  We describe an imaging method that makes use of interference filters to
provide integrated stellar spectral indices for spiral disks to faint surface
brightness limits. We use filters with bandpasses ~60A FWHM, centered on the Mg
and Fe features (5176A and 5270A respectively) allowing the determination of
the spatial distribution of the Lick indices Mg2 and Fe5270. These two indices
have been extensively modeled by different groups and used in the past mainly
for the study of elliptical galaxies, bulges, and globular clusters. Azimuthal
integration of the underlying smooth stellar signal, after removal of the
signature of the spiral arms and associated extreme pop. I structures, provides
measurements of these spectral indices useful to radial distances where the
surface brightness of the galaxy reaches ~24uV. As a first example of this
technique and its possibilities we conduct a preliminary study of the SABbc
galaxy NGC 4321 (M 100). We present spectral gradients for the inter-arm
stellar population to about 4 exponential scale lengths. There is some evidence
for a discontinuity in the run of the Mg2 index near corotation, which we
interpret as evidence for bar- driven secular evolution. There is also evidence
that Mg is overabundant with respect to Fe in the inner regions of the
projected image.

Astrophysics