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GNIRS Imaging

GNIRS can be used for near-infrared (1.0-2.5µm) imaging science and is in fact quite sensitive. However, as a photometer/imager it has the following limitations.

  • The order-blocking filters have non-standard bandpasses. Only the H order-blocking filter approximates the Mauna Kea photometric filter for its band.

  • For the order blocking and narrow band filters the field of of view is the acquisition "keyhole," whose dimensions are shown below. For the Y, J, and K Mauna Kea filters the un-vignetted field is much smaller, as also shown (the Observing Tool does not yet show this smaller field).

  • Currently the best image quality that has been achieved by GNIRS + adaptive optics (ALTAIR) is ~0.12 arcsec (fwhm); this is twice the diffraction-limited fwhm in the K band.

Observing proposals that involve infrared imaging should request NIRI, not GNIRS. If it is not known whether NIRI will be available, Phase 2 may involve creating observations for both instruments. GNIRS also may be used to perform rapid Target of Opportunity (ToO) imaging programs if NIRI is unavailable. Prospective users should be aware that the Gemini IRAF package contains no dedicated support for GNIRS imaging reductions.

The basic detector properties are given here. See also the known issues page. Approximate sensitivities and maximum exposure times (shallow well, except where noted) for imaging though several filters in GNIRS are given in the table below. Sensitivity values are for 0.15" pixels without AO; they are expected to be similar for 0.05" pixels with AO. The Y filter is not yet ready for use.

Filter Bandpass (µm) Point Source magnitude maximum
exposure timea
Y (Mauna Kea) 0.97-1.07 tbd tbd
X (order blocking) 1.03-1.17 23.9 50 s
J (Mauna Kea) 1.17-1.33 23.5 90 s
J (order blocking) 1.17-1.37 23.7 50 s
H (order blocking and Mauna Kea) 1.49-1.80 23.0 12 s
K (Mauna Kea) 2.03-2.37 22.6 30 s
K (order blocking) 1.90-2.49 23.1 10 s
H2 2.11-2.14 21.1 300 s
PAH 3.27-3.32 15.7 0.8 s (deep well)

aMaximum exposure times to avoid saturation on the sky background. If these exposure times are used, low-level residual images of the acquisition field of view will be present in subsequent long exposures. PIs wishing to follow imaging with spectroscopy are advised to take this into account in choosing their exposure times.