Transmission curve of the NB1257 filter. The black dots show the empirical profile measured in the laboratory to guarantee FWHM  =  11 nm and central wavelength λ_c =  1.257 μm (dashed vertical gray line) at cryogenic temperature. The sky emission of La Palma is overplotted in blue. This sky spectrum was obtained with the EMIR instrument at the GTC. The atmospheric absorption was computed based on the Cerro Paranal Advanced Sky Model and is plotted in red. The filter lies in a wavelength range with minimum sky continuum emission and OH emission of the atmosphere. The inset shows the comparison between the transmission curve of our NB1257 filter (solid black line) and the BB filters WFC3/F125W (solid green line) and J standard (solid orange line) from the Maunakea set of filters.