Context :

Aims : In archival 2.2m MPG-ESO/WFI data we discovered several mass peaks through weak gravitational lensing , forming a possible supercluster at redshift 0.45 .
Through wide-field imaging and spectroscopy we aim to identify the supercluster centre , confirm individual member clusters , and detect possible connecting filaments .

Methods : Through multi-colour imaging with CFHT/Megaprime and INT/WFC we identify a population of early-type galaxies and use it to trace the supercluster network .
EMMI/NTT multi-object spectroscopy is used to verify the initial shear-selected cluster candidates .
We use weak gravitational lensing to obtain mass estimates for the supercluster centre and the filaments .

Results : We identified the centre of the SCL2243-0935 supercluster , MACS J2243-0935 , which was found independently by Ebeling et al .
( 2001,2010 ) .
We found 13 more clusters or overdensities embedded in a large filamentary network .
Spectroscopic confirmation for about half of them is still pending .
Three ( 5 - 15 ) h _ { 70 } ^ { -1 } Mpc filaments are detected , and we estimate the global size of SCL2243 to be 45 \times 15 \times 50 h _ { 70 } ^ { -1 } Mpc , making it one of the largest superclusters known at intermediate redshifts .
Weak lensing yields r _ { 200 } = ( 2.06 \pm 0.13 ) h _ { 70 } ^ { -1 } Mpc and M _ { 200 } = ( 1.54 \pm 0.29 ) \times 10 ^ { 15 } M _ { \odot } for MACS J2243 with M / L = 428 \pm 82 , very similar to results from size-richness cluster scaling relations .
Integrating the weak lensing surface mass density over the supercluster network ( defined by increased i -band luminosity or g - i colours ) , we find ( 1.53 \pm 1.01 ) \times 10 ^ { 15 } M _ { \odot } and M / L = 305 \pm 201 for the three main filaments , consistant with theoretical predictions .
The filaments ’ projected dimensionless surface mass density \kappa varies between 0.007 - 0.012 , corresponding to \rho / \rho _ { crit } = 10 - 100 depending on location and de-projection .
The greatly varying density of the cosmic web is also reflected in the mean colour of galaxies , e.g . \langle g - i \rangle = 2.27 mag for the supercluster centre and 1.80 mag for the filaments .

Conclusions : SCL2243 is significantly larger and much more richly structured than other known superclusters such as A901/902 or MS0302 studied with weak lensing before .
It is a text-book supercluster with little contamination along the line of sight , making it a perfect sandbox for testing new techniques probing the cosmic web .