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Optical static Fourier spectrometer

OPTICAL STATIC FOURIER SPECTROMETER FOR "WARNING" PROJECT

Static Fourier spectrometers are often used for spectral measurements of the ionosphere emissions. There are two main advantages of Fourier spectrometers (FS) - large geometric factor and high spectral resolution; these parameters are two orders better than for grating spectral instruments. These advantages are very important for analysis of the shape of spectral lines for low level light. In our case the wide free spectral range is main parameters for observations of very faint source with moderate spectral resolution. Echelle spectrometers give broad free spectral range but geometric factors are low. Contrary, Fabry-Perot interferometers have aperture ratios as the Fourier interferometer under limited free spectral range. Only Fourier interferometer satisfies all requirements for the measurements of the emissions of night ionosphere in the spectral range 420 - 700 nm with spectral resolution of 0.5 nm.

Static Fourier spectrometer is designed without any moving part, it is a real advantage for onboard instruments. But static spectrometer requires the use of arrays as detectors. The number of sensitive elements m in array must be twice as large as the number of resolved spectral elements due to the Nyquist theorem, i.e. m = 2R, where R is spectral resolution. In practice it is better to have m >3R for increasing S/N ratio due to decrease secondary maximum of interferograms and for shifting of readout noise to high frequency. For getting spectral resolution R = 500 it is necessary to use the array with m>1500 pixels.

Spatial variations of emissions can be registered for a spectral range with CCD imaging device. One dimension of the CCD array is used then for spatial coordinates, the second – for wavelength. Spatial resolution for altitude must be 2 - 4 km in the 70 -150 km limits, therefor the number of pixels for spatial coordinates needs to be 40 - 20. There are no CCD arrays with 1500x40 pixels (usually the arrays are square-shaped or close to it). It is necessary to use method of parallel simultaneously readout of several lines in CCD array of standard format at once. Such method is possible and is better than readout method line by line with subsequent adding signal for parallel pixels due to increasing of S/N ratio.

The main disadvantage of Fourier spectrometers is the presence in output signal of constant level equal I/2, where I is average input intensity. This constant signal results in a high level of Schottky noise of detector. In scanning Fourier spectrometer optical and electric modulation of signal is used to avoid this problem, for static Fourier spectrometer there are no such ways. Estimation of the Schottky noise level gives limit for threshold light intensity » 80 Rayleighs for spectral range 420-700 nm in the case of limb observations and » 1.5 Rayleighs for nadir.
Block - scheme of static Fourier spectrometers is shown in Fig. 3.

Fig.3 Schematic diagram of static FS optical system: (a)-limb channel, (b)-nadir channel.

Gregory telescope is used as entry optic in limb channel. Interferometer is built as a single unit on the base of 90 degree prisms with secondary mirror for increasing mechanical stability of device. FOV diaphragm is placed onto surface of prism as a hole in mirror film. Optical filter is glued onto the entry surface of first prism. Flat scanning mirror allows to compensate inaccuracy of the orientation system of vehicle and to scan the FOV in orbit plane within the limit +10to-40 degree to the direction of the limb. Such scanning is useful for restoring by tomography methods the emission intensity altitude distributions. Collector lens is focusing the interference picture on CCD detector; cylinderlens forms the image parallel to the Earth's limb. Diameter of the primary mirror is 100 mm, focal distance - 250 mm, geometric factor (Aef x W) is 4 cm2sr., FOV is 1.6 x1.6 degree.

Nadir channel is equipped with two band filters - 420-700 nm and 700-900 nm without telescope optics, FOV is 7.2 x 7.2 degree. Flat scanning mirror is used to measure spatial distributions of emissions intensity in direction perpendicular to the vehicle orbit track. The scheme of scanning is shown in Fig. 4.

Fig.4 The view geometry of the FS nadir channel. The height layer of emission is 100 km.

Therefore, CCD imaging detector used in Fourier spectrometers allows measuring spatial (altitude) distribution of emission intensity for one dimension within the FOV. Fig. 5 illustrates this conception for limb channel.

Fig.5 Schematic diagram window placement CCD limb channel of FS.

Ivchenko V.N., Lapchuk V. P. Optical measurements in the project "Poperedzhennya" ("Warning"). Proceeding SPIE, vol. 3237, p.44-49.

Laboratory model of static Fourier spectrometer

CAMERA ELECTRONICS FOR THE CCD FOURIER SPECTROMETER.

CCD table PMT table Photometer Night glow emissions

1 Dec. 1998
Last update 7 Feb. 2000