradation in this channel stays mostly within 2%, except for the channel overlaps. A similar trend can be observed in the other channels in the visible range. Spectrally dependent throughput monitoring may become an additional operational task once the operational availability of the required input products is ensured. (fig. 4-7, fig. 4-8)

 

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fig. 4-7:

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Monitoring results for the nadir light path using measurements with the internal WLS in channel 2. All available data have been interpolated to a daily grid. (Graphics: IUP-IFE, University of Bremen)
 


fig. 4-8:

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As fig. 4-7, but results for channel 4. (Graphics: IUP-IFE, University of Bremen)
 

 

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