  We present the observational results of an L' and M band Adaptive Optics (AO)
imaging survey of 54 nearby, sunlike stars for extrasolar planets, carried out
using the Clio camera on the MMT. We have concentrated more strongly than all
other planet imaging surveys to date on very nearby F, G, and K stars,
prioritizing stellar proximity higher than youth. Ours is also the first survey
to include extensive observations in the M band, which supplement the primary
L' observations. Models predict much better planet/star flux ratios at the L'
and M bands than at more commonly used shorter wavelengths (i.e. the H band).
We have carried out extensive blind simulations with fake planets inserted into
the raw data to verify our sensitivity, and to establish a definitive
relationship between source significance in $\sigma$ and survey completeness.
We find 97% confident-detection completeness for 10$\sigma$ sources, but only
46% for 7$\sigma$ sources -- raising concerns about the standard procedure of
assuming high completeness at 5$\sigma$, and demonstrating that blind
sensitivity tests to establish the significance-completeness relation are an
important analysis step for all planet-imaging surveys. We discovered a
previously unknown, approximately 0.15 solar-mass stellar companion to the F9
star GJ 3876, at a projected separation of about 80 AU. Twelve additional
candidate faint companions are detected around other stars. Of these, eleven
are confirmed to be background stars, and one is a previously known brown
dwarf. We obtained sensitivity to planetary-mass objects around almost all of
our target stars, with sensitivity to objects below 3 Jupiter masses in the
best cases. Constraints on planet populations based on this null result are
presented in our Modeling Results paper.
