SOPHIE Thorium spectrum gets filter

Thorium double spectrum before insertion of filter

Thorium double spectrum after insertion of filter

On 10 April 2007 a custom high-pass interference filter was installed in the Cassegrain bonnette in front of the Thorium lamp so as to eliminate all emission from this lamp redwards of ~ 700 nm, mostly due to Argon, which, though not directly imaged on the CCD, was causing multiple internal reflections within the spectrograph and contaminating stellar spectra taken in the simultaneous Thorium mode.

Insertion of this filter not only suppressed the bothersome saturated Ar lines, but also dramatically reduced diffuse light throughout the CCD frame. Order 39 of Thorium (only orders 0-38 are extracted) is severely attenuated and the red edge of order 38 is partly attenuated redwards of 691 nm, well into the region dominated by telluric water absorption in stellar spectra.