We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space Telescope to search for multiple populations within Magellanic Cloud star clusters at various ages .
In this paper , we introduce this survey .
As first results of this programme , we also present multi-band photometric observations of NGC 121 in different filters taken with the WFC3/UVIS and ACS/WFC instruments .
We analyze the colour-magnitude diagram ( CMD ) of NGC 121 , which is the only `` classical '' globular cluster within the Small Magellanic Cloud .
Thereby , we use the pseudo-colour C _ { F 336 W,F 438 W,F 343 N } = ( F 336 W - F 438 W ) - ( F 438 W - F 343 N ) to separate populations with different C and N abundances .
We show that the red giant branch splits up in two distinct populations when using this colour combination .
NGC 121 thus appears to be similar to Galactic globular clusters in hosting multiple populations .
The fraction of enriched stars ( N rich , C poor ) in NGC 121 is about 32 % \pm 3 % , which is lower than the median fraction found in Milky Way globular clusters .
The enriched population seems to be more centrally concentrated compared to the primordial one .
These results are consistent with the recent results by Dalessandro et al .
( 2016 ) .
The morphology of the Horizontal Branch in a CMD using the optical filters F 555 W and F 814 W is best produced by a population with a spread in Helium of \Delta Y =0.025 \pm 0.005 .