Instrument design

The design of the GMTIFS instrument encompasses two key aspects:

  • the optical design of the instrument, and
  • the mechanical design of the instrument.

Optical design

GMTIFS is a near-infrared, combined integral-field spectrograph (IFS) and imager with diffraction-limited angular resolution. It will be located on the Gregorian Instrument Platform of the GMT and is fed at f/8 by the direct telescope beam from the adaptive secondary mirror. It accepts a 20.4 mm square science field centred within a 180 mm diameter circular guide field.

The cryostat window is tilted and carries a dichroic coating that transmits near-infrared wavelengths and reflects visual wavelengths to the external adaptive optics (AO) wave-front sensor system.

An Offner reimaging system is used to relay the 20.4 mm square science field at the telescope focus to an accessible position for the IFS and Imager. A rotating pupil image is formed on the second mirror of this system, which is masked and rotated to act as the cold stop. The long beam run from the third mirror is used to accommodate the atmospheric dispersion corrector (ADC).

The Imager is always available for field acquisition and tip-tilt guiding while the IFS is in use. This is achieved by means of selectable dichroic beam splitters that reflect the chosen science-band wavelengths to the IFS and transmit the off-band wavelengths to the Imager. The Imager can also be used exclusively by deploying a transmissive focus corrector plate in place of the beam splitters. Both instruments operate over the Z,J, H, and K bands (0.94-2.42 µm).

A precision On-Instrument Wave-Front Sensor (OIWFS) Feed is used to select an adaptive-optics guide star from within the guide field and relay its light to the OIWFS. Light from the guide field passes through a field lens into the OIWFS Feed system. A beam-steering mirror located at a pupil image is used to steer the image of a natural guide star to a fixed pick-off mirror located on the back of the science field pick-off mirror. A reimaging system then relays the captured guide-star image to an accessible position at the input to the OIWFS.

A calibration unit mounts on the front of the cryostat. It includes a deployable mirror that projects calibration light into the cryostat through the cryostat window.

The unfolded layout of the optical systems that are housed within the cryostat is shown below.

Isometric view of the unfolded GMTIFS optical systems that are housed within the

Figure 1 - Isometric view of the unfolded GMTIFS optical systems that are housed within the cryostat.

Light enters the cryostat through the tilted window on the left. The window is immediately followed by a second tilted plate. Both plates are slightly wedged, and together they act to eliminate the aberration that would be caused by a single tilted plate.

The off-axis guide star beam then passes through the doublet field lens near the telescope focus, and progresses through a corrector lens to the beam-steering mirror, where the field lens has formed a pupil image. The beam-steering mirror has a concave spherical surface with its center of curvature close to the telescope focal plane, and so forms a return image of the guide star near the telescope focus. The mirror is tipped and tilted about its vertex to steer this image through a hole in the field lenses. A tilted pick-off mirror mounted from the field lenses then reflects the beam downwards and through a relay. Only the first collimating doublet lens for this is shown in the figure, with the rays being terminated at the pupil image formed below it. The optical system beyond this point is part of the OIWFS.

The pick-off mirror that reflects the guide star beam downwards is double-sided and sized to match the 20.4 mm square science field. It therefore reflects the central science beam upwards where it passes through the Offner science-field relay. The first mirror of this relay forms a pupil image on the second mirror, which is used as the cold stop. The converging beam from the third mirror of the science-field relay then passes through the atmospheric dispersion corrector (ADC).

After atmospheric dispersion correction, the beam passes to a suite of four science-selector dichroic beam splitters that separate the Imager and IFS fields. These define different pass bands for the IFS and supplant IFS filters. Each beam splitter consists of two tilted and slightly wedged plates that correct aberration in the transmitted beam in the same way as the corrected cryostat window. The front face carries the dichroic coating with the reflected beam going to the IFS, and the transmitted beam passing to the Imager.

Integral-field spectrograph design

The science relay delivers an f/8 focus to the IFS at one of four interchangeable rectangular field masks. These are appropriate for the different IFS field scales, ranging in size from 0.28x0.56 arcsec for the finest scale to 2.23x4.46 arcsec for the coarsest, with the step size being a factor of two. Each of these interchangeable field masks is followed immediately by a matching anamorphic projector. The four projectors are geometrically similar, differing only in that their size is proportional to the field size.

The GMTIFS IFS integral-field unit is based on an image slicer that divides the field into contiguous slitlets that are reformatted into one dimension so that the other can be used for dispersion. The image slicer lies at the output focal plane of the anamorphic projector. The image slicer field is 90 mm square, and divided into 45 slitlets each 2 mm wide. The integral-field unit uses concentricity about the image slicer fanning axis to make all 45 channels identical and thereby avoids off-axis aberrations. The mirror elements of the pupil- and field-mirror arrays are distributed along circular arcs that are perpendicular to, and centred on the same fanning axis, so that all integral-field unit channels are radial.

The collimator mirror surface is spherical with its center of curvature also on the fanning axis. It therefore reflects the beams from the field-mirror array back to a pupil image on the fanning axis, where the VPH grating is located. The field-mirror array is midway between the fanning axis and the collimator, so the beams to the grating are collimated.

A suite of VPH diffraction gratings is proposed that provide spectral resolving powers of 5,000 and 10,000. With the proposed 4k pixel square detector, the individual Z, J, H, and K bands only use about half the detector in the spectral direction at the lower resolving power. Three gratings are therefore proposed that span selected spectral features in adjacent bands (mZJ, mJH and mHK), using the entire detector. For the higher resolving power, four gratings will be provided; these being the hZ, hJ, hH, and hK gratings.

A Three-Mirror Anastigmat (TMA) camera is used to relay light from the grating to the detector. The TMA employs three diamond-turned surfaces.

Imager design

The Imager optics relay the image from the input field mask to the detector with a magnification of 3:1, thereby filling the 4kx4kx15 µm pixel detector with the 20.4 arcsec square science field at a scale of 5 mas pixel-1.

The Imager is a simple refractive system that uses two doublets and a field flattener. A pupil image is formed between the doublets, providing a convenient location for filters. The filter suite is distributed between two wheels placed on either side of the pupil image.

Two supplementary functions are provided by means of three sets of deployable optics.

  • The pupil imager forms an image of the pupil on the detector that allows misalignment of the cold stop to be measured. Rotational misalignment can then be corrected with the cold stop drive. Translational misalignment can be corrected by adjusting the cryostat orientation on its mount. The pupil imager does not introduce any significant aberration, but the distortion produced by the cryostat window is significant and faithfully reproduced.
  • The extender and retractor allow curvature wave-front analysis. They displace the focus in opposite directions such that out-of-focus star images are recorded at the Imager detector.

Mechanical design

The GMTIFS cryostat will be located on the Gregorian Instrument Platform of the GMT. The instrument is positioned such that the optical axis is 1,000 mm above the instrument platform floor, resulting in 340 mm of clearance between the top of the instrument and the instrument space ceiling. Similarly, the clearance between the Instrument Platform and the instrument is 40 mm.

GMTIFS positioned on the Instrument Platform within its assigned envelope.

Figure 2 - GMTIFS positioned on the Instrument Platform within its assigned envelope.

GMTIFS positioned on Instrument Platform - front view.

Figure 3 - GMTIFS positioned on Instrument Platform - front view.

On-instrument wave-front sensor feed

The instrument input field is reflected off the tertiary mirror and enters the cryostat through a tilted dichroic window and its tilted corrector plate. The corrector plate is cooled by straps connected to the cold work surface and is attached to the window mount with four titanium plates. Thermal conductive flux between the window mount and the corrector plate is minimized through the use of fiberglass pads.

The guide-field light then passes through doublet field lenses, positioned in the central compartment of the CWS, towards the beam-steering mirror that patrols the guide field to acquire natural guide stars. The reflected light is guided through the central hole in the field lens doublet to be folded by the guide field pickoff mirror and reflected towards the On-Instrument Wave-Front Sensor (OIWFS) in the bottom section of the cryostat.

Guide field path towards the On-Instrument Wave-Front Sensor.

Figure 4 - Guide field path towards the On-Instrument Wave-Front Sensor.

Dichroic window with corrector plate, and the doublet field lens and guide-field

Figure 5 - Dichroic window with corrector plate, and the doublet field lens and guide-field pick-off mirror.


Science fore-optics

The science field is captured by the science pick-off mirror, which reflects the central 20.4×20.4 mm of the input field. The science pick-off mirror forms the front side of the guide-field pick-off mirror presented earlier. The light is folded to the upper part of the cryostat where the fore-optics system is situated.

Science field input towards the fore-optics system.

Figure 6 - Science field input towards the fore-optics system.

After the first fold mirror, the science beam is sent towards the science-field relay, which consists of three mirrors (M1-M3). The telescope pupil is reimaged at the science field relay M2 mirror, where the rotating cold stop system is located. Following the relay, the science beam passes through the atmospheric dispersion corrector (ADC) lens system. These are grouped in two doublet pair groups, and require two independent rotary mechanisms to fulfill their function.

The last element in the fore-optics system is the science selector in the form of a six-position rotary mechanism. The wheel carries five sets of dichroic/corrector plate doublets that reflect light to the integral-field spectrograph (IFS) and transmit light to the Imager. The sixth wheel position is occupied by a single parallel plate that only transmits light to the Imager (i.e., the Imager only operation mode).

Fore-optics component layout - top view.

Figure 7 - Fore-optics component layout - top view.

Fore-optics component layout - isometric view.

Figure 8 - Fore-optics component layout - isometric view.

Integral-field spectrograph

The input science field is reflected into the IFS by the science selector dichroic. The first elements in the IFS are the focal-plane wheel and the anamorphic projector mechanism. The focal-plane wheel mechanism carries 16 different masks and there are four sets of anamorphic projector optics, one for each of the IFS scales. These are located in a monolithic mount that allows precise positioning and alignment of the anamorphic projector mirrors. This mount is attached to a linear slide mechanism with sub-micron positioning accuracy. Each of the anamorphic projectors carries its own exit fold mirror that sends the IFS beam through the CWS, to the bottom section of the cryostat where the primary IFS elements are located.

IFS focal-plane wheel and anamorphic projector mechanism.

Figure 9 - IFS focal-plane wheel and anamorphic projector mechanism.

The beam from the anamorphic projector passes through the Cold Work Surface (CWS) to the IFS. It is first reflected by the 2nd IFS fold mirror, which sends the beam parallel to the CWS towards the image slicer. The sliced and fanned field is reflected back to the field and pupil mirror arrays and then further towards the collimator mirror. After the collimator, the beam is fanned by the 3rd fold mirror towards the VPH gratings. The gratings are carried on a tilted wheel that can accommodate up to seven transmissive grating plates up to 100×60 mm in size.

Following the gratings, the formed spectra are passed towards a TMA (three-mirror anastigmat) camera and finally imaged on the IFS detector. The IFS HAWAII-4RG detector package is mounted on a linear mechanism that accommodates necessary focus adjustment.

IFS layout - top view.

Figure 10 - IFS layout - top view.


The science beam enters the Imager optics after being reflected by the Imager fold mirror, which tilts the input beam by 20 degrees. The Imager light passes through the first doublet lens, a dual filter wheel, the 2nd doublet lens, utility wheel, and finally it is recorded by the Imager detector (HAWAII-4RG). The filter wheels have 10 positions each, potentially accommodating 17 filters, in addition to one blocked and two clear positions (one in each wheel). The utility wheel has 4 positions; one clear, two for focus extender/retractor lens doublets, and one for pupil re-imaging lenses. The detector focus stage carries the Imager detector and a field-flattener lens.

Imager layout.

Figure 11 - Imager layout.