Measuring the Near-IR Airglow Continuum with Stray Light Reduced Spectrograph

We present an experimental spectrograph based on a new type of a white pupil echelle spectrograph designed for interline sky continuum measurement. This is an application which requires extremely low instrumental scattered light background and high spectral fidelity due to the faintness of the signal and the fact that the night sky is illuminated by bright hydroxyl emission lines from the red-end of visual spectrum through near infrared wavelengths. The conceptual design is a white pupil echelle spectrograph using a single grating in double pass. The xy-plane of the optical beam is rotated between the two consecutive dispersions thus redirecting the scattered 'grating grass' light from the grating to different direction respective to the spectrum. A mask in the secondary focus prevents this scattered light reaching the camera. The beam rotation is achieved with a normal right-angle prism allowing us to use off-the-shelf optical components and classical geometrical optics to mitigate the issue of hydroxyl airglow emission lines.