  We compare the canonical bases of level-$1$ quantised Fock spaces in affine
types $A^{(1)}$ and $A^{(2)}$, showing how to derive the canonical basis in
type $A^{(2)}_{2n}$ from the the canonical basis in type $A^{(1)}_n$ in certain
weight spaces. In particular, we derive an explicit formula for the canonical
basis in extremal weight spaces, which correspond to RoCK blocks of double
covers of symmetric groups. In a forthcoming paper with Kleshchev and Morotti
we will use this formula to find the decomposition numbers for RoCK blocks of
double covers with abelian defect.
