On Sunday, 03 September 2017, the filter wheel stopped working. It was possible to get the filter to J by increasing the motor torque limits, so observing proceeded for the night. The next day, the system was investigated and it was found that the lower wheel was moving roughly by hand indicating that the problem was internal. The instrument was removed from the telescope and allowed to warm up the next day; 5 September 2017.

After fully warmin the instrument, it was disassembled and it was found that the sapphire ball bearings in the lower filter wheel were all pretty much destroyed. It is likely that one ball failed and the rest were damaged during troubleshooting attempts. The race was also damaged by the failure. The inner bearings (stainless steel) were unaffected. In addition, the broken sapphire was completely confined to the lower wheel.

Everything was thoroughly cleaned and the lower, outer race and sapphire bearings were replaced. The upper sapphire bearings were replaced proactively. The lower, Teflon ball cages were not replaced as there were not sufficient spares available. They were thoroughly cleaned and put back into service.

Initial tests of the lower wheel were that it did not move smoothly. Measurements of the races showed that the gap on the inner circle was smaller than for the upper (original) races. This meant that the lower wheel was riding on the Teflon ball cages and not on the bearings. The cages were machined down from 1.8 mm thick to ~1.4 mm thick.

These changes worked. The system was reassembled on 12-13 September and tested warm on 15 September. The system was tested cold on 19 September.

Beginning with the WIRCam camera run 11Bw02 (Sept 6, 2011), Chip #60 (top, left chip in array) is showing a low level (< 1% in ADU counts) step offset running along the y-direction (dec axis). The location of the step changes from one image to the next, and the ADU value is marginally correlated with the exposure time; the offset is seen in all filters, so the origin is likely related to the bias voltage.

Various tests were carried out to trace the origin of this offset but the exact cause is still unclear at the time of writing (Feb 2012). The offset level is low and the effect on photometry in stacked images is well below the percent level. In addition, a correction using the median value effectively removes the offset in processed images. Given the low level of the offset, the effective correction method and the marginal effect of the photometry, the engineers have decided to monitor the chip's performance and not to attempt any correction at present.

All science data observed from 11Bw02 onward are corrected for this offset before being distributed to the PIs.

On the first night of the WIRCam camera run 11Bw02 (Sept 6, 2011), it was noted that the Master and Slave cables had been swapped by mistake during the detector work carried out in August. This lead to significant noise features seen especially as horizontal bands in Chip #54 (bottom, left). The cables were reconnected properly the next morning and data taken on the night of 7 Sept no longer showed the banding.

Twilight flats are not available for the science images taken during the night of Sept 6/7. However, the full set of domeflats in all filters, taken at the time of the instrument exchange, are available. Therefore all the science data taken that night have been processed with the processed master domeflats only. On inspection, the processed images show only low level residual banding; the PIs of the affected programs are being informed of this at the time of data distribution.