A quantitative analysis of factors affecting Balmer-index photometry

An analytical expression has been developed which allows the profile of a narrow-band filter to be calculated according to the geometry of the hollow coned beam provided by a telescope. With this refinement, filter profiles have been convolved with a set of hypothetical stellar line profiles to allow expected β-indices to be calculated. From the generated data several important and new conclusions have been drawn concerning the relationship between β-indices and equivalent width, the curvatures in β-index transformations, the role the telescope plays in defining a standard β-index system, the influences of stellar rotational velocity and general radial-velocity effects on β-indices, the desired thermal stability of the photometry and band-pass fluctuations caused by variations of aperture illumination contributing to the noise on β-indices, particularly with the smaller telescopes. (author)