NIFS has a number of different guiding options, which can be summarized as follows:

 

• Without Adaptive Optics: Seeing-limited guiding using Peripheral WaveFront Sensor 2 (PWFS2). This mode can be used if there is no suitable guide star available for ALTAIR (e.g., no nearby bright star and your target is too faint or extended to guide with).
• 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 NIFS. The degree of correction depends on the brightness and off-axis distance of the guide sources used.
• With Adaptive Optics and OIWFS: NIFS is also equipped with an On-Instrument WaveFront Sensor (OIWFS) that can provide slow guiding to correct for flexures between NIFS and ALTAIR. This mode is recommended for long integrations, and for coronography.

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) andoptimal seeing (IQ ≤ 70%). ALTAIR can guide on fainterstars (to R~15 mag for A0 stars), but this will result in poor correction.

 

Guide Probe	Separation from the science target	Guide star brightness
ALTAIR	< 25" (1)	R <~12 mag (optimal)
OIWFS	Between 12 and 60" (2)	K <~ 14.5 mag
PWFS2	3.5' to 6.'5	R <~ 15 mag

⁽¹⁾ The Altair patrol field is not exactly circular or concentric with NIFS. The precise position and non-circular shape of the patrol field is correctly displayed with the Altair overlay in the Observing Tool image displayer.
⁽²⁾ The maximum radius decreases to 25" when the ALTAIR field lens is used.