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We present precision transit observations of the Neptune-sized planets K2-28band K2-100b, using the Engineered Diffuser on the ARCTIC imager on the ARC 3.5mTelescope at Apache Point Observatory. K2-28b is a $R_{p} = 2.56 R_\oplus$mini-Neptune transiting a bright (J=11.7) metal-rich M4 dwarf, offeringcompelling prospects for future atmospheric characterization. K2-100b is a$R_{p} = 3.45 R_\oplus$ Neptune in the Praesepe Cluster and is one of fewplanets known in a cluster transiting a host star bright enough ($V=10.5$) forprecision radial velocity observations. Using the precision photometriccapabilities of the diffuser/ARCTIC system, allows us to achieve a precision of$105^{+87}_{-37}$ppm, and $38^{+21}_{-11}$ppm in 30 minute bins for K2-28b, andK2-100b, respectively. Our joint-fits to the K2 and ground-based light-curvesgive an order of magnitude improvement in the orbital ephemeris for bothplanets, yielding a timing precision of 2min in the JWST era. Although we showthat the currently available broad-band measurements of K2-28b's radius arecurrently too imprecise to place useful constraints on K2-28b's atmosphere, wedemonstrate that JWST/NIRISS will be able to discern between a cloudy/clearatmosphere in a modest number of transit observations. Our light-curve ofK2-100b marks the first transit follow-up observation of thischallenging-to-observe transit, where we obtain a transit depth of $819 \pm 50\mathrm{ppm}$ in the SDSS $i^\prime$ band. We conclude that diffuser-assistedphotometry can play an important role in the TESS era to perform timely andprecise follow-up of the expected bounty of TESS planet candidates.

Diffuser-assisted Photometric Follow-up Observations of the Neptune-sized Planets K2-28b and K2-100b