4MOST Wide Field Corrector checks out

by Joar Brynnel on 2025-03-06

During the 4MOST hardware integration campaign on Paranal in October of last year, one of the verification measurements done on Paranal during the incoming inspection was measurement of the Wide Field Corrector optical throughput. At the time, the results of these measurements indicated a loss of optical throughput compared to reference measurements done in Potsdam before shipment. Nevertheless, the WFC was installed on the VISTA telescope and telescope re-commissioning has been ongoing since then.

Possible reasons for the measured loss of throughput have been intensively investigated over the past months, however, since we had no physical access to the WFC, it was not possible to make tests to better understand the root cause of the problem. After discussions between the Consortium and ESO, it was decided to remove the WFC from the VISTA telescope and move it to a clean room on Paranal to investigate the issue. Removing the WFC from the telescope is a complex procedure involving removal of the Cable Wrap system (CaCW), the 4MOST test camera (FSTT), the Guiding and WFS unit, and finally the WFC itself. On March 2, a team from AIP arrived on Paranal to carry out this work in collaboration with ESO staff.

On March 4, the WFC optical throughput was re-measured. We are very pleased to report that the results confirm the Potsdam measurements, and the WFC performance is now verified to be nominal and in specifications. Over the next couple of days, we will reinstall all the components on the telescope and the final steps of VISTA re-commissioning can be carried out as planned.

We used the opportunity in the clean room to CO2 snow clean the L1 front and L4 back surfaces to remove fine dust, see attached photo.