  We re-analyse all of the archive observations of the Ophiuchus dark cloud
L1688 that were carried out with the submillimetre common-user bolometer array
(SCUBA) at the James Clerk Maxwell Telescope (JCMT). For the first time we put
together all of the data that were taken of this cloud at different times to
make a deeper map at 850 microns than has ever previously been published. Using
this new, deeper map we extract the pre-stellar cores from the data. We use
updated values for the distance to the cloud complex, and also for the internal
temperatures of the pre-stellar cores to generate an updated core mass function
(CMF). This updated CMF is consistent with previous results in so far as they
went, but our deeper map gives an improved completeness limit of 0.1 Mo (0.16
Jy), which enables us to show that a turnover exists in the low-mass regime of
the CMF. The L1688 CMF shows the same form as the stellar IMF and can be mapped
onto the stellar IMF, showing that the IMF is determined at the prestellar core
stage. We compare L1688 with the Orion star-forming region and find that the
turnover in the L1688 CMF occurs at a mass roughly a factor of two lower than
the CMF turnover in Orion. This suggests that the position of the CMF turnover
may be a function of environment.
