The hazards of imaging spectrophotometry with interference filters

Narrowband-filter imaging spectrophotometry is a powerful tool for studying the spatial behavior of physical parameters, such as interstellar reddenin g, excitation, electron temperature, and densities or chemical abundances a cross extended objects such as nebulae and galaxies. We explore the hazards and technical limits of the method of spectral imaging with narrowband int erference filters. We discuss the specific application of statistical error s involved in the estimation of abundance gradients from nebular line flux measurements and the impact of those errors on interpretation. We demonstra te that a sample of at least 16 H II regions is needed for a meaningful and robust description of radial abundance gradients in disk galaxies. We also discuss the systematic errors introduced by the uncertainty related to the subtraction of the underlying stellar continuum from monochromatic emissio n-line images; we demonstrate that the quality of subtraction of the underl ying continuum is the main limit on the precision of imaging spectrophotome try with narrowband filters. Finally, we investigate the impact of selectio n effects on the derivation of physical parameters from measurements involv ing the ratio of two or more spectral lines.