This document describes the final design of the Gemini Near-infrared Integral Field Spectrograph to be presented at the Critical Design Review (CDR) at the Research School of Astronomy and Astrophysics (RSAA) of the Australian National University in Canberra, Australia, on 19-20 April 2001.
The Gemini Near-infrared Integral Field Spectrograph (NIFS) will be used with the ALTAIR adaptive optics system on the Gemini North 8 m telescope to perform near diffraction-limited, integral field spectroscopy at near-infrared wavelengths. The primary purpose of NIFS is to study moderate surface brightness structures around discrete objects that are revealed at high spatial resolution by ALTAIR. To do so, NIFS will use a reflective Integral Field Unit (IFU) to divide its 3.0″×3.0″ field-of-view into 29 slitlets each 0.1″ wide and 3.0″ long and simultaneously obtain spectra for each 0.04″ pixel along each slitlet. The NIFS spatial sampling is chosen to sample at close to the Gemini diffraction-limit while maintaining a modest field-of-view. Most NIFS programs will require good natural seeing to perform high spatial resolution observations. These programs will benefit from queue scheduling. NIFS will operate in the wavelength range from 0.94-2.50 μm where ALTAIR delivers its greatest gains. NIFS will use a two-pixel spectral resolving power R ~ 5300 which will permit measurements between strong near-infrared OH airglow emission lines where the near-infrared sky background is low.
The NIFS instrumental parameters are summarized in the following list:
· Wavelength range: 0.94-2.5 μm.
· Spatial pixel size: 0.04″×0.10″ on sky.
· Spectral resolving power: ~ 5300 (two pixels).
· Focal plane occulting masks: 0.1″, 0.2″, 0.5″ diameter circular.
· Neutral density filter.
· Gratings: Z, J, H, K.
· Order blocking filters: Z, J, H, K.
· Polarimetry: wire grid with K filter only.
· Direct view mirror for target acquisition.
· Detector: Rockwell 2048×2048 HgCdTe HAWAII-2, 18 μm pixels.
· Near-infrared on-instrument wavefront sensor.
- Wavelength range: 0.95-2.5 μm.
- Steerable over 180″ FOV (120″ used with ALTAIR).
- Image scale: 0.170″/pixel.
- Filters: J, H, K.
- Instantaneous field-of-view: 2″ diameter.
RSAA is fast-tracking development of NIFS by re-using many of the designs already developed for the Gemini Near-InfraRed Imager (NIRI) by the Institute for Astronomy (IfA) of the University of Hawaii. The NIFS spectrograph will be mounted in a duplicate of the NIRI cryostat and use a duplicate of the NIRI On-Instrument Wavefront Sensor (OIWFS) to sense tip-tilt and focus corrections. The NIFS science instrument will use the same mechanism encoding architecture used in NIRI and will use the same cryogenic stepper motors adopted for NIRI. This commonality of mechanical designs means that the NIRI mechanism control system hardware and temperature control system hardware can also be duplicated for NIFS with minimal change. The EPICS Instrument Sequencer (IS), Components Controller (CC), and engineering interface software developed for NIRI and the CC for the OIWFS also have been re-used for NIFS with only minor modification.
Duplication of NIRI components has proceeded at RSAA since the Conceptual Design Review (CoDR) in parallel with the NIFS spectrograph design. The detector system for the duplicate OIWFS will be developed at the IfA. This approach has clearly led to significant savings in schedule and budget.
NIFS was proposed to the Gemini Instrument Forum at its March 1999 meeting. It was conceived as a fast-tracked instrument to quickly provide a near-infrared spectroscopic capability in addition to GNIRS at the Gemini telescopes. The CoDR was held on 15-16 March 2000. The CoDR documents are available on-line[1]. The report of the CoDR committee is also available on-line[2]. The review committee had no major criticisms of the proposed instrument. They recommended that:
· The concentric IFU design be adopted.
· A prototype IFU mirror array be manufactured.
· The R ~ 5300 spectrograph option be adopted.
· The optical design be modified to accommodate a 5 μm cutoff detector.
· Active focus control not be provided for the spectrograph or the OIWFS.
· A HAWAII-2 PACE detector be ordered from the Rockwell Science Center.
All of these recommendations have been implemented in the final design. The review committee also raised concerns about the impact of persistence and fringing on performance and the impact of an imperfect knowledge of the off-axis point spread function on the interpretation of scientific data.
Changes to the NIFS design since the CoDR are :
· The grating wheel has been replaced by a grating turret which is supported by a shaft attached to the Cold Work Surface plate in a manner similar to that planned for GNIRS. This provides much stiffer support for the gratings and a degree of wavelength range adjustment by varying the grating angle. However, this change does compromise wavelength repeatability slightly when gratings are interchanged.
· A flip mirror has been included to bypass the grating and deliver undispersed images to the detector. This is necessary so that faint objects can be acquired without moving the grating and hence altering the wavelength calibration.
· The silica field flattener in the spectrograph camera is now thermally isolated from other components and cooled to 65 K by a cold strap in order to prevent 4-5 μm radiation illuminating the detector. This blocking is sufficient to permit the future use of a 5 μm cut-off science detector without modification to the spectrograph optics.
· The detector system for the NIFS spectrograph is being designed jointly by the Research School of Astronomy and Astrophysics and the Institute for Astronomy.
· A neutral density filter will be provided in the Focal Plane Mask Wheel to permit measurements of bright objects.
· A wire grid and K filter will be included in the Filter Wheel to permit spectropolarimetric observations in the K band only.
· All spectrograph mirrors are to be diamond machined.
Additionally, it has proven to be cost effective for the duplication of NIRI components to occur at RSAA, not IfA as was proposed at the CoDR.