WiFeS: Wide-Field Spectrograph Overview

• WiFeS is an optical integral field spectrograph that acquires contiguous spectra over a 25×38 arcsec field via advanced image-slicing optics for high throughput and spatial fidelity.
• It features dual blue and red channels with VPH gratings offering resolutions of R≈6800 and R≈2900, enabling precise kinematic mapping and spectral analysis.
• Its efficient nod-and-shuffle mode and robust data reduction pipeline ensure superior sky subtraction and stable calibration for a wide range of astrophysical applications.

The Wide-Field Spectrograph (WiFeS) is an optical integral field spectrograph (IFS) deployed on the Australian National University (ANU) 2.3 m telescope at Siding Spring Observatory. Designed and built at the Research School of Astronomy and Astrophysics (RSAA), WiFeS enables simultaneous acquisition of spectra over a contiguous $25 \times 38$ arcsec$^2$ field using advanced image-slicing optics, with the specific aim of maximizing throughput, stability, and spatial fidelity. Its architecture comprises dual optimized channels (blue and red), volume phase holographic (VPH) gratings offering both moderate and high spectral resolution, and operation modes (including nod-and-shuffle) designed for efficient sky subtraction and flexibility in astrophysical applications. It has become a foundational instrument for a range of extragalactic, Galactic, and transient surveys, including AGN integral-field surveys, supernova followup, planetary nebulae kinematic studies, and the construction of high-fidelity spectral libraries of resolved stellar populations.

1. Optical and Mechanical Design

WiFeS is a dedicated image-slicing integral field spectrograph, leveraging several technical features to achieve wide field, high throughput, and spatially uniform performance:

• Field Slicing and Detector Format:

The instrument accepts a $25 \times 38$ arcsec field, divided by 25 image-sliced slitlets, each 38 arcsec long and 1 arcsec wide. The slicer optics remap this 2D field onto $25 \times$ 76 spaxels (spatial pixels), preserving a spatial sampling of $0.5$ arcsec per unbinned pixel. The dispersed spectra are imaged onto a $4096 \times 4096$ pixel CCD.

• Dual-Arm Configuration and Dichroics:

WiFeS employs two parallel cameras—optimized for blue and red wavelengths—split by selectable dichroics (e.g., RT480, RT560, RT615). This configuration permits simultaneous, virtually contiguous coverage from 3300 Å to 9050 Å, depending on grating and dichroic combinations.

• Volume Phase Holographic (VPH) Gratings and Spectral Resolution:
  • High resolution: $R = \lambda / \Delta\lambda \approx 6800$ (velocity resolution $\sim$50 km/s).
  • Low resolution: $R \approx 2900$ (velocity resolution $\sim$105 km/s).
  • The measured arc line FWHM is 2.2 pixels (variation $<$5% across the field), indicating that instrumental effects (slit width, aberrations, camera imperfections) are effectively managed.
• Nod-and-Shuffle Sky Subtraction:

An interleaved nod-and-shuffle capability—further refined through the sub-aperture variant—yields highly efficient ($\approx$93.4%) observations with quantum noise-limited sky subtraction. By repositioning the object within the slit, continuous on-target observation and superior sky subtraction are achieved.

• Field Distortion Control:

Optical design, including camera roll adjustment, reduces field distortions so that the dispersed spectrum lies along a single CCD row. A seventh-order polynomial enables wavelength calibration with $<$0.05 pixel accuracy.

• Thermal Compensation and Mechanical Stability:

Cameras exhibit $<$0.2 pixel focal shift for a $5^\circ$C change. Nonetheless, thermal expansion in VPH gratings induces a drift of $\sim$1 pixel over 4 hours in the red arm, which is mitigated by nod-and-shuffle mode.

2. Performance Metrics and Instrumental Stability

The instrument's performance envelope has been validated on-sky and through laboratory characterization:

Performance Parameter	Value/Description	Notes
Resolution (High/Low)	$R \approx 6800 / 2900$	50 km/s and 105 km/s velocity, respectively
Throughput (end-to-end)	$>20$\% (4000–8000 Å), max $>$45\% (spectrograph)	$70$–$80$\% of lab expectations in some modes
Wavelength Stability	Drift $<$0.2 pixel (camera), $\sim$1 pixel/4 h (red grating)	Red arm drift linked to VPH grating pitch vs. temp.
Field-of-View	$25\times38$ arcsec$^2$	Covers small galaxies, nuclear regions, or extended nebulae

Throughput curves are grating/dichroic specific. End-to-end throughput includes telescope, atmospheric transmission, optics, and detectors, peaking over 20% across much of the optical. Notable is the observed $\sim$250 Å redward shift in dichroic passbands post-manufacture (attributed to coating annealing), which is benign due to spectral overlap in dichroic configurations, except for modest U-band sensitivity loss.

$R = \lambda/\Delta\lambda$

with $\Delta\lambda$ determined by slit and instrumental line profile.

Stability is exceptional: camera focal planes drift by $<$0.2 pixel per $5^\circ$C, with the limiting factor being VPH grating thermal response.

3. Data Reduction Framework

The WiFeS data reduction pipeline is built on the NOAO IRAF framework, adapted from the Gemini NIFS IFS pipeline to accommodate WiFeS’s fixed-format and high multiplex density. The reduction sequence includes:

• File Format Conversion:

Conversion from single-extension FITS to Multi-Extension FITS (MEF) with organized calibration/science lists.

• Calibration Frames:

Bias subtraction, flat-fielding (dome flats preferred over internal QI lamps), spatial wire calibration, and arc lamp exposure processing.

• Science Frame Processing:

Calibration solution application, cosmic-ray rejection, spectral extraction, sophisticated sky subtraction (including telluric and nod-and-shuffle), yielding final rectilinear datacubes ready for scientific analysis.

A process flow (see figure “Flow_Chart” in (Dopita et al., 2010)) ensures complete, reproducible reduction from raw exposure to science-ready cubes. The pipeline explicitly addresses the reduction of 25 long-slit spectra (1900 spaxels/camera/exposure), managing the data structure for spatially resolved spectroscopy.

4. Science Use Cases and Early Results

WiFeS's spatially and spectrally resolved datacubes enable:

• Kinematic Mapping:

Velocity and dispersion fields (e.g., [N II] emission in NGC 4696) are extracted, with line intensity contours superimposed, yielding insight into galaxy and nebular gas dynamics.

• Synthetic Imaging:

Multi-color images are created by combining spectral slices (e.g., [N II] 6484 Å, H$\alpha$, [O III] 5007 Å for NGC 3918), allowing the construction of line ratio maps and metallicity diagnostics.

• Sensitivity:

Limiting magnitudes for point sources ($\sim$21.5 mag for appropriate bands) and extended objects validate suitability for faint object spectroscopy.

• Exposure Time Calculation:

Exposure curves as a function of surface brightness and signal-to-noise guide observational planning.

These results showcase WiFeS’s versatility for wide-ranging astrophysics, from galactic nuclei to extended emission nebulae.

5. Instrumental Limitations and Operational Challenges

Although the instrument achieves its primary design objectives, several secondary effects and mitigations exist:

• Thermally-Induced Grating Drift:

The red arm is susceptible to ~1 pixel drift over $4$ hours due to VPH grating expansion with temperature. This induces low-level systematics in sky subtraction (notably for classical mode), but nod-and-shuffle mode addresses the drift by interleaving object and sky in temporal proximity.

• Dichroic Shift:

Passband shifts (e.g., RT560 moving 250 Å redward) are present but non-critical, as spectral arm overlap compensates in most observing configurations.

• CCD Artifacts:
  • Bias drift is managed with frequent bias calibration and low-order fitting.
  • Quadrant cross-talk ($\sim$150 counts/pixel) is symmetrical and flagged during processing.
  • Littrow ghosts (intensity $\sim3\times10^{-5}$ of signal) remain below detectability thresholds due to anti-reflection measures.

All such artifacts are monitored and, where relevant, handled in the reduction pipeline.

6. Comparative Context and Impact

WiFeS is distinguished from comparable IFS instruments by its field-of-view, dual-arm/dichroic configuration, and flexible operational modes. For example, in the context of NIR IFS (such as WIFIS or KMOS (Sivanandam et al., 2012)), WiFeS is optimized for visible wavelengths and higher spatial resolution at moderate aperture, making it especially effective for optical studies of galaxies, resolved stellar populations, and transients.

Its adoption as the workhorse of programs such as AWSNAP (supernovae (Childress et al., 2016)), the S7 AGN survey (Scharwächter et al., 2015), planetary nebulae IFU surveys (Danehkar, 2022), and the WAGGS integrated spectra library (Usher et al., 2017) attests to its strategic role in advancing the understanding of diverse astrophysical processes across the local universe.

7. Summary of Capabilities

WiFeS delivers a unique combination of:

• Fully sampled $25 \times 38$ arcsec field,
• Simultaneous dual-arm (blue/red) coverage,
• Spectral resolutions up to $R=6800$,
• End-to-end throughput $>$20% across much of the optical,
• Efficient nod-and-shuffle observational modes for accurate sky subtraction,
• Stable, high-fidelity spatial and spectral calibrations.

Limitations (thermal drift, dichroic shifts, minor detector artifacts) are minor relative to the instrument’s overall stability and have been mitigated through operational best practices and reduction strategies.

Through these characteristics, WiFeS has established itself as a robust, general-purpose integral field spectrograph supporting a wide array of scientific investigations in optical astronomy (Dopita et al., 2010).