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BL Lacertae (BL Lac) objects are a subclass of blazar, distinguished by theirfeatureless optical spectrum. The featureless spectrum presents a challenge inmeasuring the redshift of the BL Lacs. In this paper, we measure the redshiftof BL Lacs using the photometric dropout technique. The space-based telescope\emph{Swift} and the ground-based SARA telescopes are employed to providemagnitudes in the $uvw2,\ uvm2,\ uvw1,\ u,\ b,\ v,\ g',\ r',\ i',\ z'$ filters.We observe 60 BL Lacs and report reliable redshift upper limits for 41 of them.We discover three new high-$z$ BL Lacs ($z>1.3$) at $1.74_{-0.08}^{+0.05}$,$1.88_{-0.03}^{+0.07}$, and $2.10_{-0.04}^{+0.03}$, bringing the number ofhigh-$z$ BL Lacs found by this method up to 19. Discussions are made on theimplications for the blazar sequence, the \emph{Fermi} blazar divide, and thegamma-ray horizon based on an analysis of the 4LAC catalog and all high-$z$ BLLacs found with the photo-$z$ technique.

Revealing high-z Fermi-LAT BL Lacs using Swift and SARA data with photometric analysis