An Image-based Tapered Gridded Estimator (ITGE) for the angular power
  spectrum
  We present the Image-based Tapered Gridded Estimator (ITGE) to measure the
angular power spectrum $(C_{\ell})$ of the sky signal directly from the
visibilities measured in radio-interferometric observations. The ITGE allows us
to modulate the sky response through a window function which is implemented in
the image plane, and it is possible to choose a wide variety of window
functions. In the context of the cosmological HI 21-cm signal, this is useful
for masking out the sky signal from specific directions which have strong
residual foregrounds. In the context of the ISM in external galaxies, this is
useful to separately estimate the $C_{\ell}$ of different parts of the galaxy.
The ITGE deals with gridded data, hence it is computationally efficient. It
also calculates the noise bias internally and exactly subtracts this out to
give an unbiased estimate of $C_{\ell}$. We validate the ITGE using realistic
VLA simulations at $1.4 {\rm GHz}$. We have applied the ITGE to estimate the
$C_{\ell}$ of HI 21-cm emission from different regions of the galaxy NGC 628.
We find that the slope of the measured $C_{\ell}$ in the outer region is
significantly different as compared with the inner region. This indicates that
the statistical properties of ISM turbulence possibly differ in different
regions of the galaxy.
