With a single WFC3/UVIS pointing, we propose to obtain UV photometry for over 600 globular clusters in M87, the massive cD galaxy hosting ~10,000 globulars at the center of Virgo. When combined with deep archival ACS optical photometry of the same field, we will be able to characterize the contribution of hot horizontal branch stars to the integrated light of these clusters. In the Milky Way, these evolved stars are prevalent in those globular clusters that host complex stellar populations, because both phenomena are symptoms of sub-populations which are highly enriched in helium. The discovery of multiple populations in Galactic globular clusters, driven by the photometric accuracy of HST, has revolutionized globular cluster research, in particular concerning their very formation in the early Universe that must have been a much more complex phenomenon than ever imagined before. This large, fully representative sample of globular clusters with UV-to-optical photometry will reveal to what extent globular cluster formation has proceeded in a distinct fashion in a giant elliptical environment, in particular with respect to the multiple population phenomenon. Our observations are extremely straightforward. We will obtain 10 orbits of WFC3/UVIS F275W imaging, with two exposures per orbit. Although shorter exposures would reduce the number of cosmic rays that must be masked per exposure, our image stack will provide excellent cosmic-ray masking, and shorter exposures suffer from two problems in the UV, where the sky background is low. First, co-adding a larger number of short exposures would increase the effective readout noise of the image stack – a significant impact on the faint limit in the low sky regime. Second, charge transfer inefficiency (CTI) is worse at low background levels and worse for faint sources, so it is preferable to allocate the source and sky counts over fewer images. Even though WFC3/UVIS is the newest camera on HST, it is apparently suffering from somewhat accelerated radiation damage, compared to earlier instruments. Fortunately, our team is well-equipped to address WFC3 instrument issues. J. Anderson has been working with the WFC3 team to characterize CTI losses and to construct a pixel-based correction for it, similar to the correction that he and L. Bedin developed for ACS. Each exposure will have its own unique but small dither, such that no GC is placed on the same pixel twice, enabling resampling of the point spread function, and also providing mitigation of cosmic rays and detector artifacts (e.g., hot pixels and dead pixels). The flat- field correction is more complex and uncertain at shorter wavelengths, so the large number of dithers will average out residual errors in the flats. Although all of the exposures will execute at the same orientation (to maximize the area covered at full depth), the particular orientation is unconstrained, to maximize schedulability. We have estimated the total integration time required by inspecting the extant NUV data for 16 M87 GCs (Sohn et al. 2006). These GCs were observed with the NUV/F25QTZ bandpass on STIS, which overlaps significantly with the UVIS/F275W bandpass on WFC3, allowing a secure prediction of the WFC3 countrates. The Sohn et al. clusters are as faint as mSTIS/NUV = 25 STMAG, and we want to obtain WFC3 photometry of these clusters at a SNR of at least 10. We assumed a hot (Teff = 28,000 K) spectrum and normalized it to mSTIS/NUV = 25 STMAG, and then calculated the required exposure time in the WFC3 ETC, with iterations to ensure that the number of readouts was consistent with two exposures per orbit. Given the fact that the GCs in M87 are not quite point sources, we conservatively calculate the SNR within a 6 pixel radius, and find that a SNR of 10 can be reached with 30,000 s (10 orbits) and 20 exposures. Note that for a given mSTIS/NUV magnitude, there is only weak dependence on the assumed spectral energy distribution (i.e., our faint limit is not significantly changed if we assume a different Teff over 20,000 K). Our estimate of the expected GC sample size in M87 is straightforward. The STIS/NUV imaging in the center of M87 found 16 GCs in an area of 625 square arcsec. Our WFC3/UVIS pointing covers an area ∼40× larger to the same depth as the STIS/NUV imaging. Although the underlying light of the galaxy is sharply peaked toward the center, the ACS optical images demonstrate that the spatial distribution of GCs over the WFC3/UVIS area is fairly uniform, such that we expect at least 600 M87 GCs to be detected in the near-UV (F275W). UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE 12 false 0.0 0.0 false false false false SUPPORTED GALAXY ELLIPTICAL J2000 J2000 WFC3/UVIS Imaging No No F275W Same orient constraint links visits (01 02 03 04 05) The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. 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