Cross-disperser gratings

cross-disperser wheel
(click for larger)

One of the major aspects of the CRIRES+ upgrade can be seen in this image. It shows the cross-disperser wheel that holds six reflective gratings, one for each of the near-infrared bands YJHKLM. Visible are the ones for K, L and M bands. The wheel will rotat to choose the band to be observed. The light that reflects off the grating will be dispersed by wavelength before it reaches the main grating with much higher resolution.

Since the direction of cross-dispersion is perpendicular to the main dispersion, we can fit 9 spectral orders onto the detectors (see simulations), compared to a single one before. The simultaneous wavelength coverage and efficiency of CRIRES+ will therefore go up by about an order or magnitude.

Polarizer progress

The image above shows the now assembled turret that will hold the polarizing beam splitters. It rotates around it central vertical axis, as seen in the video below. Additional motors on the sides will allow us to exchange the two beams with each other, by rotating the optical elements. The turret is less than 15 cm in diameter and will soon be transported from Uppsala to Garching for more testing and integration.

This short video shows Alexis Lavail demonstrating the turret rotation, together with a short explanation by Nikolai Piskunov:

Slit decomposition

“Slit decomposition” is the algorithm that we use in the new data-reduction system to extract 1D-spectra from the detector images. This works by approximating a cut-out from a spectral order (top-left panel) by two one-dimensional vectors (black lines), one being the spectrum (middle panel), the other the slit illumination (right panel). From these two one can reconstruct a 2D-model, which is shown in the lower left panel. The animation shows how we step in overlapping chunks along the spectral order. The red dots are the individual pixel values from the input image, collapsed along the respective direction.