ERIS - Enhanced Resolution Imager and Spectrograph

ERIS, the Enhanced Resolution Imager and Spectrograph, is a 1-5 μm instrument for the Cassegrain focus of UT4 at the VLT, the telescope that will be equipped with the Adaptive Optics Facility (AOF).


Principal Investigator Richard Davies (MPE)
Project manager Andreas Glindemann (
Project scientist
Harald Kuntschner (
Project system engineer Angela Cortes (
Instrument approval October 17 2012
Project Status PAE passed, AIV starting in Dec 2021
Instrument Science Team Instrument science team from Consortium with H. Kuntschner as observer
Location Cassegrain focus of UT4

Baseline Specification



Spectrograph (IFU)

Wavelength range

J – Mp

J, H, K

Image Quality

>68% Strehl in K-band, on-axis, visible NGS AO sensing at MR=8

>54% Strehl in K-band, on-axis, Na LGS + visible on axis TT ref-star at MR=12


27” x 27”

54” x 54”

8" x 8", 3.2" x 3.2" and

0.8" x 0.8"

Pixel scale

0.013mas/pix, 0.027 mas/pix

250mas/spaxel, 100mas/spaxel and 25mas/spaxel


Imaging, coronagraphy, Sparse Aperture Masking, L-, M-band longslit spectroscopy

Integral field spectroscopy


  • First light at the telescope – 2022
  • Preliminary Acceptance Europe (PAE) – 2021
  • Final Design Review (FDR) – May 2017
  • Preliminary Design Review (PDR) – February 2016
  • Project partner selection concluded – Dec 2015
  • ESO internal project approval – October 17, 2012 
  • Phase A study review meeting – May 24, 2012 

Instrument Description

ERIS, the Enhanced Resolution Imager and Spectrograph, is a 1-5 μm instrument for the Cassegrain focus of UT4 at the VLT. It will benefit from the Adaptive Optics Facility (AOF) using the deformable secondary mirror (DSM) and one of the laser guide stars (LGS). The ERIS instrument combines an
imager (NIX) and an integral-field spectrograph (SPIFFIER).  

As of FDR (see Figure 1), ERIS consists of three main instrumental modules:

  • The AO module, which will use the AOF Deformable Secondary Mirror (DSM), and one AOF laser, providing NGS and LGS visible wavefront sensing with real-time computing capabilities. The baseline choice for the NGS wave front sensor (WFS) and for the LGS WFS is a Shack-Hartmann. The AO module will allow for single-conjugate adaptive optics (SCAO) operations. Both IR science instruments, the imager (NIX) and ERIS-SPIFFIER, are fed by  a dichroic beamsplitter which reflects the visible light to the AO module.
  • The imager providing diffraction limited imaging, sparse aperture masking (SAM) and coronagraphy capabilities from 1-5 μm (i.e. J-Mp), either in “standard” observing mode or with “pupil tracking” and “cube” readout mode. The imager is a cryogenic instrument and its 2k×2k detector is cooled at 40K by means of a closed cycle cooler (CCC).
  • The spectrograph ERIS-SPIFFIER is a refurbished version of SPIFFI, the 1-2.5 μm integral field unit currently on-board SINFONI,  modified to be integrated into ERIS. Its observing modes are identical to those of SINFONI, with the goal of adding a high resolution (R=8000) grating. SINFONI will be removed in ~June 2019 from Paranal.

ERIS uses and depends on the AOF infrastructure to perform the AO correction. The ERIS concept maximizes the re-use of existing sub-systems and components. In particular, the AO correction is provided by the AOF Deformable Secondary Mirror (DSM) and the artificial Laser Guide Star (LGS) is generated by the 4LGSF system. The wavefront sensor camera detectors are identical to the ones used for GALACSI and GRAAL (the two GLAO systems of the AOF) and the Real-Time Computer (RTC) is a modified version of SPARTA.

Figure 1: Chart showing the main components of the ERIS baseline design. The coloured boxes are for sub-systems belonging to ERIS, the white ones are external to the instrument.

Scientific Objectives

The main scientific drivers for ERIS are drawn from experiments benefitting from near-diffraction (8m) limited imaging and spectroscopy in the near to mid IR. Naturally, the science programs span a wide range including:

  • Solar system science – measurements of asteroid shapes, sizes and SED’s; characterization of satellites around giant planets (e.g. volcanism on Io at Jupiter, or cloud formations on Titan at Saturn)
  • Stellar disk science – studying the circumnuclear disks of young forming stars from the primordial stages to debris disks
  • Exoplanet studies – basic characterization of exoplanet SEDs, specifically in the L- and M-bands
  • Initial Mass Function (IMF) determinations – direct investigations of the IMF in AO-resolved starburst clusters
  • Galactic Centre science – Monitoring of stellar orbits around Sagittarius A* to measure the distance to and mass of the black hole (BH) to percent-accuracies. Constraining the physics of the accretion flow and investigating the star-formation in the central pc potentially influenced by the BH.
  • Centres of nearby galaxies – characterization of the circumnuclear regions at the highest possible spatial scales and the influence of the BH on star-formation in nearby (active) galaxies 
  • Medium to high redshift (z=0.4 to 2) galaxies – studying the structure, content and kinematics of distant galaxies in order to address fundamental questions on galaxy formation and evolution, and the role of BHs therein            

ERIS will combine state-of-the-art near IR imaging and spectroscopy with an advanced SCAO system to deliver superb on-axis Strehl ratios. The increased sensitivity of the wave front sensors, together with powerful (20W) laser-driven AO, allow for the use of fainter TT-stars further away from the science target. This will greatly increase the number of potential targets accessible to this new instrument. ERIS is designed to replace several of the most important NaCo capabilities; most notably, the diffraction-limited imaging of the Galactic Centre.

Given that ERIS combines a general purpose near to mid IR camera and a versatile J-K-band integral field spectrograph with a powerful AO system, we expect that it will not only fulfill its main scientific objectives, but will also be used for a wide variety of other science cases with hopefully new and unexpected results.