Webb’s NIRSpec Acquires Multiple Targets

The Webb team has now approved 10 out of 17 science instrument modes; since last week we added (14) MIRI imaging, (2) NIRCam wide-field slitless spectroscopy, and our final NIRISS mode, (10) single-object slitless spectroscopy. As we ramp down the final commissioning activities, some openings in the schedule have appeared. The team has started to take some of the first science data, getting it ready to release starting July 12, 2022, which will mark the official end of commissioning Webb and the start of routine science operations.

This week we asked Tracy Beck, Tony Keyes, and Charles Proffitt, all NIRSpec instrument scientists at the Space Telescope Science Institute (STScI), to tell us about how Webb gets the targets lined up 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. 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.

A simulation of the NIRSpec MSA-based Target acquisition, showing reference stars
A simulation of the NIRSpec MSA-based Target acquisition process, demonstrated on the NIRSpec Sharpness Check Image. NIRSpec uses “Reference Stars” observed through the fixed slits in the central area and the MSA to carefully correct the small x – y and position angle (rotation) offsets of the observatory so that the science targets will be aligned properly with their shutters across the entire NIRSpec MOS field of view. Credit: NASA, ESA, and the NIRSpec Team

“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!).

An optimized high-resolution simulation of a star at wavelength 2 microns seen through a NIRSpec microshutter (100×200 microns in size).
An optimized high-resolution simulation of a star at wavelength 2 microns seen through a NIRSpec microshutter (100×200 microns in size). For proper intensity estimation of NIRSpec science spectra, we need to accurately know the positioning of the targets to within 1/10th of the shutter width. Credit: NASA, ESA, and the NIRSpec Team

“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.”

–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


Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard
Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

Webb’s Mid-Infrared Spectroscopy Will Reveal Molecules, Elements

NASA’s James Webb Space Telescope team continues to work its way through the 17 science instrument modes. This week they checked off numbers (5) NIRCam grism time series and (4) imaging time series, both used to study exoplanets and other time-variable sources; (12) NIRISS aperture masking interferometry mode, for direct detection of a faint object that is very close to a bright one; (11) NIRISS wide-field slitless spectroscopy, for studying distant galaxies; and (9) NIRSpec bright-object time series, for studying exoplanets. That totals seven modes approved to date, with 10 still to go.

This week we are featuring MIRI’s medium-resolution spectroscopy mode and sharing our first spectroscopic engineering data. We asked two of the MIRI commissioning team members – David Law, of the Space Telescope Science Institute (STScI), and Alvaro Labiano, of the Centro de Astrobiologίa (CAB) – to explain this mode to us:

“One of Webb’s most complex instrument modes is with the MIRI Medium Resolution Spectrometer (MRS). The MRS is an integral-field spectrograph, which provides spectral and spatial information simultaneously for the entire field of view. The spectrograph provides three-dimensional ‘data cubes’ in which every pixel in an image contains a unique spectrum. Such spectrographs are extremely powerful tools to study the composition and kinematics of astronomical objects, as they combine the benefits of both traditional imaging and spectroscopy.

“The MRS is designed to have a spectral resolving power (observed wavelength divided by the smallest detectable wavelength difference) of about 3,000. That is high enough to resolve key atomic and molecular features in a variety of environments. At the highest redshifts, the MRS will be able to study hydrogen emission from the first galaxies. At lower redshifts, it will probe molecular hydrocarbon features in dusty nearby galaxies and detect the bright spectral fingerprints of elements such as oxygen, argon, and neon that can tell us about the properties of ionized gas in the interstellar medium. Closer to home, the MRS will produce maps of spectral features due to water ice and simple organic molecules in giant planets in our own solar system and in planet-forming disks around other stars.

“In order to cover the wide 5 to 28 micron wavelength range as efficiently as possible, the MRS integral field units are broken up into twelve individual wavelength bands, each of which must be calibrated individually. Over the past few weeks, the MIRI team (a large international group of astronomers from the USA and Europe) has been focusing primarily on calibrating the imaging components of the MRS. They want to ensure that all twelve bands are spatially well aligned with each other and with the MIRI Imager, so that it can be used to place targets accurately into the smaller MRS field of view. We show some early test results from this alignment process, illustrating the image quality achieved in each of the twelve bands using observations of the bright K giant star HD 37122 (located in the southern sky near the Large Magellanic Cloud).

These are the first engineering data cubes for each of the twelve MRS spectral bands, illustrating the astrometric registration and image quality for observations of HD 37122. In each panel the dashed cyan circle shows a 1 arcsecond radius region around the expected location of the star in celestial coordinates. While the star is bright at short wavelengths it fades toward longer wavelengths, where the MRS also detects thermal emission from Webb’s primary mirror. Credit: NASA, ESA, and the MIRI Consortium.

“Once the spatial alignment and image quality of the several bands are well characterized, the MIRI team will prioritize calibrating the spectroscopic response of the instrument. This step will include determining the wavelength solution and spectral resolution throughout each of the twelve fields of view using observations of compact emission-line objects and diffuse planetary nebulae ejected by dying stars. We show the exceptional spectral resolving power of the MRS with a small segment of a spectrum obtained from recent engineering observations of the active galactic nucleus at the core of Seyfert galaxy NGC 6552. Once these basic instrument characteristics are established, it will be possible to calibrate MRS so that it is ready to support the wealth of Cycle 1 science programs due to start in a few short weeks.”

This portion of the MIRI MRS wavelength range shows engineering calibration data obtained of the Seyfert galaxy NGC 6552 (red line) in the constellation Draco. The strong emission feature is due to molecular hydrogen, with an additional weaker feature nearby. The blue line shows a lower spectral resolution Spitzer IRS spectrum of a similar galaxy for comparison. The Webb test observations were obtained to establish the wavelength calibration of the spectrograph. Credit: NASA, ESA, and the MIRI Consortium.

—David Law, AURA associate astronomer, STScI

—Alvaro Labiano Ortega, Telespazio UK  for ESA, CAB (Consejo Superior de Investigaciones Cientificas – Instituto Nacional de Técnica Aeroespacial)


—Jonathan Gardner, Webb deputy senior project scientist, NASA Goddard

—Alexandra Lockwood, project scientist for Webb science communications, STScI

—Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard

Scheduling Webb’s Science

In the lead-up to the release of Webb’s first full-color images and spectroscopic data on July 12, the Webb team is now in the last phase of commissioning the science instruments. The first two instrument modes, NIRCam imaging and NIRISS imaging, have been declared ready for science; watch the “Where is Webb” page as the team works their way through the other 15 instrument modes.

After commissioning is finished, the fun – and discoveries – will start: implementing the hundreds of peer-reviewed science programs that have been selected for Webb’s first year. The area on the sky that Webb can see at any given time is called the field of regard. Deciding which observations to make on which day is a complicated process designed to optimize observational efficiency and manage the observatory’s resources. We asked Christine Chen, science policies group lead at the Space Telescope Science Institute (STScI), to tell us how Webb’s schedule comes together.

“Webb will soon transition from commissioning to regular operations when Webb’s time will be devoted to scientific observations.

“Webb’s first year of observations (Cycle 1) has already been selected. There are three types of scientific programs planned: General Observer (GO), Guaranteed Time Observer (GTO), and Director’s Discretionary Early Release Science (DD-ERS). The GO and DD-ERS programs include scientists from all over the world whose programs were selected in a dual anonymous peer review process. The GTO programs are led by scientists who made key contributions to the development of the observatory.

“All of the observations in approved Cycle 1 programs are available for scheduling at the beginning of regular operations. However, the DD-ERS observations have been given priority during the first five months because the DD-ERS programs are designed to help the scientific community understand Webb’s performance for typical scientific observations as soon as possible.

“Webb’s Long Range Planning Group (LRPG) has created a 12-month+ Observing Plan, including all of the approved observations, with the goal of creating the most efficient plan. Even though a Webb Observing Cycle is defined as a 12-month period, more than one year’s worth of observations have been approved for Cycle 1. This over-subscription will enable a smooth transition between cycles as well as provide a repository of flight-ready observations that can be moved earlier, if a window opens up. At the current time, before the start of Cycle 1, the Observing Plan is not yet completely filled. This allows the schedulers to accommodate late-breaking Targets of Opportunity (ToOs) and Director’s Discretionary (DD) programs. ToOs and DDs typically include ’unplanned for‘ events such as interstellar comets, gravitational wave sources, and supernovae.

“During regular operations, the Short Term Scheduling Group (STSG) will create detailed weekly schedules to be executed by the observatory during the following week. These Short Term Schedules will take into account several factors, including observing constraints, data volume limits for the onboard data recorder, momentum buildup on the observatory’s reaction wheels, etc. At the beginning of each week, the Flight Operations Team will uplink the week’s Short Term Schedule to Webb. At the end of each week, the LRPG will update the Observing Plan to reflect the actual programs that were executed, and to identify priorities for the following week. In this way, the LRPG and STSG work synergistically together throughout the observing cycle to maximize the scientific return from the observatory.”

— Christine Chen, Webb science policies group lead, STScI,
and David Adler, Long Range Planning Group lead, STScI


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

 

Webb: Engineered to Endure Micrometeoroid Impacts

Micrometeoroid strikes are an unavoidable aspect of operating any spacecraft, which routinely sustain many impacts over the course of long and productive science missions in space. Between May 23 and 25, NASA’s James Webb Space Telescope sustained an impact to one of its primary mirror segments. After initial assessments, the team found the telescope is still performing at a level that exceeds all mission requirements despite a marginally detectable effect in the data. Thorough analysis and measurements are ongoing. Impacts will continue to occur throughout the entirety of Webb’s lifetime in space; such events were anticipated when building and testing the mirror on the ground. After a successful launch, deployment, and telescope alignment, Webb’s beginning-of-life performance is still well above expectations, and the observatory is fully capable of performing the science it was designed to achieve.

Webb’s mirror was engineered to withstand bombardment from the micrometeoroid environment at its orbit around Sun-Earth L2 of dust-sized particles flying at extreme velocities. While the telescope was being built, engineers used a mixture of simulations and actual test impacts on mirror samples to get a clearer idea of how to fortify the observatory for operation in orbit. This most recent impact was larger than was modeled, and beyond what the team could have tested on the ground.

“We always knew that Webb would have to weather the space environment, which includes harsh ultraviolet light and charged particles from the Sun, cosmic rays from exotic sources in the galaxy, and occasional strikes by micrometeoroids within our solar system,” said Paul Geithner, technical deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We designed and built Webb with performance margin – optical, thermal, electrical, mechanical – to ensure it can perform its ambitious science mission even after many years in space.” For example, due to careful work by the launch site teams, Webb’s optics were kept cleaner than required while on the ground; their pristine cleanliness improves the overall reflectivity and throughput, thereby improving total sensitivity. This and other performance margins make Webb’s science capabilities robust to potential degradations over time.

Furthermore, Webb’s capability to sense and adjust mirror positions enables partial correction for the result of impacts. By adjusting the position of the affected segment, engineers can cancel out a portion of the distortion. This minimizes the effect of any impact, although not all of the degradation can be cancelled out this way. Engineers have already performed a first such adjustment for the recently affected segment C3, and additional planned mirror adjustments will continue to fine tune this correction. These steps will be repeated when needed in response to future events as part of the monitoring and maintenance of the telescope throughout the mission.

To protect Webb in orbit, flight teams can use protective maneuvers that intentionally turn the optics away from known meteor showers before they are set to occur. This most recent hit was not a result of a meteor shower and is currently considered an unavoidable chance event. As a result of this impact, a specialized team of engineers has been formed to look at ways to mitigate the effects of further micrometeoroid hits of this scale. Over time, the team will collect invaluable data and work with micrometeoroid prediction experts at NASA’s Marshall Space Flight Center to be able to better predict how performance may change, bearing in mind that the telescope’s initial performance is better than expected. Webb’s tremendous size and sensitivity make it a highly sensitive detector of micrometeorites; over time Webb will help improve knowledge of the solar system dust particle environment at L2, for this and future missions.

“With Webb’s mirrors exposed to space, we expected that occasional micrometeoroid impacts would gracefully degrade telescope performance over time,” said Lee Feinberg, Webb optical telescope element manager at NASA Goddard. “Since launch, we have had four smaller measurable micrometeoroid strikes that were consistent with expectations and this one more recently that is larger than our degradation predictions assumed. We will use this flight data to update our analysis of performance over time and also develop operational approaches to assure we maximize the imaging performance of Webb to the best extent possible for many years to come.”

This recent impact caused no change to Webb’s operations schedule, as the team continues to check out the science instruments’ observing modes and prepares for the release of Webb’s first images and the start of science operations.


— Thaddeus Cesari, NASA Goddard.

The Modes of Webb’s NIRISS

The Webb team continues to commission the 17 science instrument modes. This week we asked Nathalie Ouellette of the Université de Montréal to give more detail about the modes of the Near-Infrared Imager and Slitless Spectrograph (NIRISS), Canada’s scientific instrument on Webb.

“NIRISS will be able to capture both images and spectra from different types of celestial objects in near-infrared light, at wavelengths up to 5.0 microns. The NIRISS team has developed four instrument modes to collect different kinds of data that are well-suited for different targets and scientific objectives.

Single Object Slitless Spectroscopy (SOSS)

With SOSS mode, the NIRISS instrument will be able to study the atmospheres of exoplanets as they pass in front of their star using a technique called transit spectroscopy. The spectrum observed by NIRISS will act like an alien barcode, indicating the presence of certain atoms and molecules. The above illustration shows how absorption features due to sodium (Na) and potassium (K) can be seen in the visible light spectrum; Webb’s infrared light observations will be sensitive to other features such as water vapor, carbon dioxide, and methane. (Credit: European Southern Observatory)

“The SOSS mode on NIRISS allows the Webb telescope to obtain high-precision spectra from one bright object at a time. This mode is optimized to carry out time-series observations, which are ideal for studying a phenomenon that changes over the length of a typically hours-long observation, such as an exoplanet transiting in front of its host star.

“Using a technique called transit spectroscopy, the NIRISS instrument can collect a spectrum of an exoplanet’s atmosphere, which contains different markers that allow astronomers to determine its composition, temperature, potential habitability signatures, and other important characteristics.

Wide Field Slitless Spectroscopy (WFSS)

“The WFSS mode on NIRISS allows Webb to obtain spectra but for thousands of objects, such as galaxies, at the same time over the detector’s entire field of view (4.84 arcmin2). The spectra of thousands of galaxies will enable measurement of their distances, ages, and other physical parameters to trace how galaxies evolve over the lifetime of the universe. In the simulated example shown in the figure, the galaxy cluster acts like a cosmic lens that magnifies and stretches the images of faint background galaxies, so they can be studied in even greater detail.

“Since NIRISS can collect so many spectra at a time using the WFSS mode, individual spectra can overlap if their sources are too close. There are thus two orthogonal grisms, GR150C and GR150R, that can produce spectra horizontally and vertically, respectively, which helps to disentangle blended spectra from different galaxies.

Aperture Masking Interferometry (AMI)

A prototype of a mask used in the Canadian NIRISS instrument in AMI mode, showing the layout of the seven hexagonal holes in the mask with respect to the Webb primary mirror segments and secondary mirror supports. (Credit: Anand Sivaramakrishnan/Space Telescope Science Institute)

“The AMI mode on NIRISS allows Webb to study objects that are very close together on the sky, using a special technique called interferometry. A mask inside the instrument allows light from only certain parts of the primary mirror to pass through. Astronomers can increase the resolution of the telescope by a factor of nearly 2.5 by looking at the patterns created as the carefully chosen beams of light interfere with each other. This allows two objects that are close to each other that would otherwise look like a single blurred point, like an exoplanet orbiting a star, to appear as two distinct points of light in a Webb image. The mask blocks out a large portion of the light, so the observed objects must be bright in order to detect them. The AMI mode will be used to observe exoplanets, brown dwarfs, and protoplanetary disks. This is the first time that such a mask is being used in space.

NIRISS Imaging

“Because of the importance of near-infrared imaging to Webb’s scientific success, NIRISS includes an imaging capability that functions as a backup to NIRCam imaging. This capability can be used in parallel, with NIRCam and NIRISS simultaneously taking images of two closely separated fields of view, imaging a larger area of an extended source.”

— Nathalie Ouellette, Webb outreach scientist, Université de Montréal


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