2021AJ....161..166L

Starlight subtraction algorithms based on the method of Karhunen-Loeve eigenimages have proved invaluable to exoplanet direct imaging. However, they scale poorly in runtime when paired with differential imaging techniques. In such observations, reference frames and frames from which starlight is to be subtracted are drawn from the same set of data, requiring a new subset of references (and eigenimages) for each frame processed to avoid self-subtraction of the signal of interest. The data rates of extreme adaptive optics instruments are such that the only way to make this computationally feasible has been to downsample the data. We develop a technique that updates a precomputed singular value decomposition of the full data set to remove frames (i.e., a "downdate") without a full recomputation, yielding the modified eigenimages. This not only enables analysis of much larger data volumes in the same amount of time, but also exhibits near-linear scaling in runtime as the number of observations increases. We apply this technique to archival data and investigate its scaling behavior for very large numbers of frames N. The resulting algorithm provides speed improvements of 2.6x (for 200 eigenimages at N = 300) to 140x (at N = 10⁴) with the advantage only increasing as N grows. This algorithm has allowed us to substantially accelerate Karhunen-Loeve image projection (KLIP) even for modest N, and will let us quickly explore how KLIP parameters affect exoplanet characterization in large-N data sets.