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We accurately determine a new Cepheid distance to M101 (NGC 5457) usingarchival HST/ACS V and I time series photometry of two fields within thegalaxy. We make a slight modification to the ISIS image subtraction package toobtain optimal differential light curves from HST data. We discovered 827Cepheids with periods between 3 and 80 days, the largest extragalactic sampleof Cepheids observed with HST by a factor of 2. With this large Cepheid samplewe find that the relative distance of M101 from the LMC is delta {\mu}LMC =10.63 \pm 0.04 (random) \pm 0.07 (systematic) mag. If we use the geometricallydetermined maser distance to NGC 4258 as our distance anchor, the distancemodulus of M101 is {\mu}0 = 29.04 \pm 0.05 (random) \pm 0.18 (systematic) magor D = 6.4 \pm 0.2 (random) \pm 0.5 (systematic) Mpc. The uncertainty isdominated by the maser distance estimate (\pm 0.15 mag), which should improveover the next few years. We determine a steep metallicity dependence, gamma,for our Cepheid sample through two methods, yielding gamma = -0.84 \pm 0.22(random) \pm 0.07 (systematic) mag dex-1 and gamma = -0.72+0.20 -0.22 (random)\pm 0.06 (systematic) mag dex-1. We see marginal evidence for variations in theWesenheit P-L relation slope as a function of deprojected galactocentricradius. We also use the TRGB method to independently determine the distancemodulus to M101 of {\mu}0 = 29.05 \pm 0.06 (random) \pm 0.12 (systematic) mag.

A New Cepheid Distance to the Giant Spiral M101 Based On Image Subtraction of HST/ACS Observations