Powerful New Eyes to Explore the Universe with TMT
  Luc Simard
  Association of Canadian Universities for Research in Astronomy (ACURA)

What kind of scientific instruments does one need to take full advantage of a 30-meter telescope's ability to reach across the vastness of space and time?

To answer this question, the Thirty Meter Telescope (TMT) project recently completed a round of studies that involved nearly 200 scientists and engineers at 46 U.S., Canadian and French institutions. The instrument concepts that emerged from these studies were proven to be feasible even though they break major new ground in terms of physical size and complexity.

Astronomical instruments have two basic observing modes: imaging and spectroscopy. These modes capture light from astrophysical objects at different angular resolutions (how well two sources on the sky can be separated) and spectral resolutions (how well two wavelengths Δλ apart can be separated at an observing wavelength λ). The spectral resolution is defined as λ/Δλ (the "R parameter"). The angular resolution of some instruments is limited by the blurring effect (known as "seeing") from atmospheric turbulence above the observatory, while others can make use of the full resolution of a giant telescope aperture with the help of adaptive optics.

Two "seeing-limited" instruments were studied for TMT: the High-Resolution Optical Spectrograph (HROS) and the Wide-Field Optical Spectrograph (WFOS). Both spectrographs work in the optical window of the electromagnetic spectrum (wavelengths between 0.31 and 1.0 microns). The HROS concept was studied by different teams at the University of California at Santa Cruz and the University of Colorado. It focuses on collecting high-resolution spectra (R < 100,000) of single targets for precise measurements of internal motions and chemical abundances. WFOS, studied by ACURA and the Herzberg Institute of Astrophysics, trades spectral resolution (R < 7500) for a huge multiplexing advantage, since it can record hundreds of spectra simultaneously. It is ideally suited for large surveys of the Universe.

Both HROS and WFOS isolate the light from their targets using slit apertures. In addition to these optical spectrographs, a third slit spectrograph, the Mid-InfraRed Echelle Spectrograph (MIRES), studied by the National Optical Astronomy Observatory and the University of Hawaii, will measure high-resolution (R < 100,000) spectra at wavelengths between 8 and 28 microns, where astrophysical processes in very cold environments emit most of their light.

Figure 1: The proposed Wide-Field Optical Spectrograph (WFOS) for TMT is a good example of the scale and complexity of the 30-meter observatory’s planned science instruments. WFOS is eight meter in diameter and nine meters long. In other words, WFOS is as big as the largest telescopes in the world today! The DEIMOS spectrograph at the Keck Observatory, one of the largest such instruments in operation, is shown in the foreground (solid black form) for comparison. WFOS will be able to simultaneously observe hundreds of astronomical targets and produce a spectrum for each one of them.

Another class of spectrograph called integral field units (IFUs) can dissect the incoming light into information about position on the sky and wavelength. In essence, IFUs produce “datacubes” containing a spectrum at each position on the sky within their field-of-view. The InfraRed Imaging Spectrograph (IRIS) for TMT, studied by the University of California at Los Angeles and Caltech, is an IFU spectrograph that works at near-infrared wavelengths between 1.0 and 2.5 microns. IRIS will use an array of tiny lenses to split its field-of-view into a datacube. It will be assisted by adaptive optics, and its datacubes will have an angular resolution ten times better than images from the Hubble Space Telescope.

IRIS is a single IFU spectrograph, and the next logical step is to ask whether an instrument could sport multiple, deployable IFUs. This intriguing concept is known as the InfraRed Multi-Object Spectrograph (IRMOS). Its utility for TMT was studied by two teams: one from Caltech and the Laboratoire d’Astrophysique de Marseille (LAM), and another from the University of Florida and the Herzberg Institute of Astrophysics. The Caltech/LAM team came up with an innovative tile of steerable micro-mirrors that can select sub-regions of interest in the field-of-view of the spectrograph. The UF/HIA team opted to use robust robotic arms to position tiny pick-off mirrors on interesting targets. Both IRMOS concepts can simultaneously produce between 16 to 20 datacubes.

Last but not least is the ultimate imager—the Planet Formation Imager (PFI) instrument. PFI is the brainchild of scientists and engineers at Lawrence Livermore National Laboratory, the Jet Propulsion Laboratory, the University of California at Berkeley and the Université de Montréal. PFI uses highly sophisticated adaptive optics image corrections coupled to a very tight control of diffraction effects to directly image planets around other stars with unprecedented angular resolution and contrast. PFI will be capable of imaging a planet that is 100 million times fainter than its parent star at an angular distance of 0.03 seconds of arc (equal to the angle subtended by a penny viewed from a distance of 70 kilometers!)

The TMT project and its Science Advisory Committee are now absorbing what was learned in these studies and planning the next phase of instrument studies. In the near future, scientists and engineers will work eagerly on more detailed designs for these instruments. The ultimate construction of these powerful eyes will surely transform many of the views that we now hold regarding our place in the Universe.

For much more detail on the work done so far to plan the first and second generations of TMT instruments, see this overview presented at the June 2006 SPIE meeting. (pdf 2MB)