Optics and efficiency

The following are the main characteristics of CUBES new optical design:

Main drivers: manufacturability (slices and lenses) and optical transmission while including less movable parts as possible

Image slicer:

  • 6 slices each with a width of 0.25arcsec on sky
  • Slit width on sky 1.5arcsec, no slit loss
  • Physical slice width 0.5mm for ease of manufacture cf Keck Cosmic Web Imager
  • Smaller beam size at grating (applies to any spectrometer design)
  • Allows pupil to be reimaged on the grating
Fig.1 Two views of the preliminary design of the image slicer for CUBES. Two arrays (slicer mirrors and camera mirrors) of six spherical mirrors each are used to generate the spectrograph slit.

ADC:

  • 2 silica prisms
  • Deviation removed using tip-tilt mirror which can also be used for image stabilization
  • Greater observational flexibility and minimal, unchanging, angular offset to slit viewing wavelength
Fig.2 Top: foreoptics layout – telescope focus to image slicing mirrors.Bottom: detail of the ADC

Spectrograph:

  • Split into 3 bands using dichroics with separate collimators, gratings and cameras
  • Reduced AW allows
  • 3 silica lens camera
  • All surfaces spherical except for one surface with conic = -0.28
  • Same set of components for each camera
  • Chromatic focus offset by lens spacings and detector tilt
  • Number of detectors depends on packaging and folding of optical paths
  • Bands have 6nm overlap, so complete wavelength coverage in one observation
Fig.3 Layout of the 3 spectrometers including the foreoptics and image slicerTop: View looking towards the telescope. Bottom: Side view showing beam from the telescope, plane of the main optics and detectors below.

Grating:

  • Footprint on grating 160mm (along grooves) x 195mm (dispersion direction
  • Optimised for maximum transmission within each band
  • No mosaicing necessary

The transmission of the sub-systems and overall telescope focus to detector transmission are listed in Table 1.

Table 1 Sub-system and overall transmissions.