The Double Monochromator (DM) is the focal instrument of SOLARNET. It has powerful instantaneous imaging capabilities in the FUV and UV. As shown hereafter, the DM uses two gratings (G1 & G2), the first one introducing a dispersion of the image on an intermediate slit (S) while the second, G2, recombines the dispersed light on the CCD in output. This allows to have a filtered image (40" x 40" field-of-view) as narrow as allowed by the slit width and aberrations (20-40 mÅ or so).
The SOLARNET Double Monochromator is implemented using, first, two gratings with 3600 l/mm for the FUV spectral region and, second, two gratings 2400 l/mm for the UV spectral region. The UV/FUV splitting is made using a Wadsworth flat mirror which rotates synchronously with the gratings, always sending the zero order reflected by the first DM ("green beam") at the same angle for a second DM working in the UV. The overall size is reasonable for a proper structural control: 790 x 310 x 150 mm3.
