Instrument Description

NOTE: The definitive source for specifications, performance, and limits of the ERIS instrument is the user manual!

AO Performance and Limits

The instrument can be operated in four different AO modes:

  • Natural Guide Star (NGS) mode: using either the target star or a slightly off-axis (<30" recommended) guide star.
  • Laser Guide Star (LGS) mode: using one of the four lasers of the 4LGSF, and a tip-tilt star for truth sensing.
  • Seeing Enhancer (SE) mode: LGS without a tip-tilt star. Only higher-order modes are corrected, uses VLT stabilization.
  • No AO (noAO) mode: seeing-limited mode, VLT stabilization only.

Note that the minimum allowed distance between the NGS/TT star and the moon is 25 degrees.

  Gaia BP Gaia RP Off-axis? K-band
NGS <=19 NGS: <11

0" - 60"

(<30" recommended)

< 0.85 <1.9 <=70% (<=1.15") N/A
LGS <=19 TT star: 7 - 18 0" - 60" < 0.6 <1.9 <=85% (<=1.4") <=0.5 for TT mag > 16
LGS-SE N/A N/A N/A TBC <1.9 <=30% (<=0.8") N/A
noAO N/A N/A N/A N/A <2.9 Any N/A
ERIS NGS/LGS K-band Strehl ratio
Simulated K-band Strehl ratio as a function of NGS magnitude (blue) and LGS TT star magnitude (red).

IFS Performance and Limits

Observations with the IFS can be taken in any one of twelve grating configurations and three plate scale configurations. Note that the spaxels are rectangular, and the name of the plate scale configuration corresponds to the long axis, twice the length of the short.

Grating configurations

Band λc (um) λ range (um) Resolution
J_low 1.25 1.09--1.42 ~5000
H_low 1.66 1.45--1.87 ~5200
K_low 2.21 1.93--2.48 ~5600
J_short 1.18 1.10--1.27 ~10000
J_middle 1.26 1.18--1.35 ~10000
J_long 1.34 1.26--1.43 ~10000
H_short 1.56 1.46--1.67 ~10400
H_middle 1.67 1.56--1.77 ~10400
H_long 1.76 1.66--1.87 ~10400
K_short 2.07 1.93--2.22 ~11200
K_middle 2.20 2.06--2.34 ~11200
K_long 2.33 2.19--2.47 ~11200

Plate scale options

Tag Spaxel size Field of view
25mas 12.5 x 25 mas 0.8" x 0.8"
100mas 50 x 100 mas 3.2" x 3.2"
250mas 125 x 250 mas 8.0" x 8.0"

Brightness limits

To avoid the effects of persistence there is a limit to the brightest star that can be observed with the IFS. These limits are described in the User Manual, and the Phase 2 pages.

NIX Performance and Limits

NIX offers a variety of observing modes that can be used in conjunction with the AO modes described above:

  • Short-wavelength imaging: Imaging with the 13 or 27mas camera (FoV of approx 27"x27" and 55"x55") with any filter in the J, H, and K bands.
  • Long-wavelength imaging: Imaging with the 13mas camera (FoV of approx 27"x27") with any filter in the L and M bands.
  • Apodizing phase plate (APP) coronagraphy: Pupil-plane coronagraphy with K, L-band narrowband filters.
  • Focal plane coronagraphy (FPC): Focal-plane coronagraphy with a vortex coronagraph and the L and M-band broadband filters.
  • Sparse aperture mask (SAM) imaging: Sparse aperture masking with one of three aperture masks and any narrowband filter.
  • Long-slit spectroscopy (LSS): Medium-resolution (R~900) L-band slit spectroscopy.

Valid instrument configurations and filters for NIX

This table summarises the filters available in NIX.

Filter λc (um) FWHM (um) Average
transmission (%)
transmission (%)
J 1.28 0.20 82  
H 1.66 0.31 93  
Ks 2.18 0.39 87  
Short-Lp 3.32 0.43 80  
L-Broad 3.57 1.04 83  
Lp 3.79 0.60 78  
Mp 4.78 0.58 80  
Pa-b 1.282 0.021 75 83
Fe-II 1.644 0.020 86 94
H2-cont 2.068 0.064 80 90
H2-1-0S 2.120 0.020 67 77
Br-g 2.172 0.020 65 75
K-peak 2.198 0.099 83 89
IB-2.42 2.420 0.049 71 82
IB-2.48 2.479 0.051 65 78
Br-a-cont 3.965 0.108 82 91
Br-a 4.051 0.023 69 80

A plot of the filter transmission curves is available here, and the data used to create this plot are available in this archive. The total throughput of the instrument as a function of wavelength can be obtained as an output of the ERIS ETC ("Select Plots" > check the various througput efficiencies).

This table summarises the valid combination of the aperture, camera, filter and pupil wheel, as well as the valid tracking modes. A description of the various pupils can be found in the User Manual.

  Aperture Camera Filter Pupil Tracking
Short imaging Small 13mas-JHK Any < 2.5um JHK-pupil, Blocking*, ND Field
  Large 27mas-JHK Any < 2.5um JHK-pupil, Blocking*, ND Field
Long imaging Small 13mas-LM Any > 2.5um LM-pupil, ND Field
  Small 13mas-LM Any > 2.5um Spider* Pupil*
APP Small 13mas-JHK H2-cont, H2-1-0S, Br-g, K-peak
IB-2.42, IB-2.48
APP Pupil
  Small 13mas-LM Br-a, Br-a-cont APP Pupil
FPC AGPM-L 13mas-LM Short-Lp, Lp, Br-a, Br-a-cont Lyot, Lyot-ND Pupil
  AGPM-M 13mas-LM Mp Lyot, Lyot-ND Pupil
SAM Small 13mas-JHK < 2.5um (narrow only for SAM-23) SAM-7/9/23 Pupil
  Small 13mas-LM > 2.5um (narrow only for SAM-23) SAM-7/9/23 Pupil
LSS Slit mask 13mas-LM L-Broad Grism Field

* Not offered in P114

This table further details the valid pupil wheel and filter wheel combinations.

Pupil/Filter J H Ks Short-Lp Lp L-Broad Mp Pa-b Fe-II H2-cont H2-1-0S Br-g K-peak IB-2.42 IB-2.48 Br-a-cont Br-a

* Not offered in P114

Brightness limits

To avoid the effects of persistence there is a limit to the brightest star that can be observed with NIX. These limits are described in the User Manual, and the Phase 2 pages. Very bright stars can be observed with the ND filter to attentuate the flux by between 4.5 and 6.0 magnitudes, depending on the wavelength of the observation.

Bad pixels

The NIX detector suffers from several regions of highly clustered bad pixels. The figure below shows the number of neigbouring bad pixels that each pixel has. The regions in red should be avoided when planning dither patterns around the detector. A representative bad pixel map is also available to download here that can be used to help plan observations.

NIX bad pixel map
Number of identified bad pixels neighboring each pixel in NIX detector. Pixels which are not identified as bad pixels have a value of 0.

Short/Long-wavelength Imaging

Details regarding limiting magnitude and off-axis performance to be added after commissioning is completed.

Apodizing Phase Plate (APP) Coronagraphy

APP can be used for high-contrast imaging science to detect faint companions or structures at small angular separations. The APP mask is used in conjunction with a quater-wave plate and Wollaston prism to generate a deep null in a D-shaped region on opposite sides of two complementary PSFs generated by the Wollaston. Astrometric and photometric calibration can be achieved with the three calibration spots between the two PSFs.

This mode is currently only offered with the narrow band filters within the K and L-bands.

Left: On-sky PSF obtained with the APP mask in NIX, showing the central faint reference PSF and the two main PSFs with opposite D-shaped high contrast regions. Right: contrast curves measured in a 12-minute dataset on an L=2.2 star.

Sparse Aperture Mask (SAM)

There are three sparse aperture masks that can be used for imaging of companions and circumstellar material to nearby, bright stars. The choice of mask depends on the objectives of the observation and the target brightness. Simple systems (e.g. binaries) and faint targets are better suited for the SAM-7 mask, whereasm SAM-9 and SAM-23 are more suitable for complex and bright systems.

This mode is offered for all filters for SAM-7 and SAM-9, and for narrowband filters for SAM-23. Observations must be made with the 13mas plate scale.

Left: SAM-7 point spread function with the Ks filter. Right: Fourier transform of the same PSF.

Focal Plane Coronagraph (FPC)

ERIS employs an annular groove phase mask (vortex coronagraph) for focal plane coroanagraphy to provide 360 deress suppression around a point source whilst allowing light from circumstellar disks and companions to be transmited to the final focal plane. This mode can be used in either L or M band (including the two narrow-band Br-a filters). The performance of this mode as characterized during commissioning is described in the User Manual.

Improvement of the rejection of the target as the centering is improved at the end of the QACITS acquisition sequence.

Long Slit Spectroscopy (LSS)

NIX can also take data in a long slit spectroscopy mode. This has a single configuration with a wavelength range of 3.05-4.05 microns and a resolution of approximately 900.

Left: image of a raw 2D L-band spectrum showing the background and a stellar trace. Right: the extracted spectrum is dominated by atmospheric absorption features.