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The success of the Systemic Infrastructure Initiative proposal for the upgrade of the ANU and UNSW telescopes at Siding Spring Observatory (SSO) enabled the design and construction of an innovative, powerful and unique hyper-spectral instrument. It will be mounted at the Nasmyth A focus of the 2.3m telescope and will take maximal advantage of the properties of that telescope. This instrument, the Wide Field Spectrograph (WiFeS) will provide 950 simultaneous spectra with high efficiency over the full field of view accepted by the instrument. It has a data gathering capability of 10 to 100 times the rate of the existing spectrographs.

The science mission of this instrument was defined in the WiFeS Science Requirements Document, developed through a call for submissions to the full Australian astronomical community in early 2002. This emphasised the need for both wide spectral coverage and wide field. At the Conceptual Design Review (Structural), held in March 2004, its capabilities were further enhanced by a doubling of the proposed detector format - an option found to be both feasible and desirable within the baseline design concept that had been developed for the spectrograph cameras. On December 6 2005 the final WiFeS design passed its Critical Design Review. However, a full costing indicated that only the red-arm of the spectrograph could be built within the available SII funds. Because much of the important science cannot be done with the red arm alone, here we seek funding from the LIEF Program to construct the blue arm and so capture the full science potential of this innovative instrument for the Australian user community.

2The WiFeS Instrument.

The 2.3m telescope is a modest sized telescope by modern standards and offers only a rather restrictive field of view (about 6 arc min.). The science mission of any new instrument must therefore be motivated by the question, “Can this enable exciting, innovative and internationally competitive science?” Given the rich breadth of research interests of the Australian astrophysical user community of the 2.3m telescope, and the need for the instruments on the 2.3m to enable effective student research training at a national level, there is a clear requirement for an efficient spectrograph versatile enough to provide outstanding capability across a very broad range of scientific objectives.

The WiFeS spectrograph is designed to provide a unique 3-D spectroscopic functionality, enabling astronomers to obtain complete spectral and spatial coverage in a single exposure. A spatial field of 25x38 arc sec is divided into 1.0 arc sec slices on the sky and 0.5 x 0.5 arc sec. pixels at the focal plane. Two intermediate spectral resolution options are available, R=3000 and R=7000. At R=3000 the spectral coverage is 3290-5900Å (blue arm) and 5100-9500Å (red arm) with two fixed gratings. At R=7000 the spectral coverage is 3290-5580Å and 5290-9120Å with two fixed gratings per camera. This fully integral field spectrograph providing spectra in 950 spatial elements is highly complementary to the new intermediate resolution fibre-fed spectrograph AAOmega on the Anglo-Australian Telescope (which is designed to observe 392 spatially disjunct stars or galaxies across a 2 degree field).

In the design of the spectrograph great efforts were made to maximize its data gathering efficiency to the point where it competes with 8m telescopes working on extended objects. This is done in two ways.

Maximisation of throughput through use of :

  • High-efficiency Volume-phased Holographic (VPH) gratings operated at peak efficiency.

  • High-transmission lens optics in preference to mirrors, where possible.

  • Wide-band anti-reflective coatings on all air-glass surfaces, reducing losses to < 1% per surface.

  • Enhanced reflectivity (>96%) multi-layer coatings on all reflective surfaces.

  • Double-beam design to double data collection rates and optimize the throughput.

  • Large-format, wavelength-optimized, and highly-efficient CCD detector technologies.

Fig 1. The expected end-to-end throughput of WiFes in each observing mode. This is more than two times higher than competing instruments of this type. The bold lines are for the R=3000 modes; the thinner lines are the R=7000 modes with the two sets of gratings for each camera. All the spectra shortward of 590 nm will be taken with the optimized blue-arm of WiFeS proposed in this LIEF bid.

Maximisation of spectral multiplex advantage and science efficiency through.

  • A double beam design to maximize the number of independent spectral elements.

  • An innovative reflective image slicing design which maximized the number of spatial elements, and ensures a science field shape which is well-matched to the expected science targets.

  • Use of reflective image slicing which ensures good spectrophotometric characteristics.

  • A stationary spectrograph body, which both eliminates flexure and provides a stable thermal environment. This ensures excellent spectral stability over long periods.

  • Implementation of “interleaved nod-and-shuffle” allowing perfect sky background subtraction.

  • Minimization of scattered light and ghost image intensity through careful design.

The addition of a blue arm is essential to the WiFeS science mission. With both blue and red cameras, the full optical range can be covered in only one or two exposures. Both cameras are required to measure important diagnostic ratios. For stars, many important features can only be observed with the blue-arm. These include the Balmer discontinuity and spectral features such as the CaII H and K lines and the CN bands in the UV. Likewise in galaxies and nebulae, the inclusion of a blue-arm allows the important measurement of the dust reddening, and provides fundamental density, temperature and excitation diagnostic line ratios of many abundant elements. In summary, the addition of the blue arm will double the data gathering capability of WiFeS. It will more than double the science product of the instrument by enabling one to measure all the spectral features shortward of 4200 Å which are essential for stellar and galaxy population abundance analyses. It will enhance the sensitivity of any observations made between 4200A and 5900A through its optimized coatings and detector. Finally, the blue and red spectra can be obtained simultaneously, enabling increased spectro-photometric precision, and giving the ability to monitor time-dependent spectral phenomena on short time scales.
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