HI 21 cm opacity fluctuations power spectra towards Cassiopeia A
  The angular power spectrum of HI 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 HI 21 cm opacity
fluctuation power spectrum towards the supernova remnant Cas A from
interferometric data with spacial resolution of 5" and spectral resolution of
0.4 km/s. The power spectrum has been estimated using a simple but robust
visibility based technique. We find that the power spectrum is well fit by a
power law P_tau(U) = U^{alpha} with a power law index of alpha ~ -2.86 +/- 0.10
(3 sigma 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 - 15730 au) for that in the Local arm. This
estimated power law index is consistent with earlier observational results
based on both HI 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 beta =
0.2 +/- 0.6 (3 sigma error).
