NASA: 'Webb’s NIRSpec Acquires Multiple Targets' What to expect?
Article by: Andacs Robert Eugen, on 25 June 2022, at 08:56 am Los Angeles time
NASA returns with great news for the famous James Webb telescope, launched last year on December 25.
In addition that the team coordinating Webb has approved 10 of the 17 modes of scientific instruments, NIRSpec is starting to acquire targets.
This means that the James Webb telescope is beginning to make the first scientific observations, preparing for the first clear and full-color image that will be shown by NASA, ESA, and CSA on July 12, 2022.
Tracy Beck, Tony Keyes, and Charles Proffitt, all NIRSpec instrument scientists at the Space Telescope Science Institute (STScI) explain more in a new NASA post:
"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. Study of the intensity or brightness of light across the wavelengths can provide key diagnostic information about the nature of various objects across the universe - from extrasolar planets 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 gratings and a prism, the NIRSpec GWA also has a mirror that is primarily used to 'acquire' targets - to image them and place them at the proper locations in the instrument to observe a spectrum. NIRSpec has two methods for 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 wide 'S1600A1' fixed slit to determine its position on the sky as seen through the instrument. The software on-board the Webb telescope autonomously calculates an offset to move the telescope and accurately position either this target or another nearby target at the optimal location in NIRSpec to spread the light into a spectrum. During instrument commissioning, the excellent performance of WATA has been demonstrated on the sky for all four of the NIRSpec observing 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 of dozens to hundreds of science targets will be observed at one time. This requires specialized apertures that can be configured by opening and closing specific tiny doorways (microshutters) of the 250,000 total that are arranged in a rectangular grid in the MSA, allowing individual targets to be observed with little contamination from nearby objects or background light.
"During MSATA, a set of target acquisition reference stars are imaged through open microshutters. The stellar positions are calculated autonomously by Webb's on-board software and used to correct the initial spacecraft pointing and position angle (rotation). To allow accurate correction of the observed spectra for the 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 1/10th of a NIRSpec shutter width - or just 20 milli-arcseconds on the sky (the approximate size of a bumblebee, 1.5 cm, viewed from 150 km away!).
"The recent confirmation of NIRSpec target acquisition and additional work on the four science modes primes the NIRSpec team for our last activities of commissioning. We cannot wait to see the first NIRSpec science observations coming this summer!
"NIRSpec was built for the European Space Agency (ESA) by a consortium of European companies led by Airbus Defence and Space (ADS) with NASA's Goddard Space Flight Center providing its detector and microshutter subsystems."