NASA’s James Webb’s NIRSpec Acquires Multiple Targets

The Webb team now has 10 of the approved 17 Scientific Instrument Modes† since last week we added (14) MIRI imaging, (2) NIRCam wide field slitless spectroscopy and our latest NIRISS mode, (10) single object slitless spectroscopy. As we wind down the final commissioning activities, some gaps have appeared in the schedule. The team has begun taking some of the first science data and preparing it for publication from July 12, 2022, which will mark the official end of Webb’s commissioning and the start of routine science operations.

This week we asked Tracy Beck, AURA Observatory and Webb NIRSpec Instrument Scientist, STScI; Tony Keyes AURA scientist and Webb NIRSpec instrument scientist, STScI; and Charles Proffitt, AURA Observatory and Webb NIRSpec Instrument Scientist, STScI – all NIRSpec instrument scientists from the Space Telescope Science Institute (STScI), to tell us how Webb is aligning the targets for observation with the NIRSpec instrument.

“The Near infrared spectrograph (NIRSpec) is the instrument on the Webb telescope that observes spectra of astrophysical and planetary objects at near infrared wavelengths. The NIRSpec Grating Wheel Assembly (GWA) uses diffraction gratings or a prism to separate the wavelengths of incoming light into a spectrum. Studying the intensity or brightness of light across the wavelengths can provide important diagnostic information about the nature of various objects in the universe – from planets outside the solar system around distant stars to faint galaxies at the edge of the universe and objects in our own solar system. NIRSpec will observe them all.”

“In addition to the grids and a prism, the NIRSpec GWA also has a mirror that is mainly used to ‘acquire’ targets – bringing them into focus and placing them in the correct locations in the instrument to observe a spectrum. NIRSpec has two methods of target acquisition (TA): the Wide Aperture Target Acquisition (WATA) and the Micro-Shutter Assembly (MSA) based Target Acquisition (MSATA)

“The WATA process takes an image of a single astrophysical target through the broad ‘S1600A1’ fixed gap to determine its position in the sky as seen through the instrument. The software onboard the Webb telescope autonomously calculates an offset to move the telescope and accurately position this target or another nearby target at the optimal location in NIRSpec to scatter the light in a spectrum. During the commissioning of the instrument, WATA’s excellent sky performance was demonstrated for all four NIRSpec observation modes: integral field unit imaging spectroscopy, fixed-slit spectroscopy, bright object time series, and multi-object spectroscopy.

“NIRSpec includes the multi-object spectroscopy (MOS) mode, where spectra from tens to hundreds of scientific targets are simultaneously observed. This requires specialized openings that can be configured by opening and closing specific small doorways (microshutters) out of a total of 250,000 arranged in a rectangular grid in the MSAallowing individual targets to be observed with little pollution from nearby objects or background light.

During MSATA, a series of reference stars for target acquisition are imaged by open microshutters. The stellar positions are calculated autonomously by Webb’s built-in software and used to correct the initial spacecraft direction and position angle (rotation). To provide accurate correction of the observed spectra for the To allow centering of each source in its shutter, this process must place the MOS science targets across the full span of the NIRSpec field of view with an accuracy of 1/10e of a NIRSpec shutter width – or just 20 milliarcseconds in the sky (the approximate size of a bumblebee, 1.5 cm, seen from 150 km away!).

“The recent confirmation of NIRSpec target acquisition and additional work on the four science modes sets the NIRSpec team ready for our final commissioning activities. We can’t wait to see the first scientific observations of NIRSpec this summer!

“NIRSpec was built for the European Space Agency (ESA) by a consortium of European companies led by Airbus Defense and Space (ADS) with NASA’s Goddard Space Flight Center providing its detector and microshutter subsystems.”

The James Webb Space Telescope is the world’s largest, most powerful and most complex space science telescope ever built. Webb will solve mysteries in our solar system, look further into distant worlds around other stars, and explore the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency

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