The $\theta$-$\theta$ Diagram: Transforming pulsar scintillation spectra
  to coordinates on highly anisotropic interstellar scattering screens
  We introduce a novel analysis technique for pulsar secondary spectra. The
power spectrum of pulsar scintillation (referred to as the "secondary
spectrum") shows differential delays and Doppler shifts due to interference
from multi-path propagation through the interstellar medium. We develop a
transformation which maps these observables to angular coordinates on a single
thin screen of phase-changing material. This transformation is possible without
degeneracies in the case of a one-dimensional distribution of images on this
screen, which is often a successful description of the phenomenon. The double
parabolic features of secondary spectra are transformed into parallel linear
features, whose properties we describe in detail. Furthermore, we introduce
methods to measure the curvature parameter and the field amplitude distribution
of images by applying them to observations of PSR B0834+06. Finally, we extend
this formalism to two-dimensional distributions of images on the interstellar
screen.
