Instruments


Ground-based Fourier-Transform Spectrometers (FTS) are used to determine zenith column amounts of trace gases like H2O, HDO, CH4, N2O, CFC-11, CFC-12, O3, NO, NO2,  HNO3, ClONO2, HCl, and HF. The used FTS are a BRUKER® IFS 120M and a BRUKER® IFS 120HR having apodized spectral resolutions of 0.0035 cm-1 and 0.0025 cm-1, respectively. Absorption spectra are measured in the spectral region from 2 micrometer to 14 micrometer using the sun or the moon as natural sources of radiation. The high spectral resolution is needed to separate lines of different species and to allow profile retrieval for some species. To make sure that the line shape is reproduced correctly by the instrument cell measurements are made and analysed with LINEFIT.

2 Fourier Transform InfraRed (FTIR) spectrometer from BRUKER® (120 M and 120 HR) and the solar tracker are shown. First of all, the principle of an FTIR is described briefly (see next figure).

An FTIR spectrometer consists of a beamsplitter which devides the incoming radiance into 2 beams. One of them is reflected by a fixed mirror or retroreflector while the other one is sent to a moving one causing a variable optical path difference. At the beamsplitter again, they recombine and interfere according to their wavelength and optical path difference. A detector measures the intensity of the interfering beam in dependence on the optical path difference. The product is an interferogram. The optical path difference is measured with a monochromatic laser. The interferogram is converted to a spectrum by a Fourier Transformation.
 
 


Fig.2: Bruker® 120 M

Specifications for Bruker® 120 HR and 120 M [in brackets, if different to 120 HR]:

Spectral range: 620 - 4800 cm-1 ; Optional: 20 - 43000 cm-1
OPD = 372 cm (reso approx. 0.002 cm-1)
[M: OPD = 250 cm (reso approx. 0.003 cm-1)]
Scan velocity typ. 2.5 cm/s (scan time about 100 s @ 250 cm opd)

Beam splitter: KBr + Coating,  30deg relative to beam axis
Beam diameter  63.5 mm
Internal sources: Glowbar, Tungsten

1. Detector chamber (for solar observations):
2 channels simultaneously, dichroic mirror:
R (80%): 2080 - 4000 cm-1, T (80%): 690 - 1610 cm-1
MCT : D* = 5.4 x 1010 [W-1cmHz1/2]
InSb   : D* = 5.2 x 1011 [W-1cmHz1/2]
NDSC - filter set in 2 Filter wheels

2. Detector chamber (for lunar observations):
MCT : D* = 6.0 x 1010 [W-1cmHz1/2]
InSb  : D* = 6.1 x 1011 [W-1cmHz1/2] with cold filter
External Detectorport

Data aquisition with transputers and PC Pentium,
OS/2®, OPUS®,
raw data (interferograms and spectra) stored on CD - ROM

Size [cm]: 320 x 160 x 100  [M: 200 x 80 x 30]
Weight: 550 kp + 70 kp (Pump) [M: 100 kp + 30 kp (Electronics)]
 
 


Fig.3: Bruker® 120 HR


Solar tracker:

The solar tracker consists of 2 rotation stages (from Newport®): one to move the entire setup in azimuthal, the other one to move the first mirror (in elevation). The position of the motor stages is calculated by a computer code. In addition, a quadrant detector is added to check the accuracy of the tracking. If the intensity is sufficient and the intensity of the elements differ the position of the beam will be corrected.
The solar tracker and the control software STRAC was made by Stefan Huster within his Diploma thesis.
 
 


Fig.4: Solar tracker


Atmospheric Trace Constituents and Remote Sensing 
Atmosphärische Spurenstoffe und Fernerkundung