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Spatial Sampling and Image Quality

Introduction

NIFS is an image-slicer integral-field spectrograph, meaning that the sky is sampled by a stack of long, thin mirrors, each of which is rotated with respect to its neighbours. Each of these narrow mirrors acts similarly to a slit in a conventional spectrograph, providing a slice across the two-dimensional field that is then dispersed to provide a spatially-resolved spectrum. Since there are almost no gaps between the slices (or 'pseudo-slits'), they can be re-combined to provide a contiguous field, in which there exists a spectrum at every position.

Field of View

During science verification, the NIFS field of view was measure as 2.99"×2.97", measured across and along the slices respectively The field is divided into 29 individual slices, each of which is sampled by around 69 detector pixels running along the length of the slice. Distortions within the NIFS field are very small. The orientation of the NIFS field is such that the slices are aligned North-South for an instrument position angle of 0º.

Spatial Sampling

A consequence of the image-slicer design is that the spatial sampling is determined differently across the slices than along them. In the direction along the slices, the sampling is determined by the detector. Across the slices, the sampling is determined by the width of the slice as projected onto the sky. For NIFS, this results in rectangular spatial pixels (or 'spaxels') of dimension 0.103"×0.043" across and along the slice respectively.

Image Quality

The image quality of NIFS is determined primarily by the guiding method. Without the use of the ALTAIR adaptive optics system, NIFS is seeing limited, which is typically 0.4" FWHM or larger. If ALTAIR is providing full correction, using a bright NGS or LGS with a bright tip-tilt star, the PSF is close to the diffraction limit of the telescope, which is undersampled by the NIFS 0.1" slices. Thus along the slit, the PSF will be well sampled and near-diffraction limited. Across the slit, the PSF will be affected by the sampling of the NIFS slits. NIFS image quality with AO can be affected by a number of issues that are generally not important for seeing-limited observations. For example, flexure or changes in atmospheric refraction during the integration, the quality of the image-reconstruction via the data reduction process, etc. Nominal FWHM values for NIFS+ALTAIR observations with good correction are 0.1"-0.15", although precise PSF size and shape will depend on conditions and ALTAIR configuration.