Deputy Instrument Scientist:
KPF is focussed using the guide camera (CRED2). This can be accomplished via either the "Mira" process or the "autofoc" process.
The OA can perform a Mira focus on request. They will choose a star of an appropriate brightness in J-band, slew to that star, and execute a Mira sequence. The Mira sequence will obtain images of the star in "PMFM" mode in which the 36 individual segments of the primary mirror are tilted to separate their images. Focus os measured geometrically using the positions of those 36 star images at two different tilt settings of the segments.
The advantage of Mira is that it will measure and correct the tip and tilt of the secondary mirror to zero out coma in the star image in addition to setting the piston position of the secondary mirror to optimize focus.
The autofoc process is also available on KPF and will also use the guide camera images. It performs a more traditional focus routine in which the secondary mirror piston position is moved and the affect on the FWHM of the star is measured. Depending on conditions, this will be iterated a few times to arrive at the best focus value.
The advantage of the autofoc process is that it can be a bit faster than Mira, especially because it can be performed on any star which has significant signal to noise on the guider image. This includes the ability to focus on the science target as long as the guide system has reasonable signal to noise.