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Centre for Advanced Instrumentation

GNIRS-IFU

An Integral Field Unit For The GEMINI Near InfraRed Spectrography

Introduction

The Astronomical Instrumentation Group (AIG) of the University of Durham are currently developing for GEMINI an Integral Field Unit (IFU) for the GEMINI Near-InfraRed Spectrograph (GNIRS).

GNIRS is one of the initial phase instruments for Gemini North and is currently under construction at the National Optical Astronomy Observatories (NOAO) of the USA. In its baseline form, GNIRS provides a powerful capability for low to medium resolution spectroscopy over the 1 to 5 mm wavelength range using two different cameras which provide image scales of 0.05 and 0.15 arcsec per pixel respectively. With the addition of the IFU, GNIRS will gain a powerful and innovative capability for Integral Field Spectroscopy (IFS).

The design of the GNIRS IFU is based on the Advanced Image Slicer Concept developed as a result of research conducted under the auspices of the Durham Instrumentation R&D Program. This innovative, all-reflective slicer-based system has many advantages over fibre-based designs, especially for cryogenic instruments such as GNIRS. An introduction to the principles of Integral Field Spectroscopy in general and the Advanced Image Slicer concept in particular is presented in a variety of papers published by our group.

The IFU acts as a coupler between the telescope and the spectrograph by reformatting a rectangular field into a quasi-continuous pseudo-slit located at the entrance focal plane of the spectrograph. It slices the original two-dimensional image into a number (21) of contiguous narrow sub-images which are re-imaged side by side to form a long one-dimensional image at the spectrograph input. This forms the entrance aperture to the spectrograph in the same way as a slit in conventional aperture spectroscopy (AS) mode, so that the light from each slice is dispersed to form spectra on the detector. In this way, a spectrum can be obtained simultaneously for each spatial sample within the IFU field.

The IFU is optimised for use with tip-tilt corrected images, and will be used in conjunction with the short GNIRS camera. The IFU is a fully independent, self-contained module which will be located at the telescope focal plane inside the slit slide mechanism, which also accommodates the slit mask for conventional aperture spectroscopy. This enables GNIRS to change remotely between either of the conventional aperture spectroscopy mode and the IFS mode by simply inserting or withdrawing the appropriate module from the beam.

The optical and mechanical components for the Low Resolution IFU are currently being manufactured at the Laboratory for Precision Machining (LFM) of the University of Bremen, Germany. Delivery of these components is scheduled for April 2003, and we will subsequently proceed to Integration and Acceptance Test of the IFU.

In addition to the baseline Low Resolution (LR) IFU, we are planning to study the feasibility of a High Resolution (HR) IFU which will be optimised for use with fully adaptively corrected images to provide higher spatial resolution over a smaller field of view. Like the LR IFU, this will be a fully independent, self-contained module, which could be added to GNIRS at a later date.

Specifications

The GNIRS IFU’s system level requirements, which have been derived from the scientific objectives established by GEMINI, are summarised as follows:

  • Wavelength range: 1.0 to 2.5 mm requirement; 1 to 5 mm target;
     
  • Spatial sampling scale: 0.04” (HR mode) to 0.15” (LR mode) to take full advantage of the superb images provided by the GEMINI telescopes in the near infrared;
     
  • Maximum field of view and spectrum length (limited by the detector's available real estate);
     
  • The IFU should degrade a critically sampled image by no more than 10% (not taking into account the performance of the main spectrograph optics);
     
  • Good background subtraction, combined with a moderately high spectral resolution (R > 6000) to eliminate the effects of the relatively bright sky (i.e. the OH airglow lines in the J and H windows), even between the OH lines.

The following table summarises the derived system specifications consistent with the system level requirements.

IFU MODE Low Resolution (LR) High Resolution (HR)
Spatial Sampling (IFU Pixel) 0.15” x 0.15” 0.04” x 0.04”
Number of Slices 21 26
Field of View 3.15” x 4.46” 1.04” x 1.50”
Number of Spatial Elements 625 972
Spectrum Length 1024 pixels 1024 pixels
Slice Dimensions 10.876 x 0.788 mm2 9.4 x 0.5 mm2
GNIRS Camera Employed Short Camera Long Camera

Design Features

The salient features of the GNIRS IFU's optical and mechanical design are summarised below:

  • The IFU contains 6 optical elements, arranged in two optical subsystems: the fore-optics consisting of a flat pick-off mirror and two aspheric re-imaging mirrors, and the image slicer consisting of the slicing mirror assembly and the monolithic pupil- and slit mirror arrays;
     
  • The fore-optics introduce an anamorphic magnification of the field so that each spatial resolution element projects onto 1 x 2 pixels at the detector (i.e. the width of each slice corresponds to 2 pixels) to ensure correct (Nyquist) sampling in the dispersion direction;
     
  • The (spherical) slicing mirrors "slice" the re-imaged field and form a real image of the telescope pupil on their corresponding (spherical) pupil mirrors. Each pupil mirror then re-images this slice of the original field on its corresponding (toroidal) slit mirror located at the spectrograph's focal plane (slit plane), which re-images the telescope pupil onto the spectrograph pupil stop;
  • The design is optimised for use in a cryogenic environment to enable operation in the near-infrared wavelength range;
     
  • The IFU offers an excellent image quality (see spot diagrams below);
     
  • The slicer-based design facilitates good background subtraction since the PSF and throughput of the system vary smoothly across the field;
     
  • The design employs all reflective (and thus achromatic) optics;
     
  • All components (including optical components) are made from aluminium in order to eliminate effects of differential thermal expansion and thus ensure thermal compatibility with the GNIRS instrument;
     
  • All optical surfaces are diamond machined to provide excellent surface quality and roughness in order to minimise scattered light, which would otherwise increase the thermal background;
     
  • All optical surfaces are gold-coated to maximise their reflectivity in the (near) infrared;
     
  • The use of monolithic arrays (slicing, pupil and slit mirror arrays) facilitates integration of the IFU, as it is no longer necessary to align the individual slicing-, pupil- and slit mirrors into their respective assemblies, but instead their relative alignment is assured through accurate machining. Datum surfaces are machined in the same operation where possible;
     
  • The design facilitates accurate alignment of the optical components;
     
  • Compact (200 x 100 x 53 mm approx.) and light-weight (0.9 kg approx.) design;
     
  • Baffles are used to ensure that there will be no direct path through the IFU for stray light. All passive surfaces are painted black with Nextel;
     
  • A three-point mount fixes the IFU inside the GNIRS slit slide mechanism; a reference (datum) edge defines the module's alignment. If necessary, the unit can be aligned through shimming;
     
  • The input focal plane of the IFU is coincident with the output focal plane of the Offner relay, which is the plane of the slit. The input and output of the IFU will be parallel with the optical axis of the GNIRS instrument as defined at the slit centre. Deployment of the IFU will not require a change in GNIRS focus.

Project Team

The GNIRS IFU Project Team at Durham comprises the following key AIG personnel:

David J. Robertson Work Package Manager
Jeremy R. Allington-Smith Work Package Scientist
Marc Dubbeldam Project Management 
Systems Engineering 
Mechanical Design
Robert Content Optical Design
Colin Dunlop Assembly, Integration and Test
Phil Armstrong Mechanical Workshop Manager

References

  • Content R., “A New Design for Integral Field Spectroscopy with 8-m Telescopes”, SPIE Proceedings, Vol. 2871, pp. 1295-1305, 1997.