Below is a list of bibliographic references for SPHERE and its various sub-systems. Please cite the publications relevant to your observations.
- General description: Beuzit et al., 2019, A&A, submitted
- General description: Fusco et al., 2006, Optics Express, 14, 7515
- Control: Petit et al., 2014, SPIE, 9148
- Wavefront sensing: Sauvage et al., 2014, SPIE, 9148
- On-Sky performances: Fusco et al., 2014, SPIE, 9148
- General description: Dohlen et al., 2008, SPIE, 2014
- Dual-band imaging mode: Vigan et al., 2010, MNRAS, 407, 71
- Long-slit spectroscopy mode: Vigan et al., 2008, A&A, 489, 1345
- Dual-polarization imaging mode, paper I: de Boer et al., 2019, A&A, accepted.
- Dual-polarization imaging mode, paper II: van Holstein et al., 2019, A&A, accepted.
- General description and performance: Schmid et al., 2018, A&A, 619, 9
SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is an extreme adaptive optics system and coronagraphic facility feeding three science instruments: IRDIS, IFS, and ZIMPOL. The instrument design is optimized to provide the highest image quality and contrast performance in a narrow field of view around bright targets that are observed in the visible or near infrared. SPHERE is installed at the UT3 Nasmyth focus of the VLT (Figure 1) and includes the following sub-systems.
CPI: The common path and infrastructure receives direct light from the telescope, and provides highly stabilized, AO-corrected, and coronagraphic beams to the three sub-instruments.
IFS: The integral field spectrograph provides a data cube of 38 monochromatic images either at spectral resolution of R~50 between 0.95 - 1.35 µm (Y-J) or at R~30 between 0.95 - 1.65 µm (Y-H).
IRDIS: The infrared dual-band imager and spectrograph provides classical imaging (CI), dual-band imaging (DBI), dual-polarization imaging (DPI), and long slit spectroscopy (LSS) either between 0.95 - 2.32 µm, with resolving power of R~50 (LRS) or between 0.95 - 1.65 µm with R~350 (MRS).
ZIMPOL: the Zurich imaging polarimeter provides diffraction limited classical imaging and differential polarimetric imaging (DPI) at <30 mas resolution in the visible.
Figure 1. Concept of the SPHERE implementation on the VLT Nasmyth platform showing the common path with the XAO system and the three science sub-systems IRDIS, IFS and ZIMPOL.
Common path with eXtreme Adaptive Optics - CPI
CPI includes the main optical bench, connects the other sub-systems to the light path, and guarantees a static alignment of SPHERE to the VLT focus (Figure 2). It feeds the cryostats and motors with power, includes servo-controlled pods, damping vibrations on the Nasmyth platform. An enclosure protects the SPHERE optics from dust and reduces temperature gradients. From the entrance focal plane up to the second focus (FP2) a reflective design propagates the complete spectral range from 450 to 2320 nm through the instrument. It includes the beam derotator, active optics, with pupil and image tilt mirrors, and the high order deformable mirror DM, which is the heart of the system providing a stable AO-corrected image.
CPI also includes visible and infrared half-wave plates for polarization switching/modulation. After FP2, light is split between visible and NIR. The NIR beam path includes refractive optics: the NIR ADC, coronagraphic components, neutral densities filters and an exchangeable splitter between IRDIS and IFS.
In the optical, atmospheric dispersion correction is applied by the VIS ADC, and then the beam is split between ZIMPOL and AO (WFS). Before ZIMPOL, the beam goes through a coronagraphic focal plane (FP4) and a pupil plane hosting the corresponding Lyot stops.
The adaptive optics module is called SAXO. It is designed to correct for the turbulence perturbation at high frequency (1.2 kHz). The turbulence is measured by a 40x40 lenslet Shack-Hartmann sensor that is equipped with a red-sensitive sub-electron noise EMCCD. It servo controls the pupil position, and fine-tunes the centering on the coronagraph by means of a dedicated differential tip-tilt sensor (DTTS).
The coronagraphs are set-up by a mask in the focal plane (FP3 for NIR and FP4 for the visible), a Lyot stop in the downstream pupil, and an apodizer in the upstream pupil (before the focal mask). The wheels for the coronagraphs include also field stops and the IRDIS long slit masks.
Figure 2. Layout of the SPHERE Common Path Infrastructure.
Infra-Red Dual-beam Imager and Spectrograph - IRDIS
The Infra-Red Dual-beam Imaging and Spectroscopy (IRDIS) sub-system has a FOV of 11"x12.5" with a pixel scale of 12.25 mas, matching Nyquist sampling at 0.95 µm. Two parallel beams are projected onto the same 2k x 2k detector and each occupies about half of the array. The IRDIS instrument layout is displayed in Figure 3. DBI provides images in two neighbouring filters, simultaneously. Various filter pairs are available for different spectral features. DPI uses crossed polarizers providing images in the two polarization directions at the same time. For LSS, the coronagraph mask is replaced by a coronagraphic slit.
Three wheels are provided within the cryogenic environment:
- The Lyot stop wheel, with Lyot stops for the coronagraphs of the CPI, LRS prism, and MRS grism.
- The common filter wheel, with blocking filters, broadband and narrowband filters.
- The DBI filter wheel, with DBI filter pairs, polarizers, and a pupil-imaging lens.
The complete list of available filters is summarized in the User Manual. Dual imaging separation is done using a beam-splitter combined with a mirror, producing two parallel beams. IRDIS achieves less than 10 nm differential aberrations between the two channels and, as a consequence, allows high contrast differential imaging.
Figure 3. Inside view of the IRDIS cryostat. The CPI beam comes from the left, goes through the common filter and Lyot stop wheels, is split in two, then goes through the dual filter wheel before landing on the infrared detector.
Infrared Integral Field Spectrograph - IFS
The SPHERE integral field spectrograph (IFS) is a lenslet-based integral field unit (called BIGRE, Antichi et al. 2009), providing a 1.73” x 1.73” FOV that is Nyquist sampled at 0.95 µm. The IFS includes a flat calibration source and filters for accurate detector calibrations. The IFS instrument layout is displayed in Figure 4. The raw data 21000 spectra are aligned with the detector columns over a hexagonal grid rotated by ~10.7° with respect to the dispersion. Each spectrum from a spaxel is projected on a rectangular area of 5.1x41 pixels on the detector.
During data reduction, the image is translated into a (x,y,λ) data cube, which has for both available spectral resolutions a constant dimension of (291,291,38). Each image is resampled by the pipeline over a square regular grid at (7.4 mas)2 / spaxel. Data outside this region are meaningless.
Figure 4. Inside the IFS. Note that the IFS optical bench is not cold, and that it has its own Lyot stop and internal calibration sources.
Visible Differential Imager - ZIMPOL
The Zurich Imaging Polarimeter (ZIMPOL) is designed as high contrast imaging polarimeter with very high polarimetric sensitivity for the visual range. It also provides at the same time imaging capabilities. Another VLT instrument, FORS2, provides a polarimetric mode for the visual range which is however complementary to ZIMPOL. FORS2 is a seeing limited Cassegrain imaging and spectrograph polarimeter for accurate absolute polarization measurements (0.1% in the field center). FORS2 has a very high throughput and is therefore suited for faint objects. ZIMPOL is a diffraction limited imaging polarimeter with a very small field of view centered on a bright target. Because ZIMPOL is a Nasmyth instrument, it provides only a limited absolute polarimetric accuracy (0.5%). The basic observing strategy for ZIMPOL is a relative polarization measurement of the immediate surroundings of a central star, which is used as a (zero)-polarization reference for the correction of the instrument polarization.
Atmospheric seeing variations are a key problem when aiming to achieve high image quality. Differential measurements such as DPI with fast (kHz) polarization modulation can solve this problem. DPI is performed quasi-simultaneously by fast modulation revealing, e.g. faint structures around a bright point source.
The ZIMPOL instrument layout is displayed in Figure 5. ZIMPOL includes a new concept for imaging polarimetry, which is based on fast polarimetric modulation and on-chip de-modulation by two CCDs. Thanks to a beam-splitter, one polarization is sent to CCD1 and the other perpendicular polarization to CCD2. Further, the polarization modulation is synchronized with an innovative reading scheme of the detectors.
By this technique, the polarization of the incoming light is converted by a polarization modulator and the following polarization beam splitter into an intensity modulation with an amplitude proportional to the polarization signal. The de-modulating CCD has every second row covered by a stripe mask and these covered rows can be used as a buffer storage area. With charge shifting on this masked CCD each “open” pixel can measure the intensity of the two alternating states. Photo-charges created during one half of the modulation cycle are shifted for the second half of the cycle to the next masked row and again back for the subsequent illumination during the next first half of the modulation cycle. In this way two frames are build up in alternating pixel rows, corresponding to opposite polarization states. After many modulations the CCDs can be readout and the raw polarization signal is then the difference between “even” and “odd” rows.
A single arm of ZIMPOL provides a full polarimetric measurement. Because a polarizing beamsplitter is used for the polarization analysis, half of the light goes to the other arm which can perform polarimetry for the same or another filter.
Figure 5. ZIMPOL schematic view showing the main elements of this visilbe polarimeter, from polarization compensator, common filter wheel, Ferroelectric Liquid Crystal (FLC) modulator to the splitter, filter wheels 1 and 2, and the two detectors.
Advantages of differential polarimetric technique are:
- Both images are created quasi-simultaneously (the modulation is faster than the seeing variations), and both images are recorded with the same pixels reducing significantly flat-fielding issues.
- Differential effects due to the storage of two images in different buffer pixels is compensated with a demodulation phase-switch between subsequent images.
- There are only very small differential aberrations between the two images with opposite polarization.
- The differential signal does not suffer from chromatic effects due to diffraction or speckle chromatism.
The efficiency of the modulation – demodulation process depends on the modulation frequency. The instrument offers a fast modulation mode with a frequency of about 1 kHz and a polarization efficiency of 80% and a slow modulation mode with a frequency of about 30 Hz and an efficiency of 90%. A simple calibration is foreseen to correct for the polarization efficiency taking into account a slight wavelength dependence and possible small temporal variations in the polarization measurement.