KMOS

Cutaway CAD model of the KMOS cryostat (image courtesy ATC)

Introduction

KMOS is a revolutionary new instrument built for the ESO Very Large Telescope by a consortium of UK and German institutions. It will primarily be used to study the properties of very distant galaxies, although it has a wide range of uses. Due to their distance most of the light from these galaxies is redshifted into the near infrared (NIR) so KMOS is designed to work in the wavelength region between 800nm and 2450nm.

The institutions involved in KMOS are:

Science background

Fiducial display of the KARMA software, used to allocate the individual IFUs onto the targets (galaxies)
Understanding how galaxies form and evolve is a long-standing goal of astrophysics. While it is possible to study nearby galaxies and infer what must have happened to them in the past, it is also possible to look directly into the past at very distant galaxies from which the light is only just arriving now.

Observing distant galaxies is made difficult by their (apparent) faintness and redshift. More distant objects are fainter on the sky, requiring longer exposures with bigger telescopes. Increasing redshift pushes the well understood optical spectral region into the NIR, where the atmosphere is less transmissive, the sky emits more radiation (creating more noise) and detectors used to be less efficient than visible CCDs.

KMOS is optimised for NIR observations and benefits from the latest technologies to reduce noise (including detectors that are as efficient as CCDs). It is also a multiplexed instrument, with 24 individual integral field units (IFUs) which allow parallel observations of targets, dramatically increasing the observing efficiency of the telescope by reducing the overall exposure time for large samples. The 3D spectral maps produced by IFUs give a complete view of the processes transforming galaxies. KMOS is therefore optimised for the study of high redshift galaxies, when the Universe was a small fraction of its present age

KMOS Instrument Overview

In order to operate at NIR wavelengths the entire working parts of the instrument are cooled to below -130C with the detector cooled even further to below -200C. To achieve this the entire instrument is contained in a vacuum within a cryostat to prevent icing and extra heat load on the fragile components.

Inside the KMOS cryostat the KMOS instrument can be divided into three sections:

A single robotic arm (Photo courtesy ATC/Durham)

  • The front section containing 24 robotic arms that patrol a 7 arcmin diameter science field. Each arm collects the light from a 2.8x2.8 arcsec section of sky
  • A central section containing filter wheels to remove unwanted light (blocking parts of the spectrum that are not required) and 24 integral field units (IFU) which take the light from the arms and reformat or slice the image of the sky to form a long slit. This section of the instrument also combines the light from 8 individual arms into a single slit so that the output from the central section is 3 long slits
  • The rear section which contains three identical spectrographs (one for each long slit). Each spectrograph has five different diffraction gratings to select one of five wavelength bands, providing versatile observing modes to address a range of astrophysics.

View of front section containing all 24 robotic arms (photo courtest ATC)

Oxford Astrophysics’s role

Grating assembly with five gold coated diffraction gratings

Roger Davies was the PI for the three identical spectrographs in the KMOS instrument.

The optical design for the spectrograph was performed by a collaboration between Oxford and Rutherford Appleton Labs and the mechanical design and analysis was performed by the Design Office in the Oxford Physics Department.

The spectrographs were constructed in the mechanical workshops in the Physics department, with optical components being outsourced. All optical surfaces were coated with antireflection coatings tuned for NIR wavelengths by the Thin Film Facility in Oxford Physics who also coated nearly all of the instruments several hundred gold surface mirrors.

CAD model of a single spectrograph

The camera barrel was assembled and aligned at RAL before being integrated into the spectrograph at Oxford ready for testing. The spectrographs were tested at the 2.2m cold test facility in Atmospheric Physics before shipping to ATC Edinburgh for full instrument integration and testing.

Many of these technologies will be used in the HARMONI instrument for the E-ELT (PI Niranjan Thatte).

Three spectrographs assembled for the first time in the rear of the main KMOS instrument cryostat

Commissioning

After full instrument integration and testing at ATC in June/July 2012, KMOS was shipped to the Paranal Observatory in Chile (the home of the Very Large Telescope) in August 2012 where, after further testing, KMOS was installed at the Nasmyth Focus of one of the four unit telescopes that form the VLT and received "first light" in November 2012 (the commissioning stage).

KMOS being lifted into place at the VLT (photo courtesy ATC)KMOS being transported from the test facility to the VLT at Paranal Observatory, Chile (photo courtesy ATC)