KMOS Overview

The K-band Multi Object Spectrograph (KMOS) is a second-generation instrument designed for operation on the VLT. The key feature of KMOS is its ability to perform Integral Field Spectroscopy in the near-infrared bands for 24 targets simultaneously.

The instrument design employs 24 configurable arms that position pickoff mirrors at user-specified locations in the Nasmyth focal plane. The sub-fields thus selected are then fed to 24 image slicer integral-field units (IFUs) that partition each sub-field into 14 identical slices, with 14 spatial pixels along each slice. Light from the IFUs is then dispersed by three cryogenic grating spectrometers which generate 14x14 spectra with ~1000 Nyquist-sampled spectral resolution elements for each of the 24 independent sub-fields.

The patrol field of the pickoffs is 7.2 arcmin in diameter, which is the diameter of the unvignetted field at the VLT Nasmyth focus, thus minimising the thermal background in the K-band. Each IFU has a square field of view of 2.8 x 2.8 arcsec; anamorphic magnification in the IFU foreoptics ensures uniform spatial sampling of 0.2 x 0.2 arcsec whilst maintaining Nyquist sampling (~2 pixel) of the spectral resolution element at the detector.

The use of focal-plane pick-off arms allows considerable flexibility in selecting targets and in particular the important capacity to deal with strongly clustered or close-paired sources. In addition to observing multiple individual sources, KMOS has the capability for integral field mapping of contiguous areas in an 8-point or 16-point dither pattern. The spectral resolution of R~3000-4000 provides velocity resolution for studies of low-mass objects and is optimal for OH-avoidance in the J & H bands. A lower resolution mode is also available in the combined H+K band.  

KMOS main specifications are summarized in the following table:

Wavelength coverage

0.8 mum to 2.5 mum

Spectral bands

IZ, YJ, H, K, H+K

Spectral resolving power

R = 3400, 3600, 4000, 4200, 2000    (IZ, YJ, H, K, H+K)

Number of IFUs


Extent of each IFU

2.8" x 2.8"

Spatial sampling

0.2" x 0.2"

Patrol field

7.2 arcmin diameter circle

Close packing of IFUs

>= 3 within 1 sq. arcmin

Closest approach of IFUs

>= 2 pairs of IFU separated by 6 arcsec