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We searched for a fast moving H$\alpha$ shell around the Crab nebula. Such ashell could account for this supernova remnant's missing mass, and carry enoughkinetic energy to make SN 1054 a normal Type II event. Deep H$\alpha$ imageswere obtained with WFI at the 2.2m MPG/ESO telescope and with MOSCA at the2.56m NOT. The data are compared with theoretical expectations derived fromshell models with ballistic gas motion, constant temperature, constant degreeof ionisation and a power law for the density profile. We reach a surfacebrightness limit of $5\times10^{-8} ergs s^{-1} cm^{-2} sr^{-1}$. A halo isdetected, but at a much higher surface brightness than our models ofrecombination emission and dust scattering predict. Only collisional excitationof Ly$\beta$ with partial de-excitation to H$\alpha$ could explain suchamplitudes. We show that the halo seen is due to PSF scattering and thus notrelated to a real shell. We also investigated the feasibility of aspectroscopic detection of high-velocity H$\alpha$ gas towards the centre ofthe Crab nebula. Modelling of the emission spectra shows that such gas easilyevades detection in the complex spectral environment of the H$\alpha$-line. PSFscattering significantly contaminates our data, preventing a detection of thepredicted fast shell. A real halo with observed peak flux of about$2\times10^{-7} ergs s^{-1} cm^{-2} sr^{-1} $ could still be accomodated withinour error bars, but our models predict a factor 4 lower surface brightness. 8mclass telescopes could detect such fluxes unambiguously, provided that asufficiently accurate PSF model is available. Finally, we note that PSFscattering also affects other research areas where faint haloes are searchedfor around bright and extended targets.

Observational and theoretical constraints for an H$\alpha$-halo around the Crab Nebula