image Arago Visible Spectrograph image Arago Visible detector image

Visible spectrograph

The Visible light exiting from the polarimeter goes through a dichroic and enters a spectrograph designed to cover the Visible wavelengths from 355 to 888 nm. The Visible spectrograph is composed by an off-axis parabolic collimator, an echelle-grating and the cross-dispersion is obtained by an immersed grating, i.e. the combination of a prism and a reflective grating. This combination is optimized to make the spatial separation between all of the orders almost constant on the detector plane. Finally, a Tessar-type lens camera focuses all the orders onto a CMOS detector.
Visible spectra with a resolution of R>=35000 are recorded over the full wavelength range from 355 to 888 nm. This range includes many useful spectral lines, in particular many hydrogen lines such as H_alpha, CaII lines, and molecules of oxygen.