  We present a GUI-based interactive Python program, VIMAP, which generates
radio spectral index maps of active galactic nuclei (AGN) from Very Long
Baseline Interferometry (VLBI) maps obtained at different frequencies. VIMAP is
a handy tool for the spectral analysis of synchrotron emission from AGN jets,
specifically of spectral index distributions, turn-over frequencies, and
core-shifts. In general, the required accurate image alignment is difficult to
achieve because of a loss of absolute spatial coordinate information during
VLBI data reduction (self-calibration) and/or intrinsic variations of source
structure as function of frequency. These issues are overcome by VIMAP which in
turn is based on the two-dimensional cross-correlation algorithm of Croke and
Gabuzda (2008). In this paper, we briefly review the problem of aligning VLBI
AGN maps, describe the workflow of VIMAP, and present an analysis of archival
VLBI maps of the active nucleus 3C 120.
