Cover Picture: Astron. Nachr. 7/2024

Abstract

First-light spectra of the Sun as a star taken by SDI-POL, the Solar Disk Integration POLarimeter feeding the PEPSI night-time spectrograph at the LBT. The top panel shows the full wavelength range (383 - 907 nm) covered by SDI-POL. The color of the Stokes I spectra indicates the individual cross dispersers; dark blue (=CD 1) to dark red (=CD 6). The line at the bottom represents the normalized Stokes V Q & U signal on the same intensity scale as Stokes I. The bottom panel is a zoom into the spectral region around the sodium D lines. The top spectrum is again Stokes I (plotted are dots per CCD pixel connected by a line), the bottom spectra are Stokes V Q U shifted by -0.10 in intensity and expanded by a factor 200 in scale with respect to Stokes I. The I Q U spectrum was taken on UT May 20, 2022, the V-spectrum on UT June 22, 2023. The first-light application detects a clear Stokes V/I profile indicative of a tiny but significant solar disk-averaged line-of-sight net magnetic field of +0.37±0.02 Gauß on Oct. 13, 2023, as reported in the article by Strassmeier et al., this issue, e240033.