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We present observations and initial analysis from an HST/STIS program toobtain the first co-spatial, UV-optical spectra of ten Galactic planetarynebulae (PNe). Our primary objective was to measure the critical emission linesof carbon and nitrogen with unprecedented S/N and spatial resolution overUV-optical range, with the ultimate goal of quantifying the production of theseelements in low- and intermediate-mass stars. Our sample was selected from PNewith a near-solar metallicity, but spanning a broad range in N/O. This study,the first of a series, concentrates on the observations and emission-linemeasurements obtained by integrating along the entire spatial extent of theslit. We derived ionic and total elemental abundances for the seven PNe withthe strongest UV line detections (IC~2165, IC~3568, NGC~2440, NGC~3242,NGC~5315, NGC~5882, and NGC~7662). We compare these new results with otherrecent studies of the nebulae, and discuss the relative merits of deriving thetotal elemental abundances of C, N, and O using ionization correction factors(ICFs) versus summed abundances. For the seven PNe with the best UV linedetections, we conclude that summed abundances from direct diagnostics of ionswith measurable UV lines gives the most accurate values for the total elementalabundances of C and N. In some cases where significant discrepancies existbetween our abundances and those from other studies, we show that thedifferences can often be attributed to their use of fluxes that are notco-spatial. Finally, we examined C/O and N/O versus O/H and He/H inwell-observed Galactic, LMC, and SMC PNe, and found that highly accurateabundances are essential for properly inferring elemental yields from theirprogenitor stars.

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Co-spatial Long-slit UV/Optical Spectra of Ten Galactic Planetary  Nebulae with HST/STIS I. Description of the Observations, Global  Emission-line Measurements, and CNO Abundances