dust-free quasars in a sample of 21 quasars, or $\approx 10\%$ of the population. [@leipski_2014] also found that $\approx 15\%$ of their sample of 69 quasars at $z>5$ are deficient in (but not devoid of) hot dust, and there is evidence of a trend toward higher dust-poor fraction with increasing redshift [@Jun2013].

Hubble Space Telescope Data and Observing Strategy
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Each of the six quasars was observed with the HST WFC3 infrared channel in the F125W (J-band) and F160W (H-band) filters. The five FIR-luminous quasars were observed for two orbits (4800s) in each filter, while was observed for four orbits (10400s) in each filter. @Windhorst2011 provides details on the WFC3 IR two-orbit sensitivity.

In addition to the quasar observations, coeval observations of a nearby PSF reference star were completed along with each epoch of quasar imaging. Although the HST PSF is stable compared to ground-based observatories, slight changes in the position of the secondary mirror cause small time-dependent focus variations. These variations are believed to be caused primarily by changes in the spacecraft thermal environment [@bely_1993; @hershey_1998; @cox_2011]. We mitigated this effect by imposing constraints on the PSF star observations, as in the pilot program [@Mechtley2012]—the (non-binary) stars were selected 