Reducing the systematic errors on the Hubble constant is still of significance and of immediate importance to modern cosmology. One of the largest remaining uncertainties in the Cepheid-based distance scale (which itself is at the foundation of the HST Key Project determination of H_o) which can now be addressed directly by HST, is the effect of metallicity on the Cepheid Period-Luminosity relation. Three chemically distinct regions in M101 will be used to directly measure and thereby calibrate the change in zero point of the Cepheid PL relation over a range of metallicities that run from SMC-like, through Solar, to metallicities as high as the most metal-enriched galaxies in the pure Hubble flow. ACS for the first time offers the opportunity to make a precise calibration of this effect which currently accounts for at least a third of the total systematic uncertainty on Ho. The calibration will be made in the V and I bandpasses so as to be immediately and directly applicable to the entire HST Cepheid-based distance scale sample, and most especially to the highest-metallicity galaxies that were hosts to the Type Ia supernovae, which were then used to extend the the distance scale calibration out to cosmologically significant distances.

Three fields in M101 will be monitored in the V and I bandpasses searching for Cepheids. A window of 45 days will be optimally sampled by 12 observations. A single orbit containing both the V and the I exposures will reach the required signal-to-noise ratio for the shortest-period (3-5 day) Cepheids COSMOLOGY 12 false 0.0 false false false false false false SUPPORTED GALAXY SPIRAL ARM SPIRAL STAR FORMING REGION This object was generated by the targetselector and retrieved from the NED database. changed to IRCS coordinate frame since GSC2 is not recognized. J2000 14 03 28.8000 +54 21 35.00 J2000 GALAXY SPIRAL ARM STAR FORMING REGION SPIRAL changed to IRCS coordinate frame since GSC2 is not recognized. J2000 14 02 51.0000 +54 20 22.00 J2000 GALAXY STAR FORMING REGION SPIRAL ARM SPIRAL These coordinates have been changed from 14 02 29.0, +54 18 20, to acquire guide star coverage. J2000 14 02 29.0000 +54 18 20.00 J2000 ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W WFPC2 Imaging No F555W WFPC2 Imaging No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F814W WFPC2 Imaging No F555W WFPC2 Imaging No F814W WFPC2 Imaging No F555W WFPC2 Imaging No F814W (Tic-Visit :proposal-id 11297 :visit-id "62" :proposal-version -1 :trans-version "20070325" :date "14-May-2007 09:35:08.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 179264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11297 :visit-id "62" :obset-id "62" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11297 :visit-id "62" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2580 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3200 :pcs-gap 0 ) false 1 0 there are np time constraints on this visit (Tic-Visit :proposal-id 11297 :visit-id "63" :proposal-version -1 :trans-version "20070325" :date "14-May-2007 09:34:45.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 179264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11297 :visit-id "63" :obset-id "63" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11297 :visit-id "63" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2580 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3200 :pcs-gap 0 ) true 1 0 (Tic-Visit :proposal-id 11297 :visit-id "64" :proposal-version -1 :trans-version "20070325" :date "14-May-2007 09:34:39.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 179264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11297 :visit-id "64" :obset-id "64" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11297 :visit-id "64" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2580 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3200 :pcs-gap 0 ) true 1 0