Chapter 4

Chapter 4, The Point Design

Section 4.7: Instrumentation

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One of the key lessons learned in working with the current-generation telescopes is the growing need to integrate instrumentation and telescope design choices. In the GSMT era, it will no longer be possible to think of the telescope and "auxiliary" instruments as somehow separate. Design choices for instrumentation impact telescope system design and vice versa. Hence, our approach from the beginning has been to develop the telescope, Adaptive Optics (AO) system, and instruments as an integrated system.

It is both impossible and perhaps unwise at this stage in the design process to study all possible instruments. Rather, we have endeavored to select instruments that serve the needs of many of the science objectives while also probing a reasonable region of parameter space. The following instruments were selected because they meet key science requirements (see Chapter 2) and pose and illustrate the kind of technical challenges that must be met in designing and building instrumentation in the 30-m era:

  1. Multi-object multi-fiber optical spectrograph (MOMFOS). This instrument is driven by the scientific application of tomography of the universe.
  2. Near-infrared deployable integral field spectrograph (NIRDIF). The tomography of pre-galaxy fragments is the primary science driver.
  3. A set of high-resolution, AO-fed IR spectrographs:
    1. Near-IR, echelle spectrograph (NIrES). 1-5 micron echelle spectrograph.
    2. Mid-IR high-dispersion AO spectrograph (MIHDAS). This 16-20 micron instrument is designed to address science regarding the origins of planetary systems.
    3. A coronagraphic capability to exploit extreme AO applications.

A detailed description will be provided for each of these instruments.

Since the first edition of this study, a design study was commissioned at the University of Durham for a detailed description of the following instrument:

A million-element integral field spectrograph

The details of this instrument are provided in Section 4.7.5 .

Additional instrument concepts were also explored, though not at a very detailed level:

An MCAO-fed, Near-IR imager
A high-resolution optical spectrograph

A brief description of what these instruments might look like, their potential impact on the telescope design, and specific technological challenges will be given.

The goal of these design studies is to see if current technologies are adequate for these instruments, and to learn whether building on extant concepts is possible. Major design challenges are defined and needed technologies are identified. In keeping with the point design philosophy, we chose to explore a single path rather than carry out either a thorough evaluation of all options or an optimization of how one might implement the concepts. The following table lists the instruments.

MOMFOSMulti-object fiber-fed prime focus spectrograph for optical spectroscopy with resolving power ranging from 2000 to 20,000.
NIRDIFDeployable integral field units for the multi-conjugate AO field of view (FOV). Spectral coverage in the near-IR with spatial resolution of about 0.1" and spectral resolving power of 2000 to 5000.
NIrESNear-IR, echelle fed by the on-axis AO system. Diffraction-limited slit with spectral resolving power of > 100,000.
MIHDASMid-IR, echelle fed by the on-axis AO system. Diffraction-limited slit with spectral resolving power of > 100,000.
MCAO ImagerNear-IR imager for diffraction-limited imaging over nearly 2 arcminute FOV.
MEIFUMillion-element integral field instrument for optical. Few by few arcminute FOV. Spectral resolving power of few hundred to 1000.
CoronagraphNear-IR coronagraphic imager for use with extremely high Strehl AO.

A summary of the technical challenges involved with the concepts studied is listed here.

InstrumentTechnical ChallengesComments
  • Adaptive 2-m mirror
  • Large gratings
  • Fiber sky performance
  • Fiber positioner
  • Fiber relay optics
  • To correct for windshake
  • 500 mm size, VPH preferred
  • 1 part in 5000
  • Builds on Echidna concept
  • Doublet micro lenses
  • Cryogenic, deployable relay arms
  • Image slicer fabrication
  • Spectrograph packaging

  • No significant technical risks
  • Probably the most straightforward of all instruments!
  • Large cryogenic chamber cooled to 10 to 20 K
  • Complex, off-axis camera
  • Utilize techniques developed for physics laboratories
  • Diamond-turned optics
MCAO Imager
  • Large detector array
  • Large cryostat
  • Large IR optical components
  • Significant cost factor
  • Possibly implement deployable imagers
  • Large number of spectrographs
  • Cost factor
  • High Strehl ratio required
  • Scattered light control
  • Is > 0.9 reasonable?
  • Good instrument for TES or STJ detector?

March 2002