  The Large Synoptic Survey Telescope (LSST) is a wide-field imaging system of
unprecedented etendue. The initial goal of the project is to carry out a ten
year imaging survey in six broad passbands (ugrizy) that cover $350 nm <
\lambda < 1.1 \mu m$. This document reports on the discussions that occurred at
workshop (held April 11-12, 2019 at Argonne National Laboratory) that was
convened to explore concepts for using the LSST system once the initial survey
is complete. Participants discussed the tradeoffs in science performance, cost,
and uniqueness for i) imaging surveys using the initial wide-field CCD
instrument, perhaps supplemented with different filters, ii) replacing the
focal plane with some alternative sensor technology, and iii) converting the
LSST system to a wide-field multi-object fiber-fed spectrograph. Participants
concluded that the fiber spectrograph option would be most effective if the
focal plane were to feed upwards of 30,000 fibers. Thermal management and power
considerations in the LSST instrument barrel make it difficult to accommodate
infrared sensors that require very low operating temperatures, and the current
generation of buttable IR sensors that would extend sensitivity to 2 $\mu$m
are, we concluded, cost-prohibitive. Procuring and using an alternative filter
set, on the other hand, is modest in cost, would take full advantage of the
LSST image reduction pipeline, and could yield considerable additional
information.
