The HB narrowband comet filters: Standard stars and calibrations

We present results concerning the development and calibration of a new set of narrowband comet filters, designated the HE filter set, which was designed and manufactured to replace aging IHW filters. Information is also presented about the design and manufacturing of the filters, including the reasoning that was used for deciding the final wavelengths and bandpasses. The new filters are designed to measure five different gas species (OH, NH, CN, C-2, C-3), two ions (CO+, H2O+), and four continuum points. An improved understanding of extended wings from emission bands in comet spectra, gained since the development of the IHW filters, was incorporated into the new design, so that contamination from undesired species is significantly reduced compared to previous filters. In addition, advances in manufacturing techniques lead to squarer transmission profiles, higher peak transmission and UV filters with longer lifetimes. We performed the necessary calibrations so that data obtained with the filters can be converted to absolute fluxes, allowing for, among other things, accurate subtraction of the continuum from the gas species. Flux standards and solar analogs were selected and observed, and the data were used to establish a magnitude system for the HE filters. The star measurements were also used to evaluate which solar analogs were best representatives of the Sun and to explore how the flux standards differed in the UV with respect to their spectral type. New procedures were developed to account for the non-linear extinction in the OH filter, so that proper extrapolations to zero airmass can be performed, and a new formalism, which can account for mutual contaminations in two (or more) filters, was developed for reducing comet observations. The relevant equations and reduction coefficients are given, along with detailed instructions on how to apply them. We also performed a series of tests involving factors that can affect either the filter transmission profiles or the distribution of the emission lines in the gas species to determine how these effects propagate through to the calibration coefficients. The results indicate that there are only two factors that are a concern at a level of more than a few percent: f-ratios smaller than f/4, and a few individual filters whose transmission profiles are significantly different from the filters used in the calibrations. (C) 2000 Academic Press.