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We present observations of the first two supernovae discovered with therecently installed Advanced Camera for Surveys (ACS) on the Hubble SpaceTelescope. The supernovae were found in Wide Field Camera images of the HubbleDeep Field North taken with the F775W, F850LP, and G800L optical elements aspart of the ACS guaranteed time observation program. Spectra extracted from theACS G800L grism exposures confirm that the objects are Type Ia supernovae (SNeIa) at redshifts z=0.47 and z=0.95. Follow-up HST observations have beenconducted with ACS in F775W and F850LP and with NICMOS in the near-infraredF110W bandpass, yielding a total of 9 flux measurements in the 3 bandpassesover a period of 50 days in the observed frame. We discuss many of theimportant issues in doing accurate photometry with the ACS. We analyze themulti-band light curves using two different fitting methods to calibrate thesupernovae luminosities and place them on the SNe Ia Hubble diagram. Theresulting distances are consistent with the redshift-distance relation of theaccelerating universe model, although evolving intergalactic grey dust remainsas a less likely possibility. The relative ease with which these SNe Ia werefound, confirmed, and monitored demonstrates the potential ACS holds forrevolutionizing the field of high-redshift SNe Ia, and therefore of testing theaccelerating universe cosmology and constraining the "epoch of deceleration".Comment: 11 pages, 8 embedded figures. Accepted for publication in Ap

Discovery of Two Distant Type Ia Supernovae in the Hubble Deep Field–North with the Advanced Camera for Surveys