Layout of the optical unit
OPTICAL UNIT
Following the light path, we find the filter wheel mechanism (FWM), the polarization modulation package (PMP), a mirror that drives the beam into the lithium niobate etalon, which is traveled through twice, a mirror that drives the light into the polarizing beam splitter, which divides the beam in two that impinge each of the two scientific cameras. The three isostatic supports that link the instrument to the Sunrise instrumental platform are clearly seen.
The first wheel hosts the F4 field stop for room and operational conditions checking and a phase diversity plate. The latter is used to restore from the effects of the PSF through a series of specific measurements. The second wheel hosts the three narrow-band pre-filters, each for one of the target spectral lines. Those for the two Fe I lines are identical. The different tuning is achieved by a differential tilt between them. Since each wheel has five vanes, room is available for polarimetric calibration optics or pinholes for alternative determination of the PSF.
As in IMaX and SO/PHI, the polarization modulation is made with a pair of nematic liquid crystal variable retarders. The polarization analysis is performed by the beam splitter. Being a dual-beam polarimeter (the two orthogonal polarization states are recorded at the same time), IMaX+ circumvents most of the problems introduced by the residual jittering of the platform and increases the signal-to-noise ratio of the measurements. The goal is for reaching a polarimetric precision of one part in one thousand in the Fe I lines and better in the Mg Ib2 line.
The spectroscopic analysis is made with the pre-filters and the Fabry-Pérot etalon in double pass to reach a profile transmission FWHM of 6.6 pm and 6.4 pm for the Fe and Mg regions respectively.