The HB Narrowband Comet Filters: Standard Stars and Calibrations
We present results concerning the development and calibration of a newset of narrowband comet filters, designated the HB filter set, which wasdesigned and manufactured to replace aging IHW filters. Information isalso presented about the design and manufacturing of the filters,including the reasoning that was used for deciding the final wavelengthsand bandpasses. The new filters are designed to measure five differentgas species (OH, NH, CN, C2, C3), two ions(CO+, H2O+), and four continuum points.An improved understanding of extended wings from emission bands in cometspectra, gained since the development of the IHW filters, wasincorporated into the new design, so that contamination from undesiredspecies is significantly reduced compared to previous filters. Inaddition, advances in manufacturing techniques lead to squarertransmission profiles, higher peak transmission and UV filters withlonger lifetimes. We performed the necessary calibrations so that dataobtained with the filters can be converted to absolute fluxes, allowingfor, among other things, accurate subtraction of the continuum from thegas species. Flux standards and solar analogs were selected andobserved, and the data were used to establish a magnitude system for theHB filters. The star measurements were also used to evaluate which solaranalogs were best representatives of the Sun and to explore how the fluxstandards differed in the UV with respect to their spectral type. Newprocedures were developed to account for the non-linear extinction inthe OH filter, so that proper extrapolations to zero airmass can beperformed, and a new formalism, which can account for mutualcontaminations in two (or more) filters, was developed for reducingcomet observations. The relevant equations and reduction coefficientsare given, along with detailed instructions on how to apply them. Wealso performed a series of tests involving factors that can affecteither the filter transmission profiles or the distribution of theemission lines in the gas species to determine how these effectspropagate through to the calibration coefficients. The results indicatethat there are only two factors that are a concern at a level of morethan a few percent: f-ratios smaller than f/4, and a few individualfilters whose transmission profiles are significantly different from thefilters used in the calibrations.
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