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

¹ Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, 5 Place Jules Janssen, 92195 Meudon, France e-mail: Pierre.Mein@obspm.fr
² Laboratoire d'Étude du Rayonnement et de la Matière en Astrophysique (LERMA), Observatoire de Paris, CNRS UMR 8112, 5 Place Jules Janssen, 92195 Meudon, France

Received: 8 November 2008
Accepted: 16 July 2009

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 (x, y, λ)/ are constant in the range . The UNNOFIT inversion is used to compare vector magnetic maps deduced from THEMIS/MSDP and slit-spectropolarimetry THEMIS/MTR data.

Results. Both results are in good agreement. In active regions, the rms of the MSDP noise, calculated over 1 arcsec², 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.