GRIS: spectropolarimetry in the near Infrared

GRIS (GREGOR Infrared Spectrograph), the grating spectrograph is installed in the 1.5-meter GREGOR telescope located at the Observatorio del Teide in Tenerife. The spectrograph has a standard Czerny-Turner configuration with parabolic collimator and camera mirrors that belong to the same conic surface. Although nothing prevents its use at visible wavelengths, the spectrograph is mostly used in combination with the infrared detector of the Tenerife Infrared Polarimeter (TIP-II) in standard spectroscopic mode as well as for spectropolarimetric measurements. A slit scanner allows to scan some 60 arcsec. The slit length also corresponds to some 60 arcsec, with a sampling of 0.13 arcsec.

It operates in the range 1.0–2.3 μm. The experience at the German Vacuum Tower Telescope (VTT), located at the same observatory, demonstrated that the most used spectral regions are those at 1.083μm (including the photospheric Si I and the chromospheric triplet He I) and at 1.565μm (with a number of iron lines including a line with g=3). As an example for the 1.565μm region: Five accumulations of 30ms exposure time each in the 4 polarimetric states yields a signal-to-noise ratio of 1000. In this case one scan step takes 3 sec.

Examples of GRIS scans from previous years can be viewed at the GRIS data archive.

GRIS is operated by the IAC. Manolo Collados is the Principal Investigator of GRIS.

GFPI: Imaging spectroscopy

GREGOR Fabry-Pérot Interferometer (GFPI)

Fast processes on the Sun require instruments capable of acquiring data in a time span comparable to the evolution time-scale of solar features, which is on the order of minutes and sometimes even on the order of seconds. Imaging spectropolarimetry with the GFPI (Puschmann et al. 2012, 2013) is ideally suited for this type of application. The instrument comprises two tunable etalons in collimated mounting, which provide a spectral resolution of ℜ ≈ 250.000. Scanning a spectral line takes a few tens of seconds to from a few minutes depending on the sampling, the number of images acquired per wavelength position, and the observing mode (spectroscopy vs. polarimetry). Two cameras with 1376 × 1024 pixel acquire images strictly simultaneously in the narrow- and broad-band channels to facilitate post-facto image restoration including simple destretching, speckle masking imaging and deconvolution, and multi-object mult-frame blind deconvolution (MOMFBD). The field-of-view (FOV) is 50″ × 38″ in the spectroscopic observing mode and about half the size for polarimetric observations. Small sunspots and substantial portions of active regions can thus be observed. The coatings of the etalons are optimized for the wavelength range 530–860 nm, while the polarimeter limits the spectral observing window to 580–660 nm. Many interesting photospheric and chromospheric spectral lines are accessible, and two of them can be observed sequentially.

GFPI can observe simultaneously with GRIS. Then the pentaprism beamsplitter feeds GFPI with wavelengths < 650 nm.

All GFPI data (i.e., raw data and high-level data products) are stored in the GREGOR archive at AIP, where they can be accessed and queried on this website (at the moment access is limited to GREGOR partners).

PI Institute: Leibniz Institute for Astrophysics Potsdam (AIP)
Contact: Carsten Denker

 

Blue Imaging Channel (BIC)

High-cadence imaging provides important context information for the  GFPI observations. Small-scale magnetic features are more more easily detected in particular wavelength regions, e.g., the Fraunhofer G-band. Furthermore, the broad Calcium H and K lines offer chromospheric diagnostics.  Two CCD cameras (pco.4000 and pco.sensicam) can be used in the BIC of the GFPI depending on the availability of the GREGOR/VTT facility cameras. Three interference filters (Ca II H λ396.8 nm, G-band λ430.7 nm, and blue continuum λ450.8 nm) are available. Recently, MPS contributed a 0.1-nanometer-wide Ca II  H λ396.8 nm interference filter of the Sunrise mission. The maximum FOV of the pco.4000 CCD cameras is limited to 75″ × 93″ (2160 × 2672 pixel) because of dichroic pentaprism splitting the light between GFPI and BIC. The FOV of the pco.sensicam CCD camera is much smaller 34″ × 26″ (1376 × 1040 pixel) but the data acquisition rate is much higher (eight vs. three frames per second). However, in both cases post-facto image restoration is feasible (Knox-Thompson, speckle masking imaging, and multi-frame blind deconvolution). In 2016, new (faster) cameras will be integrated.

All BIC data (i.e., raw data and high-level data products) are stored in the GREGOR archive at AIP, where they can be accessed and queried on this website (at the moment access is limited to GREGOR partners).

PI Institute: Leibniz Institute for Astrophysics Potsdam (AIP)
Contact: Carsten Denker