SPHERE Overview

Introduction

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 primary science goal of SPHERE is imaging, low-resolution spectroscopic, and polarimetric characterization of extra-solar planetary systems. 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 and includes the following sub-systems:

CPI: Adaptive Optics Main Specifications

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. The most important feasibility check for SPHERE observations concerns the AO performance, measured by the Strehl ratio (SR). The SR directly impacts raw contrast level.

 

 

 

AO performance

H-band Strehl Ratio

R-band Strehl Ratio

Good

> 75%

> 20%

Median

50 - 75%

5 - 20%

Poor

< 50%

< 5%

In the following table, we define three regimes with good (H-band SR > 75%, R-band SR > 20%), median (H-band SR = 50-75%, R-band SR = 5-20%) and poor (H-band SR < 50%, R-band SR < 5%) AO corrections as a function of R magnitude and seeing. Note that these values are very conservative, and the system will likely deliver better performance, although this is not yet guaranteed (see User Manual for further details).

 

NGS mag / Seeing

Seeing < 0.8"

Seeing 0.8" - 1.2"

Seeing > 1.2"

R = 0 - 9 mag

Good

Median

Poor

R = 9 - 11 mag

Median

Median

Poor

R > 11 mag

Poor

Poor

Poor

IFS: Main Specifications

The integral field spectrograph provides a data cube of 30 monochromatic images either at spectral resolution of R~50 between 0.95 - 1.35µm (Y-J) or at R~30 between 0.95 - 1.65µm (Y-H).

 

Observing mode

Integral Field Spectroscopy

Spectral range

0.95 - 1.35µm: R~50

0.95 - 1.65µm: R~30

Sampling

(12.25 mas)2 / spaxel (hexagonal grid), Nyquist at 0.95 µm.
Resampled by the pipeline on a square grid with (7.4 mas)2 / pixel.

FOV

1.73" x 1.73"

Detector type

Hawaii II RG 2048x2048

Coronagraphy

None, or with classical or apodized Lyot coronagraphs, 4QPM

Stabilization

Pupil- or Field-stabilized

WFS

Visible light

Performance estimates

http://www.eso.org/observing/etc/

 

 

 

IRDIS: Main Specifications

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 – 2.32µm, with resolving power of R~50 (LRS) or between 0.95 - 1.65µm with R~350 (MRS).

 

 

Observing modes

DBI, CI, DPI (not offered in P95)

LSS

Spectral range

0.95 - 2.32 µm: broad- and narrowband filters

0.95 - 2.32µm: R~50
0.95 - 1.65µm: R~350

FOV

11’’ x 11”

11” slit

Coronagraphy

None, or with classical or apodized Lyot coronagraphs, 4QPM

Central blocking

Stabilization

Pupil- or Field-stabilized

 Field stabilized

Sampling

12.25 mas pixels, Nyquist-sampled at 0.95µm

Detector type

Hawaii II RG 2048 x 1024

WFS

Visible light

Performance estimates

http://www.eso.org/observing/etc/
see UM for LSS

 

 

 

ZIMPOL: Main Specifications

The Zurich imaging polarimeter provides diffraction limited classical imaging and differential polarimetric imaging (DPI) at < 30 mas resolution in the visible. Different CCD read-out modes are implemented, allowing for fast read-out for observations of bright targets, or low read-out noise for observations of faint targets.

 

Observing modes

Imaging, differential polarimetric imaging

Spectral range

500 - 900 nm in broad- and narrowband filters

Sampling

(7 mas)2 / pixel, diffraction-limited at λ >600 nm

FOV

3.5” x 3.5”

Linear polarization

Instrumental polarization <1%, polarimetric sensitivity < 0.1% with fast modulation, simultaneously on two CCDs

Stabilization

- Imaging: pupil or field stabilized.
- Polarimetry: field stabilized, or fixed derotator with stable and minimized instrumental polarization.

Coronagraphy

None, or with classical Lyot coronagraphs 

WFS

Visible light shared between WFS and ZIMPOL:             
- Dichroic for R-band observation
(100% WFS outside R band / 100% R band to ZIMPOL)
- Grey beam splitter
(20% WFS / 80% ZIMPOL), with AO limit lowered by ~1.74 mag
Performance estimates

http://www.eso.org/observing/etc/

 

Instrument combinations

Dichroics allow IFS and IRDIS to take data in parallel. One dichroic setting allows IFS observations in the Y-H range, and IRDIS observations in K at somewhat reduced performance. Another allows IFS observations in Y-J range, and IRDIS observations in the H band with uncompromised performance.
For IRDIS-alone observations, a mirror is used ensuring optimal K band performance and access to the complete 0.95 - 2.32 µm spectral range.
ZIMPOL can only be used alone.

Offered Modes

FOR ESO Period 95-96, the following observing modes will be offered for visitor- and/or service-mode observations. Note that all programmes using a natural guide star (target) with R > 11 mag, should apply for visitor mode.

Since P96 and thanks to a successful completion of tests/commissioning, the IRDIS_DPI mode is offered in visitor mode only. More information about this mode is to be expected in the next version of the User Manual (P97 Phase 2).

For P97, observations using the IRDIS_LSS mode should be requested in Visitor Mode. However, if very good conditions are critical for the observations, this mode can now also be requested in Service Mode under certain conditions explained in the Late Breaking News.

News: For P98, the SPHERE IRDIS DPI mode can be requested in service mode solely with the BB_J filter for which the instrumental polarisation is always >90% regardless of the decorator angle. Polarimetric standard stars are not yet part of the calibration plan  and must therefore be provided by the user, if needed.

 

 

 

 

Observing mode
Comments
Service/Visitor mode
IRDIFS

- Pupil-stabilized (default) or Field stabilized
- Coronagraph: ALC_YJH_S
S+V
- 4QPM and other coronagraphs
- NGS R > 11 mag
V
IRDIFS_EXT

- Pupil-stabilized (default) or Field stabilized
- Coronagraph: ALC_YJH_S
S+V
- 4QPM and other coronagraphs
- NGS R > 11 mag
V
IRDIS_DBI

- Pupil-stabilized (default) or Field stabilized
- Coronagraph: ALC_YJH_S, N_ALC_Ks with K-band filters
S+V
- 4QPM and other coronagraphs
- NGS R > 11 mag
V
IRDIS_CI
- Pupil-stabilized (default) or Field stabilized
- Coronagraph: ALC_YJH_S, N_ALC_Ks with K-band filters
S+V
- 4QPM and other coronagraphs
- NGS R > 11 mag
V
IRDIS_LSS Long Slit Spectroscopy in LRS and MRS
(S*)+V
IRDIS_DPI Differential Polarimetric Imaging
S (BB_J only) + V 
ZIMPOL_P1/P2

- Derotator: P1 (static) or P2 (Field-Stabilized)
- Filters: combination of NB and BB filters not allowed
- WFS/ZIMPOL splitter: GREY or DIC-HA
- No coronagraph (allowing some saturation) or V_CLC_M_WF
- Some saturation allowed
- Detector/modulator: FastPol, SlowPol
- Field-Position: on-axis
S+V
- Other coronagraphs
- NGS R > 11 mag
- NB and BB filter combination
V
ZIMPOL_I

Same as P1/P2 but with readout mode StdImaging.
S+V
- Other coronagraphs
- NGS R > 11 mag
- NB and BB filter combination
V