Prediction model for diffuser-induced spectral features in imaging spectrometers

Wide-field spectrometers for Earth observation missions require in-flight radiometric calibration for which the Sun can be used as a known reference. Therefore, a diffuser is placed in front of the spectrometer in order to scatter the incoming light into the entrance slit and provide homogeneous illumination. The diffuser, however, introduces interference patterns known as speckles into the system, yielding potentially significant intensity variations at the detector plane, called spectral features. There have been several approaches implemented to characterize the spectral features of a spectrometer, e.g., end-to-end measurements with representative instruments. Additionally, in previous publications a measurement technique was proposed, which is based on the acquisition of monochromatic speckles in the entrance slit following a numerical propagation through the disperser to the detection plane. Based on this measurement technique, we present a stand-alone prediction model for the magnitude of spectral features in imaging spectrometers, requiring only few input parameters and, therefore, mitigating the need for expensive measurement campaigns.

Prediction Model for Diffuser Induced Spectral Features in Imaging Spectrometers

Wide-field spectrometers for Earth Observation missions require inflight radiometric calibration, for which the sun can be used as a known reference. Therefor a diffuser is placed in front of the spectrometer in order to scatter the incoming light into the entrance slit and provide homogeneous illumination. The diffuser however, introduces interference patterns known as speckles into the system, yielding potentially significant intensity variations at the detector plane, called Spectral Features. There have been several approaches implemented to characterize the Spectral Features of a spectrometer, e.g. end-to-end measurements with representative instruments. Additionally, in previous publications a measurement technique was proposed, which is based on the acquisition of monochromatic speckles in the entrance slit following a numerical propagation through the disperser to the detection plane. Based on this measurement technique we present a standalone prediction model for the magnitude of Spectral Features in imaging spectrometers, requiring only few input parameters and therefor mitigating the need for expensive measurement campaigns.

METHODS FOR IMPROVEMENT OF HIGH-RESOLUTION SPECTROMETER CHARACTERISTICS

The present paper demonstrates results of high-resolution spectrometer characteristics improvement methods. Increasing resolution, spectral range extending and illumination efficiency for the spectrometer were investigated. Obtained results will be found useful in atomic spectroscopy applications such as atomic absorption, atomic emission spectroscopy, mass-spectroscopy, chromatography and others. In order to increase spectrometer resolution it was suggested to use higher diffractive grating curvature radius. Experimentally, characteristics of both spectrometer prototypes assembled using diffractive gratings R1000 and R2000 were obtained and compared. New approach for polychromatous displacement was developed in order to extend operational spectrum range. The main feature is single spectrometer entrance slit for both UV and visible range Paschen-Runge polychromatous with beam splitting by coupled flat folding mirrors placed behind slit. Diffractive grating illumination monitor system was designed in order to provide this spectrometer by alignment control for lightning system. New spectrometer “Grand-2” was fabricated. It includes coupled Paschen-Runge polychromators for UV and visible spectral range providing 12 and 30 pm spectral resolution respectively with single entrance slit equipped with diffractive grating illumination monitoring system. This spectrometer can be used for various atomic spectroscopy applications.

Exploring short period oscillations in filaments

The authors analyse sources of false Doppler velocity signals of high frequencies (10 mHz and higher) in observations of filaments. In ground-based observations, spectrograph noise and image shifting at the spectrograph entrance slit are the main causes of the false signal. It is shown that using differential methods and telluric lines as reference lines significantly reduces the influence of the first factor. Periodical image shifting along the spectrograph slit can be compensated for during data reduction. In some cases detected high-frequency oscillations appear to be real.

Characteristics of the Belgrade Astronomical Observatory's stellar spectrograph

In this paper the main features of the new SpectraPro-750 spectrograph of Belgrade Astronomical Observatory are described. The instrumental profile of the spectrograph for the 1200 l/mm grating is determined using a fiber optic bundle with fibers arranged in a pseudo-slit pattern. This instrumental profile is compared to the instrumental profiles obtained when the same fiber optic bundle illuminates the entrance slit with different widths. From appropriate instrumental profiles the practical spectral purities and the spectral resolutions for different entrance slit widths are obtained. The variation of the reciprocal linear dispersion with wavelength in the spectral range 350 - 600 nm is determined. A proposal for a link between telescope and the spectrograph is given.