Removal of WHT Flip Ring
Removal of the WHT Flip Ring
The top end of the 4.2-m William Herschel Telescope (WHT) consists of an outer fixed ring, which is supported by eight Serrurier trusses, and an inner flip ring. The flip ring is attached to the outer ring such that it is allowed to rotate around an axis that allows it to be flipped through 180o. This flipping procedure permits the telescope to be configured with a secondary mirror or with an instrument at prime focus (see Figure 1).
Figure 1: The WHT top end showing the flip ring inside the fixed ring. The configuration is such that the secondary mirror, which is covered, is facing the primary mirror.
The WEAVE top-end assembly requires that this inner ring, along with the vanes and the prime focus corrector, is removed; an activity that had never been attempted in the history of the telescope. Prior to removal, an alignment telescope, positioned in the Nasmyth focal station, was used to record the position of the secondary mirror with respect to a reference mark within the alignment telescope (see Figure 2).
Figure 2: A collimated green beam was propagated through the crosshair of the alignment telescope, reflected off the secondary mirror and its position, with respect to a circular reference mark on the mirror, was recorded.
ING astronomers then carried out a Shack-Hartmann analysis to characterise the combined optical quality of the primary and secondary mirrors. The results of this analysis can be seen in Table 1.
Elevation (o) | Raw image RMS | Raw image 80% | Sub | Sub RMS | Sph RMS | Coma RMS | Coma extent | Astig RMS | Coma PA |
80 | 0.15 | 0.52 | 0.42 | 0.11 | 0.01 | 0.04 | 0.27 | 0.08 | 152 |
48 | 0.11 | 0.40 | 0.29 | 0.08 | 0.00 | 0.06 | 0.36 | 0.05 | 176 |
37 | 0.10 | 0.38 | 0.31 | 0.08 | 0.01 | 0.04 | 0.23 | 0.05 | -164 |
23 | 0.11 | 0.41 | 0.28 | 0.08 | 0.01 | 0.07 | 0.44 | 0.03 | 177 |
Table 1: The Shack-Hartmann test results, in arcsec, prior to removing the ring. For more information see the WHT optics log.
The following day, on the 19th of July 2016, ING engineering staff successfully removed the 5.5-tonne flip ring by separating the two bearing housings of the ring from its interface to the outer fixed ring (see Figure 3).
Figure 3: The left-hand bearing, with the dowel pins (only one shown in picture) removed, showed no movement of the bearing housing with respect to the outer ring. After removing the four bolts, a small movement (80µm) to the left of the bearing housing was seen. Before continuing with the process, it was confirmed that this displacement could be recovered.
The WHT 25-tonne crane, with a series of lifting strops, was used to lift the flip ring (Figure 4). The crane, which is supported from the dome, has a limited patrol range but was proven to be sufficient to allow the rings to be separated.