We consider testing for presence of a signal in Gaussian white noise with intensity n −1/2 , when the alternatives are given by smoothness ellipsoids with an L 2 -ball of (squared) radius ρ removed. It is known that, for a fixed Sobolev type ellipsoid Σ(β, M ) of smoothness β and size M , a squared radius ρ ≍ n −4β/(4β+1) is the critical separation rate, in the sense that the minimax error of second kind over α-tests stays asymptotically between 0 and 1 strictly (Ingster [22]). In addition, Ermakov [9] found the sharp asymptotics of the minimax error of second kind at the separation rate. For adaptation over both β and M in that context, it is known that a log log-penalty over the separation rate for ρ is necessary for a nonzero asymptotic power. Here, following an example in nonparametric estimation related to the Pinsker constant, we investigate the adaptation problem over the ellipsoid size M only, for fixed smoothness degree β. It is established that the sharp risk asymptotics can be replicated in that adaptive setting, if ρ → 0 more slowly than the separation rate. The penalty for adaptation here turns out to be a sequence tending to infinity arbitrarily slowly.