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The angular power spectrum of H  i  21 cm opacity fluctuations is a useful statistic for quantifying the observed opacity fluctuations as well as for comparing these with theoretical models. We present here the H  i  21 cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5 arcsec and spectral resolution of 0.4 km s −1 . The power spectrum has been estimated using a simple but robust visibility‐based technique. We find that the power spectrum is well fitted by a power law   P  τ ( U ) = U  α   with a power‐law index of  α∼−2.86 ± 0.10  (3σ error) over the scales of 0.07–2.3 pc for the gas in the Perseus spiral arm and 0.002–0.07 pc (480–15 730 au) for that in the local arm. This estimated power‐law index is consistent with earlier observational results based on both H  i  emission over larger scales and absorption studies over a similar range of scales. We do not detect any statistically significant change in the power‐law index with the velocity width of the frequency channels. This constrains the power‐law index of the velocity structure function to be  β= 0.2 ± 0.6  (3σ error).

H i 21 cm opacity fluctuations power spectra towards Cassiopeia A