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In order for Wide-Field Infrared Survey Telescope (WFIRST) and other Stage IVdark energy experiments (e.g., Large Synoptic Survey Telescope; LSST, andEuclid) to infer cosmological parameters not limited by systematic errors,accurate redshift measurements are needed. This accuracy can be met by usingspectroscopic subsamples to calibrate the photometric redshifts for the fullsample. In this work we employ the Self Organizing Map (SOM) spectroscopicsampling technique, to find the minimal number of spectra required for theWFIRST weak lensing calibration. We use galaxies from the Cosmic AssemblyNear-infrared Deep Extragalactic Legacy Survey (CANDELS) to build theLSST+WFIRST lensing analog sample of ~36 k objects and train the LSST+WFIRSTSOM. We find that 26% of the WFIRST lensing sample consists of sources fainterthan the Euclid depth in the optical, 91% of which live in color cells alreadyoccupied by brighter galaxies. We demonstrate the similarity between faint andbright galaxies as well as the feasibility of redshift measurements atdifferent brightness levels. Our results suggest that the spectroscopic sampleacquired for calibration to the Euclid depth is sufficient for calibrating themajority of the WFIRST color-space. For the spectroscopic sample to fullyrepresent the synthetic color-space of WFIRST, we recommend obtainingadditional spectroscopy of ~0.2-1.2 k new sources in cells occupied by mostlyfaint galaxies. We argue that either the small area of the CANDELS fields andthe small overall sample size or the large photometric errors might be thereason for no/less bright galaxies mapped to these cells. Acquiring the spectraof these sources will confirm the above findings and will enable thecomprehensive calibration of the WFIRST color-redshift relation.

Photometric Redshift Calibration Requirements for WFIRST Weak Lensing Cosmology: Predictions from CANDELS