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Guiding Options

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Natural Seeing (Non-Adaptive Optics) Guided Observations:

NIRI standalone (i.e., without AO) requires the use of a peripheral wavefront sensor for both imaging and spectroscopy. 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 NIRI.

NGS: The guide star (which may be the target itself) 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. 

LGS+P1: The laser created guide star provides information for the deformable mirror while in place of using a nearby star for  tip-tilt focus, the peripheral wavefront sensor (PWFS1 or P1) is guided on a star that provides the tip/tilt corrections. This mode does not provide diffraction-limited resolution, but instead gives “super-seeing” by reducing the natural seeing PSF FWHM by a factor of 2-3. The major benefit of this seeing-improver mode is that it increases the LGS sky coverage to almost 100%, providing a significant level of PSF correction for a wide variety of applications. Further details can be found here.

Guidelines for Selecting Guide Stars for Your NIRI 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, and thus not recommended if brighter stars are available. We note that the automatic guide star selection in the Observing Tool (OT) will pick a guide star whenever possible that satisfies the criteria below.

Guide Probe Separation from center of NIRI FOV (field-of-view) Guide star brightness
ALTAIR (NGS) < 25" (1)

R <~12 mag (optimal)

R <~15 mag (faint limit)

ALTAIR (LGS) < 25" (1)

R <~17.5 mag (bright time)

R <~18.5 mag (dark time)

LGS+PWFS1 4.75' to 7' for f/14 and f/32 R <~ 14 mag

5' to 7' for f/6

4.25' to 7'  f/14 and f/32(2)


R <~ 15 mag

(1) The precise position of the Altair patrol field is correctly displayed with the Altair overlay in the Observing Tool (OT) position editor.

(2) For guide stars closer than 5' the position editor in the Observing Tool (OT) position editor should be inspected carefully to check that the PWFS probe does not block the science field