1 Description 

Fast and Furious (F&F) is a wavefront sensing and control algorithm used to improve image quality during the night with limited interference to observations. This algorithm uses science camera PSF images, as well as the previously applied DM correction to estimate slowly varying wavefront aberrations in the focal plane. More common focal plane wavefront sensing techniques generally require phase diversity, measuring defocused PSFs to break the sign ambiguity of even modes. F&F instead uses the previous DM correction and WFS estimate. The correction is then applied as centroid offsets in the Shack-Hartmann WFS (SHWFS). 

F&F can correct static or quasi-static wavefront errors within the control space of the SHWFS. It will therefore correct non-common path errors between the SHWFS and instrument, SHWFS calibration errors, and SHWFS aliasing errors (eg. primary mirror segment edge discontinuities incorrectly reconstructed by the SHWFS). Previous on-sky tests have seen a common K band Strehl improvement of ~10% when running F&F on high-SNR images. 

F&F runs at a much slower loop than the AO system and therefore cannot correct quickly varying residual errors. The image on the science detector must remain stable between exposures to get a decent F&F correction. F&F cannot be run during offsets. If F&F is run in bad seeing conditions, the Strehl ratio will not improve as the phase estimation will be dominated by high speed errors. 

F&F is run as a PSF optimization tool and could be used as needed during the night. It can be run on a calibration star if the science subject is faint, a binary, or an extended object. It can operate in parallel with science (if that science is on a single star), but we typically have run it separately. If the Strehl ratio of the PSF visibly reduces on NIRC2, F&F can be run again as a tune up.   

Current F&F Usage Cases 

·                F&F is currently only operational for NIRC2 

·                Subject must be an isolated single star (no close binaries or extended objects). The nearest object must be no closer than 32 pixels from the main star (or ~0.32” at the smallest pixel scale offered) from the main star. 

·                There is no magnitude limit, but each integration must have relatively high SNR – it will only correct what it can reliably detect in every image. Longer integration times == longer time for the algorithm to converge. 

·                NGS operation has been fully tested 

·                NIRC2 should be in vertical angle mode (stationary pupil) 

·                NIRC2 must be in the narrow camera  

Note: F&F has been run with LGS, but is not yet operational in this mode.  

Note: F&F cogs can be used with a rotating pupil, but will only show improvements from normal AO cogs ~10degrees from the angle it was run.  

2 Modes of Operation 

F&F is in the process of being developed for multiple modes of AO observing. Currently it is only operational for NGS NIRC2 observations. Below is a table of the modes currently in progress and planned.