Capability

, Updated

Spectroscopy

GNIRS has two spectroscopic modes:

Long-slit spectroscopy

Raw K band spectral image of a crowded field, obtained using the 32 l/mm grating, short blue camera, and 0."45 x 99" long slit. Sky emission lines are roughly horizontal. Wavelength increases downward.

Long-slit, 1.0-5.4 µm spectroscopy of any individual window (X, J, H, K, most of L, or M) at R up to ~1,800, or of any portion thereof at R up to ~18,000; either without adaptive optics or with adaptive optics (XJHKL only), using slit lengths of 50-100 arc-seconds. The choices of set-ups are:

Low resolution (R~1800)

Intermediate resolution (R~5900)

High resolution (R~18000)

Cross-dispersed Spectroscopy

Typical raw argon arc lamp image showing orders 3-9 (left to right, with wavelength increasing downwards); note the central wavelength is set at 1.65µm.

Cross-dispersed, 0.85-2.5 µm short slit spectroscopy with full spectral coverage at R up to ~1,800 and partial coverage (disjoint 0.1-0.2 µm segments) at R up to ~6,000; with or without adaptive optics, using slit lengths of 5-7 arc-seconds.

Note: The sensitivity in a single order in cross-dispersed mode is reduced by about 10% compared to the sensitivity in long slit (single order) mode. Unless the science specifically involves only one order of GNIRS (e.g., J, H, or K) or the source size is more than 2-3 arc-seconds (requiring nodding to sky) it is advantageous to employ the cross-dispersed mode.

The choices of set-ups are:

Low resolution (R~1800)

Intermediate resolution (R~5900)

High resolution (R~18000)


Imaging

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. For instance, GNIRS may be used to perform rapid Target of Opportunity (ToO) imaging programs when NIRI is unavailable. Prospective users should be aware that the Gemini IRAF package contains no dedicated support for GNIRS imaging reductions.

GNIRS near-infrared (1.0-2.5µm) imaging 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.

The basic detector properties are given here. See also the known issues page.


Sensitivity & Throughput

Spectroscopy

The following table provides estimated broadband magnitudes reached at 5 sigma (per spectral pixel) in 1 hour of integration, both without and with adaptive optics (AO).

Adaptive
Optics?
Resolving power Pixel scale Slit width Diffraction Order (and waveband or wavelength)
9
0.75um
8
0.83um
7
0.93um
6 (X)
1.105um
5 (J)
1.245um
4 (H)
1.665um
3 (K)
2.19um
2 (L)
3.50um
1 (M)
4.67um
NO ~1,700(a) 0.15"(e) 0.30" 15.1 16.8 18.5 19.3 19.0 19.3 18.8 13.6 -
NO ~6,000(b) 0.15"(e) 0.30" 13.8 15.5 17.3 18.6 18.5 18.5 18.2 13.1 10.5
NO ~18,000(b) 0.05"(f) 0.10" - - - 15.7 15.6 15.6 15.3 11.9 9.3(g)
YES ~1,800(c) 0.05"(f) 0.10" - - - 15.3 15.8 17.5 17.9 9.7 -
YES ~5,000(a) 0.05"(f) 0.10" - - - 13.9 14.5 15.9 16.7 9.2 -
YES ~18,000(b) 0.05"(f) 0.10" - - - 13.0 13.8 15.0 15.6 8.0 -

(a) 32 l/mm grating
(b) 111 l/mm grating
(c) 10 l/mm grating
(e) short blue or short red camera
(f) long blue or long red camera
(g) R~13,000 (intrinsically lower than in other bands; see this table)

GNIRS sensitivity is monitored regularly. The following plots show the sensitivities measurements for standard star observations, taken at the central wavelength of each band., in the two spectral modes LS and XD. This time, the sensitivity is defined as the magnitude of an object that would provide a signal-to-noise ratio of 1 for an hour of integration time (SN1 hr).

Plots


GNIRS-XD

GNIRS-LS

GNIRS throughput is also monitored regularly. All throughput measurements are based on observations of spectrophotometric standards, collected under photometric conditions. The most current measurement, as well as an historic of the measurements are available through the links below.

Mode Latest Measurement History
XD 20170319 click
LS 20161111 click

Imaging

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.
 

GNIRS IMAGING SENSITIVITY IN GOOD SEEING AND WITH 0.15" PIXELS
(S/N=5 IN 1 HOUR, N.I. OVERHEADS)
Filter Bandpass (µm) Point Source magnitude maximum
exposure timea
Y (Mauna Kea) 0.97-1.07 23.3 240 s
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 10 s
K (Mauna Kea) 2.03-2.37 22.6 30 s
K (order blocking) 1.90-2.49 23.1 10 s
H2 2.105-2.137 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.

A zero point is defined as the magnitude of an object that would yield 1 ADU/sec at an airmass of 1. The following table contains zero points for GNIRS with its short (0.15 arc sec/pix) camera and broad band "photometric" filters. Extinction values are taken from Tokunaga, Simons & Vacca (2002 PASP 114, 180) for 2mm of precipitable water vapor. See their Table 2 for extinction values for other amounts of precipitable water.

Filter Central wavelength
(µm)
Zero point magnitude
(for 1 ADU/s)
Typical extinction
(mag/airmass)
Y 1.02 24.069 +/- 0.085
J 1.25 24.217 +/- 0.082 0.015
H 1.64 24.113 +/- 0.114 0.015
K 2.20 23.421 +/- 0.049 0.059

Zero points depend on instrument and telescope transmittances as well as detector electronics. They tend to be stable at the 3% level over many months. Measured values need to be corrected for variation of atmospheric extinction with airmass.


Guiding Options

GNIRS can be used with the standard Gemini peripheral wavefront sensors, or with the Altair adaptive optics module (in both natural and laser guide star modes).

Natural Seeing (Non-Adaptive Optics) Guided Observations:

GNIRS non-adaptive optics observations require the use of a peripheral wavefront sensor for both imaging and spectroscopy. The peripheral wavefront sensors (PWFS) operate in the optical. PWFS2 is preferred over PWFS1, as it can be used on fainter guide stars, can run at a higher frequency for a given guide star, works better under windy and cloudy conditions, and is smaller and thus vignettes less of the field of view.

Laser Guide Star (LGS) and Natural Guide Star (NGS) Adaptive Optics (AO) Guided Observations:

With Adaptive Optics: ALTAIR is used as the primary guider, providing adaptive optics correction for seeing effects. ALTAIR provides both Natural Guide Source (NGS) and Laser Guide Source (LGS) capabilities with GNIRS. ALTAIR can guide on fainter stars (to R~15 mag for A0 stars), but this will result in poor correction, and thus not recommended if brighter stars are available.

NGS: The guide star  is used to provide wavefront information for the deformable mirror as well as overall image motion for tip/tilt. More information can be found here.

LGS: The laser created guide star provides information for the deformable mirror while a nearby star must be used for tip/tilt. This star can be much fainter than an NGS star used with the deformable mirror. Further details can be found here.

Guidelines for Selecting Guide Stars for Your GNIRS Observations:

Guidelines for selecting good guide stars are summarized in the table below. Note that the guide star brightness limits are for cloudless nights (50% CC) and optimal seeing (IQ ≤ 70%). ALTAIR can guide on fainter stars (to R~15 mag for A0 stars), but this will result in poor correction. We note that the automatic guide star selection in the Observing Tool (OT) will pick a guide star whenever possible that satisfies the telescope guiding constraints. Always check the guide stars "auto-selected" by the OT Position Editor, as the guide star catalogs have many spurious entries. Extended objects and binaries should be rejected.

Guide Probe Separation from
the science target
Guide star brightness
ALTAIR (NGS) < 25"

R <~12 mag (optimal)

R <~15 mag (faint limit)

ALTAIR (LGS) < 25"

R <~17.5 mag (bright time)

R <~18.5 mag (dark time)

LGS+PWFS1 3.5' to 7' R <~ 14 mag
PWFS2 3.5' to 6.5' (1)

R <~ 15 mag

(1) In XD mode, or if some vignetting of the slit and/or acquisition field of view can be tolerated, guide stars closer to the science target can be used (check for vignetting in the OT Position Editor and please add a note alerting the observer).