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

摘要

Context. Multichannel subtractive double pass (MSDP)spectrometers produce 3D data cubessimultaneously across several line profiles. They do not suffer fromimage convolution by any slit width, and synchronous observationsacross all wavelengths avoid differential seeing effects. They are verysuitable for fast 2D spectroscopy. Aims. (1) We review specifications and capabilitiesof some existing MSDP spectrometers with respect to high-cadenceobservations. (2) THEMIS/MSDP is designed for the spectropolarimetry ofstrong lines. We propose new data reductions also suitable for thespectropolarimetry of photospheric lines. Methods. An off-line algorithm is described as a wayto increase the spectral resolution. Taking the opportunity of 3D data,spatial interpolations are used around each solar point by onlyassuming that intensity gradientsare constant in the range.The UNNOFIT inversion is used to compare vector magnetic maps deducedfrom THEMIS/MSDP and slit-spectropolarimetry THEMIS/MTR data. Results. Both results are in good agreement. Inactive regions, the rms of the MSDP noise, calculated over1arcsec2, is less than 24Gfor the LOS magnetic field and less than 52G for Bxand 32G for By.The MSDP scanning speed is 10 times the speed ofslit-spectropolarimetry. Conclusions. THEMIS/MSDP can provide vector magneticmaps with typical temporal resolutions that are less than1min for small fields-of-view and 10min for activeregions. This allows addressing a number of fast events. In the future,MSDP instruments should efficiently complement single-slit spectroscopyand tunable filters. Their main capabilities should be the multilineaspect and the high temporal and spatial resolutions. New opticaldevices, such as image slicers, should substantially increase thesignal-to-noise ratio. For polarimetric measurements, variouscompromises are possible between speed, spatial resolution, and SNR.A-posteriori image restorations, either using wide band proxies orbursts of multi-wavelength short exposures, should help improvingsignal-to-noise ratio and spatial resolution. Key words: instrumentation: spectrographs- techniques: polarimetric - Sun: magnetic fields