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Guiding Options

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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).


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