Gemini Near-Infrared Integral-Field Spectrograph (NIFS)


Research School of Astronomy and Astrophysics    AUSPACE Ltd. Institute for Astronomy


NIFS Diffraction Effects

The diffraction image FWHM at 2.2 microns on the Gemini 8 m telescope is about 0.07". Consequently, a spectrograph with a 0.10" slit is essentially diffraction limited in the K band.

[NIFS Diffraction]

The figure above shows diffraction profiles at J1 (1.05 microns), J2 (1.25 microns), H (1.65 microns), and K (2.20 microns) for an object centered in one of the 0.1" wide slitlets of the f/256 image slicer. The slitlet width is indicated by dashed lines.

Slit diffraction causes divergence in the spectral direction of rays through the slit in excess of the geometrical focal ratio; it can be thought of as focal ratio degradation speeding the beam in the spectral direction. This spreads the beam in the IFU in the direction perpendicular to the slicing direction and can be accommodated by increasing the size of each element of the pupil mirror array in this direction. Since the NIFS spectrograph already accommodates a 64 mm long reformatted slit, the NIFS collimator optics are quite capable of accepting this diffracted light. Rays outside the geometrical focal ratio then strike the grating outside the geometrical pupil. Passing this diffracted light to the camera requires using a wider grating than would otherwise be the case.

Calculation of the diffraction angle at the slit is complicated because the image slicer is at a focus. A perfect image would have no phase coherence across the slit. In practice, there will be significant phase coherence across the slit due to a combination of the telescope diffraction and seeing effects uncorrected by the adaptive optics system. Using a grating with a width twice that of the geometrical grating pupil captures an acceptably high fraction of the diffracted light.



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