Checking Out the Mechanisms in Webb’s NIRSpec Instrument

This week, the Webb team has been working on the fourth stage of mirror alignment, called Coarse Phasing, which measures and corrects smaller height differences between the mirror segments.

In the meantime this past week, Webb’s Near-Infrared Spectrograph (NIRSpec) team successfully finished the check-out and initial characterization of three crucial onboard mechanisms. Today, members of the team join us to share more about the inner workings of this instrument, which was contributed by ESA (European Space Agency):

“To work properly as a spectrograph, NIRSpec has three mechanisms: a Filter Wheel Assembly (FWA), a Grating Wheel Assembly (GWA), and a Refocus Mechanism Assembly (RMA). The gratings in the GWA spread the incoming light over its colors or wavelengths to make a spectrum. The filters in the FWA block the wavelengths that are outside the range of interest to prevent contamination between different optical paths, or ‘orders.’ The RMA adjusts the instrument focus.

This NIRSpec diagram shows the placement of the Filter Wheel Assembly (FWA), a Grating Wheel Assembly (GWA), and a Refocus Mechanism Assembly (RMA). Credit: STScI

“We operated the Filter Wheel Assembly first, cycling it through all eight of its positions in both forward and reverse directions. Those eight filter wheel positions include five long-pass order-separation filters, two finite-band target acquisition filters, and an ‘opaque’ position that serves as the instrument shutter. At each position, we recorded a set of reference data. This data showed us how well the wheel was moving and how accurately it settled into each position. Between each FWA position, we downloaded ‘high-capacity buffer’ data from the positioning sensors, and the NIRSpec team analyzed the data. The data showed that the wheel moved very well even in the first attempt.

“We then used a very similar procedure for the Grating Wheel Assembly, which also performed excellently the first time. The GWA is shaped like a miniature Ferris wheel and holds eight optical elements, consisting of six diffraction gratings, one prism, and a mirror. These dispersers separate the incoming light by wavelength, generating spectra that are detected by NIRSpec’s sensor chips.

“The Refocus Mechanism Assembly includes a linear translation stage that holds two flat mirrors. It will be used to fine-tune the instrument focus, compensating for any change in the overall focus position of the Webb telescope that may occur throughout the observatory’s lifetime. After various initial retrievals of the RMA telemetry acquisition chain, the mechanism was moved forward a few hundred steps from launch position. Just like with the FWA and GWA, we used high-capacity buffer readouts to collect reference datasets. After the initial move, we commanded the RMA mirrors to their previous best focus position; successful completions of this test showed us that the RMA is a well-behaved and healthy mechanism.

The NIRSpec thermal team from Airbus Germany of Taufkirchen and Immenstaad – Marc Maschmann (left), Martin Altenburg (right), and Ralf Ehrenwinkler (front) – at the Space Telescope Science Institute in Baltimore. Credit: STScI

“In the coming months, the NIRSpec team will continue their commissioning efforts. The whole team is very much looking forward to the start of science observations this summer!”

–Maurice Te Plate, Webb NIRSpec systems engineer, European Space Agency; Tim Rawle, Webb NIRSpec instrument scientist, European Space Agency; and Ralf Ehrenwinkler, project manager, NIRSpec post-delivery support, Airbus Defence and Space

By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center

And Alexandra Lockwood, project scientist for Webb science communications, Space Telescope Science Institute