Light path through WEAVE/LIFU

Light from an astronomical source passes through the following en route to a WEAVE detector (click on some of the figures to see full-size):

• Atmosphere (see the ING site-quality pages).
• WHT primary mirror M1, reflectivity typically 0.82 - 0.89, image quality at bare prime focus typically <~ 0.25 arcsec FWHM:
  
  
• New top end. The figure below (taken from document WEAVE-PRI-002) shows schematically, right to left, the sky-facing surface of the first lens in the corrector, the vanes supporting the corrector can, the rotator cable-wrap and the fibre-positioner: Each of the four support vanes is attached to the top-end ring via a focus-translation unit. The 4 translation units comprise the focus translation system (FTS), providing telescope focus adjustment over a range +- 3mm, accuracy ~ 5 micron.

  Click here to see the above figure left-right inverted.

• New 6-element prime-focus corrector (as-built drawing, taken from document WEAVE-PRI-014.7): The first lens along the light path, L1, is strongly curved and has diameter 1.1 m. Lenses L2-5 are a pair of counter-rotating prisms, providing atmospheric-dispersion correction. Delivered f-ratio is 2.8 on-axis, 2.9 at edge of field of view (radius 1 deg). Focal-plane scale 17.8 arcsec mm^-1. FWHM of PSF from M1 + corrector ~ 0.25 arcsec (from Shack-Hartmann tests).
• Field rotator, angular range -155 < mount PA < 155 deg (figure from document WEAVE-PRI-018):
• Pre-LIFU air-spaced doublet (2 BK7 lenses), delivering f/3.2. Scale at LIFU is 15.4 arcsec mm^-1.
• LIFU integral-field unit (90 arcsec x 78 arcsec on sky), comprising 547 hexagonally-packed science fibres, plus 56 sky fibres (all with diameter 2.6 arcsec) carrying the light from the focal plane to the LIFU slit-head in the spectrograph.
• En route to the spectrograph, the fibre bundles pass through the cable wrap on the rotator, along the prime-focus support vanes, down the telescope truss, over the elevation cable wrap, and into the refurbished GHRIL Nasmyth enclosure housing the spectrograph. The fibres are 32 m long.
• LIFU slit-head (vertically oriented) at the entrance to the spectrograph, and positioned there by the slit-exchange unit, which carries the MOS-A, MOS-B, mIFU an LIFU slit-heads in a square formation, as illustrated below (figure from document WEAVE-SPE-004):
• Spectrograph collimator: f/3.1, delivered beam diameter 190 mm. Size 645 mm x 260 mm, material Zerodur.
• Dichroic - light blueward of 5900 Å is reflected to the blue arm, redder light is transmitted to the red arm). Size = 390 mm x 308 mm x 25 mm thick.
• Blue arm:
  • Collimator corrector lenses.
  • Camera shutter (and, a few mm earlier along the light path, the Hartmann shutters, used for focusing).
  • VPH grating: LR (1385 lines/mm), HR1 (blue) (3580 lines/mm) or HR2 (green) (3056 lines/mm). The physical sizes of the low-res VPH are 192 mm x 202 mm (x 30 mm thick), and the high-res VPH are 190 mm x 309 mm (x 40 mm thick).
  • Camera optics: 8 lenses (one is aspheric), 235 - 320 mm in diameter, with gap ~ 5 - 7 mm before the cryostat window. Delivered f-ratio = 1.8.
  • Cryostat window (fused silica): diameter 180 mm, thickness 15 mm, 3 mm above detector surface.
  • Detector: 2 x 6k x 6k moasic of EEV CCD231-C6 deep-depletion CCDs.
• Red arm:
  • Collimator corrector lenses
  • Camera shutter, and Hartmann shutters (same arrangment as for the blue arm).
  • VPH grating: LR (876 lines/mm) or HR (2431 lines/mm). The physical sizes are as for the blue arm.
  • Camera optics (identical lenses to those in the blue-arm camera, but with different spacing).
  • Cryostat window: as for blue arm.
  • Detector: 2 x 6k x 6k mosaic of same model of CCDs as on the blue arm.

There is no second-order blocking filter in the red arm. Any second-order blue light passing into this arm will be strongly defocused.

The figure below, provided by Remko Stuik (NOVA, Univ. Leiden), shows the spectrograph in high-resolution mode, i.e. with the blue and red cameras positioned at the ends of the 'bananas' (shown in red) closest to the slit-exchange unit. The footprint of the spectrograph on the optical bench is ~ 2.1 m x 1.9 m in low-res mode, and 1.7 m x 1.7 m in high-res mode. The maximum height of the components above the bench is ~ 1.8 m. For an enlarged view, click on the figure.

The light path is shown from injection at the pseudo-slit (upper centre), via the dichroic (transmitting red, reflecting blue) to the blue camera (left) and the red camera (right). The shutter box in each arm includes both the fast shutter used to limit exposure times and the Hartmann shutters used for focusing. A set of internal (flat-field) calibration lamps is positioned between the camera and VPH on each arm.

Switching the spectrograph between low-resolution mode (first figure below) and high-resolution mode (second figure, same configuration as above) involves rotating the camera arms in the horizontal plane:

(these figures are taken from WEAVE internal document WEAVE-SYS-007).

In both low-res and high-res configurations, the spectrograph operates in Littrow mode, i.e. the angles of incidence and dffraction at the grating are equal: 20 deg in low-res mode, and 51.6 deg in high-res mode.

Back to LIFU home page