STScI Newsletter
2021 / Volume 38 / Issue 02

About this Article

J. MacKenty (mackenty[at], and J. Schlieder (NASA/GSFC)

Roman's Instruments

The Nancy Grace Roman Space Telescope carries two scientific instruments, the Wide Field Imager (WFI) and a coronagraph demonstration instrument.  

The WFI Instrument

The WFI instrument is designed to accomplish the surveys necessary to achieve the scientific goals for the Roman mission as defined in the Astro2010 decadal survey, and refined during the mission formulation phases. These include wide-field, multi-band, near-infrared imaging, and wide-field, slitless spectroscopic, and time-domain surveys. This instrument will address the scientific goals that motivate the Roman mission, namely precision cosmological measurements to elucidate the nature of dark energy via measurements of supernovae at cosmic distances, baryonic acoustic oscillations, weak lensing, large-scale structure, and the census of longer-period and free-floating exoplanets. Simultaneously, these surveys, combined with a robust General Investigator (GI) program, will enable a broad variety of scientific studies.

Chart showing Roman’s range for wavelength and coverage
Figure 1: WFI filter and spectroscopic element passbands. These also illustrate the overall performance of the Telescope+WFI system using a modeled "effective area" consistent with performance requirements on all elements.

The WFI consists of a large focal plane covering over 0.28 square degrees—200 times the field of view of Hubble's Wide-Field Camera 3 (WFC3) infrared camera with slightly better sampling (0.11 vs. 0.129 arc seconds/pixel) providing imaging from 0.5–2.3 microns and spectroscopy from 0.8–1.9 microns. This focal plane consists of eighteen 4K × 4K Hawaii-4RG detectors, which also provide the precision-pointing control for the spacecraft. A single element wheel provides eight imaging filters, two dispersive optics (a prism and grism), and a blank for dark calibrations. The filter and spectral passbands are shown in Figure 1. The two dispersive elements provide slitless spectroscopy at low resolution (R ~ 80–180) from 0.8–1.8 microns motivated by the need to classify supernovae found in time-domain searches and at higher resolution (R ~ 500–800). A grism element covering 1.0–1.9 microns is optimized for measuring galaxy redshifts at z ~ 1.

The WFI is expected to provide ~28 magnitude 5σ point-source sensitivity in one hour (26 in the F213 filter). For spectroscopy, one hour should yield 10σ continuum-sensitivity limits of 23 and 21 magnitude for point sources at 1.2 microns, for the prism and grism, respectively.

The focal plane, and especially its detectors, have been the focus of many years of development with final deliveries, testing, and selection of the flight devices recently completed. These detectors, which represent the next generation of devices from Teledyne Imaging Systems, feature 4096 × 4096 10‑micron pixels. They represent the next generation of detectors, which build upon the 1K × 1K device flown on Hubble's WFC3 and the 2K × 2K devices on Webb's Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSPEC), and Near-Infrared Imager and Slitless Spectrograph (NIRISS). These devices have excellent quantum efficiency, low noise, and fewer pathologies compared to prior devices, thus reaching the demanding requirements expected for Roman's precise calibration. Considerable effort has been invested in the characterization of these devices by scientists and engineers at NASA's Goddard Space Flight Center, the Institute, IPAC at Caltech, and the Roman Science Investigation teams over the past several years. This is essential to the science goals of precision cosmology, which require a detailed understanding of the behavior and calibration needs of these sensors. Figure 2 shows the focal plane Engineering Test Unit (ETU).

The WFI instrument nestled in its test bed
Figure 2: Engineering Test Unit for the WFI focal plane consisting of 18 detectors. This ETU is a path finder for the assembly, handling, and cryogenic testing of the flight unit, which is now being assembled. Note the small gaps between detectors and the overall shape of the focal plane, which maximizes the optical quality of the Roman Telescope's field of view. These factors require careful observing strategies, but are easily accommodated in the context of a survey mission such as Roman.

The WFI is more than just its detectors and optical elements. Goddard and Ball Aerospace are tasked with designing, constructing, and testing the WFI collaboratively. Other key components include custom electronic controllers and signal chains for the detectors (ACADIA devices; the next generation beyond the SIDECAR devices used in Hubble's Advanced Camera for Surveys after the 2009 repair mission, Webb, and ESA's Euclid infrared instrument. Also under construction are the structural elements and the necessary cooling systems to maintain the detectors, filters, and spectral elements at their designed cryogenic temperatures. The element wheel itself (a very large component due to the optical-element size needed to achieve Roman's large field of view) has been fabricated. During 2021, the internal calibration system for the WFI was redesigned for simplicity and programmatic needs. This system provides both flat fields for internal calibration and, most importantly, precise calibration of the count rate non-linearity in the detectors via two distinct methods. Lastly, teams are defining the operations, flight software, and ground- and flight-verification and calibration plans.

Key Milestones Passed

The Roman Space Telescope has also recently passed several other key milestones. The telescope and mission-critical design reviews were held successfully in late September following a long sequence of reviews of the various components of the mission over the prior nine months. These included the ground system critical design review in July covering the relevant work at Goddard (Mission Operations Center), STScI (Science Operations Center), and IPAC (Science Support Center), and a detailed review of the calibration plans for WFI in late July. A "Request for Information" was released seeking community input on the possible early definition of a GI science program—which, if carried out, would be done with community involvement.

Formulation Science Working Group and Science Investigation Teams

The Formulation Science Working Group and associated Science Investigation Teams (SITs) have now disbanded in advance of the upcoming selection of new science teams via the NASA ROSES opportunity expected in early 2022. These teams, after six years of dedicated effort, have provided workshops during October (Roman CGI Workshop)  and November (Science with Wide-Field Instrument), documenting their work and providing tools and simulations in support of future studies.

Read more information about the WFI and the Roman Mission at NASA, STScI, and CalTech.



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