Instrument description

The era of 8-10 m telescopes saw revolutionary advances in adaptive optics and integral-field spectroscopy that placed high angular resolution astronomy at the scientific forefront. Laser Guide Stars (LGS) adaptive optics systems made high angular resolution science on ground-based telescopes generally accessible over a broad range of fields. Coupling these adaptive optics systems to near-infrared integral-field spectrographs, such as NIFS on GeminiOSIRIS on Keck, andSINFONI on the VLT, enabled adaptive optics science to transition from descriptive morphological studies to quantitative studies of kinematics, excitation, and chemistry that probe physical mechanisms occurring in a wide range of astrophysical objects. These are now essential capabilities for the next generation of Extremely Large Telescopes (ELTs). The Giant Magellan Telescope Integral-Field Spectrograph (GMTIFS) will provide the GMT community with such a single-object adaptive-optics-corrected near-infrared integral-field spectrograph to comprehensively study detailed physical phenomena in a wide variety of discrete objects at high angular resolution.

The Giant Magellan Telescope will extend the capabilities of existing adaptive-optics-corrected integral-field spectrographs in two fundamental ways: The larger telescope aperture allows higher angular resolution diffraction-limited observations that probe structures on finer linear scales than is currently possible. And, its larger light collecting power allows studies of fainter targets at the same angular resolution that is achieved with current 8-10 m telescopes. The former advantage will be exploited in many diffraction-limited studies including those of massive black holes and star clusters in galactic nuclei, the central obscuring torus regions of Active Galactic Nuclei (AGN), and collimated outflows and planet-forming disks associated with young stars. The latter advantage will be exploited on faint targets including studies of galaxy assembly where typical gaseous clump sizes are ~ 1 kpc, which corresponds to > 100 mas at all redshifts. GMTIFS realizes both of these advantages for the GMT community.

 

Functional overview

GMTIFS will be used behind GMT's Laser Tomography Adaptive Optics (LTAO) system, and so will operate in the 1-2.5 µm wavelength region where this system performs well. The LTAO system uses GMT's adaptive secondary mirror, six LGSs, and at least one Natural Guide Star (NGS) to deliver a diffraction-limited field of ~ 30 arcsec diameter to the science instrument. The angular resolution within this field ranges from ~ 10 mas at a wavelength of 1.0 µm to ~ 25 mas at 2.5 µm. GMTIFS will have both an integral-field spectroscopy mode and an imaging mode to exploit this potential. Basic parameters for GMTIFS are listed below.

Integral-field spectrograph

IFS feed: Dichroic mirror reflection
Wavelength range: 0.9-2.5 µm
Field geometry: 45 slitlets x 88 spaxels per slitlet
Spaxel size: 50, 25, 12, 6 mas, square on sky
Field of view: 4.40x2.25, 2.20x1.125, 1.056x0.54, 0.528x0.27 arcsec
Field location: 3.64, 0.00, -1.83, -2.74 arcsec with respect to the optical axis
Spectral resolving power: 5,000 (mZJ, mJH, mHK)
  10,000 (hZ, hJ, hH, hK)
Spectral coverage: 0.89 - 1.35 µm (mZJ)
  1.19 - 1.80 µm (mJH)
  1.64 - 2.49 µm (mHK)
  0.92 - 1.13 µm (hZ)
  1.10 - 1.35 µm (hJ)
  1.47 - 1.80 µm (hH)
  2.04 - 2.51 µm (hK)
Focal-plane occulting masks: 25, 50, 100, 200 mas diameter circular
Target acquisition: Imaging mode or collapsed IFS data cube
Detector: Teledyne 4096x4096 pixel HgCdTe HAWAII-4RG-15
Guide Mode: Tip-tilt Guider or Imager On-Detector Guide Window
 

Imager

Imager feed: Direct or dichroic transmission
Wavelength range: 0.9 - 2.5 µm
Pixel size: 5 mas
Field of view: 20.4x20.4 arcsec square
Filters: JHK + intermediate bands + narrow bands
Target acquisition: Imaging mode
Detector: Teledyne 4096x4096 pixel HgCdTe HAWAII-4RG-15
Guide mode: Tip-Tilt Guider or Imager On-Detector Guide Windows
Cold pupil stop alignment: Pupil-viewing optics
Non-common path wave front Dual defocus lenses
 

Tip-tilt guider

Wavelength range: 0.9-2.5 µm
Patrol field: 180 arcsec diameter circular, excluding Imager field
Guide star selection: Beam-steering mirror at pupil image
 

Calibration

IFS dark exposures: IFS optical path blocked
Imager dark exposures: Imager optical path blocked
Wavelength calibration: Ar+Xe arc lamps
Flat field calibration: Incandescent lamp
Intensity attenuation: Neutral density filters matched to all science scales
Illuminated field of view: 20.4x20.4 arcsec Imager field
 

By their nature, integral-field spectrographs are most sensitive to objects that are compact, both in a spatial and a spectral sense. The GMTIFS Integral-Field Spectrograph (IFS) offers a range of four image scales and corresponding rectangular fields of view. The coarsest scale samples faint targets with ~ 50 mas sampling. The fine scales sample the diffraction full width at half maximum (FWHM) over a range of wavelengths. The rectangular field of view permits in-field nodding of compact science targets for accurate sky-subtraction with optimal efficiency. The use of square spaxels on the sky simplifies data analysis. Two moderate spectral resolving powers will be provided. The velocity resolution of ~ 60 km/s achieved at R ~ 5,000 samples dynamical motions in large galaxies and allows access to significant low-background wavelength regions between strong airglow emission lines. The higher resolution R ~ 10,000 gratings produce velocity resolutions of ~ 30 km/s that probe stellar kinematics in the vicinities of lower mass (104-106 MSun) black holes and nuclear star clusters.

The GMTIFS Imager has a field of view of 20.4 x 20.4 arcsec that is well matched to the adaptive-optics-corrected field of the single-conjugate LTAO system. The 5 mas pixel size of the imaging mode Nyquist samples the 10 mas diffraction FWHM at the shortest operating wavelength of ~ 1 µm and over-samples the 22 mas diffracation FWHM at 2.2 µm for greater fitting precision. Standard broad-band JHK filters are available, as well as a range of intermediate-band filters that are suitable for obtaining photometric redshifts and measuring coarse stellar population characteristics. The Imager also includes a facility for imaging the telescope exit pupil and the cold pupil mask to ensure their accurate alignment, and so maximize detected signal and minimize detected background. Achieving high Strehl ratio imaging requires that static non-common path wave-front errors within GMTIFS be measured and nulled using the adaptive secondary mirror. Two defocus lenses allow the measurement of out-of-focus stellar images that will be used with a curvature-sensing algorithm to measure non-common-path wave-front errors at the Imager detector. In addition to its scientific role, the Imager also acts as the acquisition camera for the IFS.

GMTIFS has been designed from the outset to achieve high adaptive-optics sky coverage. This makes a large variety of science routinely accessible to GMTIFS. The NGS requirements of the LTAO system are still being defined. It is expected that a NGS with R < 16.5 mag will be required to measure the fast tip-tilt information that complements the high-order correction provided by the LGSs. Diffraction-limited observations with the fine integral-field spectrograph scales or the Imager require the NGS to be located within ~ 30 arcsec of the science target. Much of this area is occupied by the GMTIFS Imager field, so tip-tilt guiding will be possible using an On-Detector Guide Window on the Imager detector. Similarly during IFS observations, tip-tilt motions can be monitored with the On-Detector Guide Window in out-of-band light from suitable science targets using a dichroic science-selector element that reflects primary science wavelengths to the IFS while passing complementary wavelengths to the Imager On-Detector Guide Window. Greater image motion (i.e., angular anisoplanatism) can be tolerated with the coarse IFS image scales. At the coarsest 50 mas/spaxel scale, NGSs can be used anywhere within the 180 arcsec diameter GMTIFS guide field that is centered on the science field.

The unprecedented high angular resolution attainable with LTAO on GMT places strong constraints on image quality. Principle among these is the need to accurately correct differential atmospheric dispersion over the science wavelength band. The GMTIFS instrument includes fore-optics that contain a multi-component atmospheric dispersion corrector for this purposes. Low backgrounds are achieved using a rotating cold pupil stop, also in the fore-optics. Flat-field and wavelength-calibration sources are also provided.

Together, the GMTIFS Integral-Field Spectrograph and Imager will form the workhorse narrow-field adaptive-optics capability of GMT. They will be used to address many of the key science drivers for the GMT telescope that are articulated in the context of GMTIFS in the GMTIFS science case.

Performance overview

Integral-field spectrograph

Accurate estimates of the performance of the GMTIFS Integral-Field Spectrograph (IFS) are needed to inform discussion of its science potential. These estimates can be made most conveniently using the GMTIFS IFS Exposure Time Calculator (ETC).

The performance of an adaptive-optics-corrected integral-field spectrograph depends on how spatially and spectrally compact is the source. An IFS is most sensitive to emission that is compact both spatially and spectrally, so it is optimally suited to measuring clumpy line emission. Adaptive-optics correction improves sensitivity to unresolved objects by placing more light in a given detector pixel. The sensitivity to uniform extended emission is unchanged by adaptive-optics correction. However, adaptive-optics correction does improve our ability to detect structure in complex systems that would otherwise appear to be uniform extended emission. The sensitivity of the GMTIFS IFS has been benchmarked assuming we require a signal-to-noise ratio of 10 in 104 s of on-source exposure, consisting of 10 individual exposures each of 1000 s duration. Limiting magnitudes for unresolved sources, extended continuum emission, and extended line emission on this basis are presented in the tables below. It is assumed that the measurement aperture is circular and two spaxels in diameter. The emission-line sensitivity assumes a line FWHM of 200 km/s.

 

Limiting Vega magnitudes of the GMTIFS IFS on unresolved sources

Spatial Scale Grating
(mas/spaxel) mZJ mJH mHK hZ hJ hH hK
50 21.9 22.1 21.4 20.4 21.2 21.5 20.9
25 22.1 22.3 21.9 20.5 21.3 21.6 21.3
12 22.2 22.6 22.4 20.5 21.5 21.9 21.7
6 22.3 21.9 21.1 20.8 21.5 21.2 20.4
 

Limiting Vega magnitudes/sq. arcsec of the GMTIFS IFS on extended continuum sources

Spatial Scale Grating
(mas/spaxel) mZJ mJH mHK hZ hJ hH hK
50 18.8 18.3 17.2 17.9 18.1 17.7 16.7
25 17.4 17.0 16.2 16.5 16.7 16.3 15.6
12 15.9 15.4 14.8 14.9 15.1 14.7 14.1
6 14.4 13.9 13.4 13.4 13.6 13.2 12.6
 

Limiting emission-line surface brightness of the GMTIFS IFS in erg cm-2 s-1 arcsec-2

Spatial Scale Grating
(mas/spaxel) mZJ mJH mHK hZ hJ hH hK
50 7.7x10-17 6.0x10-17 7.7x10-17 2.6x10-16 1.4x10-16 1.1x10-16 1.2x10-16
25 2.7x10-16 2.0x10-16 2.0x10-16 1.0x10-15 5.3x10-16 3.9x10-16 3.4x10-16
12 1.1x10-15 8.3x10-16 6.8x10-16 4.2x10-15 2.2x10-15 1.6x10-15 1.3x10-15
6 4.4x10-15 3.3x10-15 2.5x10-15 1.7x10-14 8.9x10-15 6.5x10-15 5.0x10-15

The GMTIFS IFS will be background-limited in 1,800 s exposures with the mHK and hK gratings at the 50 mas/spaxel scale and with the mHK grating at the 25 mas/spaxel scale. The IFS will be limited by detector read noise and dark current in all other configurations.

 

Imager

The performance of the GMTIFS Imager can be predicted using the GMTIFS Imager Exposure Time Calculator (ETC).

The sensitivity of the GMTIFS Imager has been benchmarked assuming we require a signal-to-noise ratio of 10 in 104 s of on-source exposure, consisting of 100 individual exposures each of 100 s duration. Limiting magnitude for point sources, extended continuum emission, and extended line emission on this basis in the tables below. It is assumed that the measurement aperture is circular and 50 mas in diameter.

 

Limiting sensitivities of the GMTIFS Imager

Source Filter
Z J H K
Star (Vega mag) 26.1 26.2 26.1 25.9
Continuum (Vega mag arcsec-2) 21.8 21.3 20.6 20.0
Line (erg cm-2 s-1 arcsec-2) 2.1x10-15 1.4x10-15 2.1x10-15 1.4x10-15

The GMTIFS Imager will be background-limited in 120 s exposures in all broad-band (and intermediate-band) filters.

 

Updated:  21 October 2017/Responsible Officer:  RSAA Director/Page Contact:  Webmaster