Wavelength Distribution at GREGOR

The image on the right shows the wavelength distribution and instruments at GREGOR foreseen for observations during the season 2021B. Two beamsplitters are located on a rotational stage. They transmit the IR and reflect the visible. The cutoff wavelength between transmission and reflection can be selected either as 900 nm or 650 nm. The IR beam goes towards GRIS. The visible beam goes towards the AO and the fast imaging cameras. When observing with the 900 nm beamsplitter, H-alpha is available for the fast cameras, but not in the slitjaw system and vice versa for the 650 nm beamsplitter. GRIS is available in IFU mode in 2021B.

 

The wavelength for the fast imaging can be selected (depending on the available filters). Commonly used filters are: 430 nm (G-band), 450 nm (continuum), or 396 (Ca line). The list of existing filters is available here.

 

GRIS will be upgraded in 2021B and therefore is only available for about half of the observing season.

GRIS: spectropolarimetry in the near Infrared (slit or IFU)

Layout of the GRIS spectrograph. The slit and polarimeter assembly exists on two interchangeable benches, one for the regular slit and one for the IFU. Image taken from Collados et al., AN 333, 872, 2012.

GRIS (GREGOR Infrared Spectrograph) is the grating spectrograph installed at the GREGOR telescope. 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, although scanning can also be performed with the AO. The slit length also corresponds to some 60 arcsec, with a sampling of 0.13 arcsec.

 

GRIS+TIP operates in the range 1.0–2.3 μm, however the polarimeter is optimized for 1–1.3 μm and 1.5–1.8 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. The instrument paper can be found here.

 

Starting in 2018B, GRIS is available in combination with the IFU built by the IAC. For a field of view (FOV) of 3" x 6" spatial and spectral information is then obtained simultaneously. The IFU is especially suitable for fast-cadence studies of a smaller FOV (e.g. evolution of granules), but it can also be used to scan across larger regions. The technical details can be found here. Changing the setup from slit-mode to IFU-mode requires maintenance time, and therefore the setup changes are minimized and only done between the two observing seasons.

 

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.

BBI: broadband context imaging

The broadband imager consists of 2 sCMOS cameras, which are usually run at 50 Hz. Context imaging is useful to study the fast evolution of the Sun. BBI is usually run at the wavelength of G-band, TiO, or any two different filters can be selected for the 2 beams.

 

The observers are responsible for their own setup, including selecting filters and reading the manual (available before start of 2021A) for a proper operation of the instrument.

 

PI Institute: Leibniz Institute for Solar Physics

 

 

High-resolution Fast Imager (HiFI)

High-cadence imaging provides important context information for the observations. Small-scale magnetic features are easily detected in particular wavelength regions, e.g., the Fraunhofer G-band. Furthermore, the broad Ca II H and K lines offer chromospheric diagnostics.

Two Imager sCMOS cameras take simultaneous broad-band images (FWHM ≈ 10 Å) in the blue part of the spectrum below λ5200 Å. Three interference filters (Ca II H λ3968 Å, G-band λ4307 Å, and blue continuum λ4508 Å) are available of which two can be selected. The field-of view is about 70″ × 60″. Fast image acquisition at 47 Hz and frame selection (best 100 image pairs out of 500) provides well-conditioned data for image restoration with the Speckle Masking method or Multi-Object Multi-Frame Blind Deconvolution (MOMFBD). The cadence of the restored data is 20+ s depending on exposure times.

Simultaneous observations with GRIS and the Hα Imagers (see below) are possible.

The observer is responsible for the conversion of Level 0 data (raw data) to Level 1 data (standard calibration) onsite and for transfer of the Level 1 data to the GREGOR data archive at AIP (gregor.aip.de). This archive also contains corresponding Level 2 data after image restoration. Level 0 data are typically deleted because of their large data footprint once calibrated data are saved.

 

Please contact the PI Institute well before observations and read the documentation (e.g., HiFI manual).

 

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

 

M-lite Imagers: Context Hα broad-band images and narrow-band filtergrams

Two Imager M-lite 2M cameras take strictly simultaneous Hα broad-band images (interference filter with FWHM = 7.5 Å) and Hα narrow-band filter (Halle Lyot filter with FWHM = 0.6 Å, typically tuned to Hα line core). The field-of view is about 75″ × 60″. Fast image acquisition at 100 Hz and frame selection (best 100 image pairs out of 500) provides well-conditioned data for image restoration with Multi-Object Multi-Frame Blind Deconvolution (MOMFBD). The cadence of the restored data is about 6 s.

Simultaneous observations with GRIS and HiFI are possible.

The observer is responsible for the conversion of Level 0 data (raw data) to Level 1 data (standard calibration) onsite and for transfer of the Level 1 data to the GREGOR data archive at AIP (gregor.aip.de). This archive also contains corresponding Level 2 data after image restoration. Level 0 data are typically deleted because of their large data footprint once calibrated data are saved.

Please contact the PI Institute well before observations and read the documentation (HiFI manual also applies here).

 

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

 

 

GFPI: Imaging spectroscopy (not offered for 2021A)

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 mount, which provide a spectral resolution of ℜ ≈ 250.000. Scanning a spectral line takes a few tens of seconds to a few minutes depending on the sampling, the number of images acquired per wavelength position, and the observing mode (spectroscopy vs. polarimetry), however only spectroscopy is currently offered. 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 (25″ x 38″). 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. Many interesting photospheric and chromospheric spectral lines are accessible, and two of them can be observed sequentially. GFPI can observe simultaneously with GRIS.

The following restrictions exist for the 2019A season: GFPI is offered only in collaboration with AIP and only in spectroscopic mode. AIP is fully responsible for support. Only 2 spectral lines maximally 100 nm apart can be observed at a time due to the focus shift induced by the non-apochromatic lenses (they are off-the-shelf achromats). A current manual does not exist yet. Intensity variations exist in the continuum, which so far are not explained and are not calibrated.

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 the website after registration.

 

PI Institute: Leibniz Institute for Astrophysics Potsdam (AIP)

Contact: Carsten Denker