Discovering high-redshift BL Lacs

BL Lacertae (BL Lac) objects are a subclass of a blazar with the relativistic jet pointing along the line of sight. Their featureless optical spectra do not allow, in most cases, a measurement of their redshift. Here we report on a photometric campaign to measure BL Lacs redshifts using Swift and SARA telescopes. Our campaign has so far discovered more than 20 rare high-z BL Lacs. In this contribution, we will interpret the findings in terms of the blazar sequence, the constraint on the extragalactic background light, and the discovery of masquerading BL Lacs.