The presented new 1% narrowband filter centered at 1254nm with a FWHM of 11nm required and achieved excellent maximum transmission and deep out of band blocking. Magnetron sputtering was used for the filter coating process. The filter was used in the fully cryogenic near-infrared camera CIRCE of the Gran Telescopio Canarias (GTC) Telescope. The goal of the project is to detect very distant galaxies at the dawn of the Universe. These remote and extremely faint galaxies are selected by their Ly-alpha emission. We report on the spectral and interferometric results of the filter, and provide the spectacular first deep image taken with the 10.4m GTC telescope.