Fast imaging spectroscopy with MSDP spectrometers. Vector magnetic maps with THEMIS/MSDP

Abstract

Context. Multichannel subtractive double pass (MSDP) spectrometers produce 3D data cubes (x, y, λ) simultaneously across several line profiles. They do not suffer from image convolution by any slit width, and synchronous observations across all wavelengths avoid differential seeing effects. They are very suitable for fast 2D spectroscopy. Aims.(1) We review specifications and capabilities of some existing MSDP spectrometers with respect to high-cadence observations. (2) THEMIS/MSDP is designed for the spectropolarimetry of strong lines. We propose new data reductions also suitable for the spectropolarimetry of photospheric lines.Methods. An off-line algorithm is described as a way to increase the spectral resolution. Taking the opportunity of 3D data, spatial interpolations are used around each solar point by only assuming that intensity gradients ∂I(x, y, λ)/∂x are constant in the range (x ± x , λ ± λ ). The UNNOFIT inversion is used to compare vector magnetic maps deduced from THEMIS/MSDP and slitspectropolarimetry THEMIS/MTR data. Results. Both results are in good agreement. In active regions, the rms of the MSDP noise, calculated over 1 arcsec 2 , is less than 24 G for the LOS magnetic field and less than 52 G for B x and 32 G for B y . The MSDP scanning speed is 10 times the speed of slit-spectropolarimetry. Conclusions. THEMIS/MSDP can provide vector magnetic maps with typical temporal resolutions that are less than 1 min for small fields-of-view and 10 min for active regions. This allows addressing a number of fast events. In the future, MSDP instruments should efficiently complement single-slit spectroscopy and tunable filters. Their main capabilities should be the multiline aspect and the high temporal and spatial resolutions. New optical devices, such as image slicers, should substantially increase the signal-to-noise ratio. For polarimetric measurements, various compromises are possible between speed, spatial resolution, and SNR. A-posteriori image restorations, either using wide band proxies or bursts of multi-wavelength short exposures, should help improving signal-to-noise ratio and spatial resolution.

New generation multichannel subtractive double pass for EST imaging spectropolarimetry

“…We can triple the resolution if we make three successive images with a displacement of the slicer corresponding to one third blade thickness. And so on ... Other not explained here methods can increase the spectral resolution with conservation of simultaneous observations at all wavelengths [38].…”

“…The speed of 2D spectro-polarimetry is a very powerful way to observe magnetic field evolutions across large solar areas [38]. This is one of the main prospects for EST/MSDP.…”

Method of composing two-dimensional scanned spectra observed by the New Vacuum Solar Telescope

“…But its shortcoming is that the observed field-of-view (FOV) is inversely proportional to the spectral resolution, which means only a moderate spectral resolution can be achieved for a reasonable FOV. For instance, the MSDP installed in Meudon Solar Tower can provide images at nine wavelength points simultaneously with an FOV of 71 ′′ (perpendicular to the slit direction) × 465 ′′ 42-2 Y.-F. Cai et al: Composing Method for the Two-dimensional Scanning Spectra (slit direction) and the wavelength distance is about 0.3 Å (Mein et al 2009). Spatial scanning is the method usually utilized together with a slit spectrograph.…”

The Influence of Filter Slit on the Imaging Performance of the Solar Grating Spectrometer Based on Adaptive Optics

“…It is well known that the evolving magnetic field causes the solar atmosphere eruption, and it produces most of the sometimes spectacular visible phenomena, such as the sunspots, prominences, flares, coronal mass ejections and so on [1] [2]. Real-time observation and forecast of the solar atmosphere activities are very important since the violent activities of the solar atmosphere and period variation will influence the living environment of human being, and the imaging grating spectrometer is an important tool to achieve this goal [3]- [5]. It not only can perform the imaging observation, but also can perform the spectral observation and identify the thermodynamics parameters of the solar atmosphere by the spectral information, such as the magnetic field, pressure and element abundance.…”