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Sky subtraction at the Poisson limit with fibre-optic multiobject spectroscopy

Monthly notices of the Royal Astronomical Society, 2010-11, Vol.408 (4), p.2495-2510 [Peer Reviewed Journal]

2010 The Authors. Journal compilation © 2010 RAS 2010 ;2010 The Authors. Journal compilation © 2010 RAS ;2015 INIST-CNRS ;ISSN: 0035-8711 ;EISSN: 1365-2966 ;DOI: 10.1111/j.1365-2966.2010.17298.x ;CODEN: MNRAA4

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  • Title:
    Sky subtraction at the Poisson limit with fibre-optic multiobject spectroscopy
  • Author: Sharp, R. ; Parkinson, H.
  • Subjects: Astronomy ; Earth, ocean, space ; Exact sciences and technology ; Fiber optics ; instrumentation: spectrographs ; methods: data analysis ; methods: observational ; Spectrum analysis ; techniques: image processing
  • Is Part Of: Monthly notices of the Royal Astronomical Society, 2010-11, Vol.408 (4), p.2495-2510
  • Description: We report on the limitations of sky-subtraction accuracy for long-duration fibre-optic multiobject spectroscopy of faint astronomical sources during long-duration exposures. We show that while standard sky subtraction techniques yield accuracies consistent with the Poisson noise limit for exposures of 1 h duration, there are large-scale systematic defects that inhibit the sensitivity gains expected on the summation of longer duration exposures. For the AAOmega system at the Anglo-Australian Telescope, we identify a limiting systematic sky-subtraction accuracy, which is reached after integration times of 4–10 h. We show that these systematic defects can be avoided through the use of the fibre nod-and-shuffle (N+S) observing mode, but with a potential cost in observing efficiency. Finally, we demonstrate that these disadvantages can be overcome through the application of a Principal Components Analysis (PCA) sky-subtraction routine. Such an approach minimize systematic residuals across long-duration exposures, allowing deep integrations. We apply the PCA approach to over 200 h of on-sky observations and conclude that for the AAOmega system, the residual error in long-duration observations falls at a rate proportional to τ−0.32 in contrast to the τ−0.5 rate expected from theoretical considerations. With this modest rate of decline, the PCA approach represents a more efficient mode of observation than the N+S technique for observations in the sky limited regime with durations of 10–100 h (even before accounting for the additional signal-to-noise ratio and targeting efficiency losses often associated with the N+S technique). This conclusion has important implications for the observing strategies of the next generation of fibre-optics redshift surveys with existing facilities as well as design implications for fibre-optic systems destined for new facilities. It argues against the use of the inherently inefficient N+S technique for faint object fibre-optic survey spectroscopy.
  • Publisher: Oxford, UK: Blackwell Publishing Ltd
  • Language: English
  • Identifier: ISSN: 0035-8711
    EISSN: 1365-2966
    DOI: 10.1111/j.1365-2966.2010.17298.x
    CODEN: MNRAA4
  • Source: Alma/SFX Local Collection
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