The Wide-Field Instrument (WFI) is the primary instrument of the Roman Space Telescope. WFI provides wide-field imaging and multi-object, slitless spectroscopy over an. 0.281 deg² field-of-view. The design of WFI incorporates an 11-position element wheel assembly, 18 optical/NIR sensitive detectors mounted to a mosaic plate assembly and linked to associated control electronics, and a relative calibration system. The WFI detectors also acquire guide star images interleaved with science data collection to enable fine pointing control. The key parameters of the WFI are summarized below.

WFI carries 8 science filters with overlapping band passes spanning 0.48 – 2.3 microns. The filters are located at the telescope exit pupil. They are housed in the element wheel and are rotated into the instrument light path for multiwavelength imaging.

Filter Point Spread Functions

Point spread functions (PSFs) for the Nancy Grace Roman Space Telescope have been created using WebbPSF, a python based package. This tool takes into account properties of the telescope and the instruments, including detector pixel scale, rotations, filter profiles, and point source spectra. These are not full optical models, simply a tool that transforms the optical path difference maps, into the resulting Roman PSFs.

The website linked above provides instructions on how to install WebbPSF, how to run it via the Python API, in addition to providing Roman specific examples.

A small subset of simulated Roman PSFs for a K0V star can be found here (wim_psf_subset.zip - 28MB). These PSFs were calculated at the centers of each SCA (1 through 18), and at the boundaries of the FOV. An 8 mas pointing jitter in each axis was included in the PRF formulation, and each PRF was oversampled by a factor of eight.

More extensive libraries, with PSFs computed at the center of each SCA and at various locations around the boundary of the FOV are also available; these have been computed with stellar spectral energy distributions for spectral types:

• F0V (wim_psf_f0v_fullset.zip - 965 MB)
• G0V (wim_psf_g0v_fullset.zip - 965 MB)
• K0V (wim_psf_k0v_fullset.zip - 965 MB)

Filter parameters

* Note: PSF FWHM in arcseconds simulated at the center of the center pixel of a detector near the center of the WFI FOV using an input spectrum for a K0V type star. Please click on the FWHM value for each filter to view the simulated PSF.

The above table provides representative PSF FWHM values for a K0V type star near the center of the WFI FOV. Simulated PSFs for different stellar types and all detectors across the focal plane are available here.

Filter Effective Area Curves

The effective area as a function of wavelength for each Roman filter is available in tabular form here (.XLS).

Imaging Sensitivity Overview

The table below gives AB magnitude limits achieved at 5σ in 1 hour or 55 seconds of integration, for twice the minimum zodiacal light background, both for point sources and for compact galaxies with half-light radius of 0.3”. In the one-hour cases, the SNR calculations assume co-addition of four 900s exposures, while the 55s case assumes a single exposure. Profile fitting photometry is assumed.

Scalings for other deep cases: The flux density limit for other deep integrations, between about ten minutes and tens of hours, can be estimated from the 1 hour depths using a flux ∝ t^-1/2 scaling. For integrations below about 10 minutes, this scaling becomes over-optimistic by > 30% due to read noise and overheads. Limits for a half-light radius of 0.2” are slightly closer to the 0.3” case than the point source case. For more extended sources, the limiting flux density f_ν,limfor fixed SNR and integration time should be scaled from the 0.3” case approximately as f_ν,lim∝r₅₀ or ΔAB_lim=2.5 log⁡(0.3"/r₅₀). Reducing the assumed zodiacal background from 2x to 1.44x minimum improves deep imaging flux limits by about 0.15 mag for the F062 through F158 filters, 0.08 mag for F184 and F146, and negligibly for F213.

Fast/Wide Limit: The final two lines in the table give magnitude limits achieved at 5σ in 55 seconds (with a single exposure). At this integration time, Roman can cover approximately 8 contiguous square degrees per hour in one spectral element, and slew-and-settle overheads slightly exceed integration time. There is little point considering faster survey speeds, because sensitivity drops rapidly for modest increases in survey speed at yet shorter integrations.

More information on the filter sensitivities is available in tabular format on the Anticipated Performance Tables page.

Zodiacal Light and Thermal Background

The tables below provides the count rate per pixel at minimum Zodiacal light in each filter and the estimated thermal background. For observations at high galactic latitudes, the Zodi intensity is typically ~1.5x the minimum. For observation into the galactic bulge the Zodi intensity is typically 2.5-7x the minimum.

Imaging Sensitivity Calculator

On this page you will find tools designed to help you, the user, calculate the exposure time required for a given source at a given signal to noise, or vice versa.

We have provided a Jupyter notebook which walks through each step of the calculation, and a python script that will open a gui in which you can input your objects information.

To run both scripts you will need to download the accompanying data files.

Both the notebook and the gui require the user to specify the filter, zodiacal light contribution, type of source, fitting method, and signal to noise. In this notebook we will be doing exposure time calculations for point sources, and extended sources with half-light radii of 0.2 arcsec or 0.3 arcsec.

• Filters: F062, F087, F106, F129, F158, F184, F146, F213
• Zodiacal light contributions (multiples of the minimum): 1.2, 1.4, 2.0, 3.5
• Source: point sources, objects with a half-light radius (HLR) = 0.2", objects with a HLR = 0.3"
  Fit with a PSF (Point source only)
  Fit with a 2 pixel circular aperture (Point source & HLR = 0.2")
  Fit with a 3 pixel circular aperture
  Fit with a 4 pixel circular aperture
  Fit with a 5 pixel circular aperture (HLR = 0.3" only)
  Fit with a 6 pixel circular aperture (HLR = 0.2" & 0.3" only)
• S/N: 5, 10, 15, 20, 50

The exposure times used within these calculations are quantized in multiples of 3 readout frames, with the number of visits/dithers being 1.

An example of using the Jupyter notebook is provided here.

WFI carries 2 dispersive elements for slitless, multi-object spectroscopy. The grism band pass spans 1.0 – 1.93 microns and has a resolution of ~600. The prism band pass spans 0.75 – 1.80 microns and has a resolution of ~100. The dispersing elements are housed in the element wheel and are rotated into the instrument light path for slitless spectroscopy across the WFI FOV.

Grism and Prism parameters

Grism and Prism Effective Area Curves

The effective area as a function of wavelength for the Grism and Prism are available in tabular form here (.XLS).

Grism and Prism zodiacal light

The table below provides the count rate per pixel at minimum zodiacal light for the grism and prism. For observations at high galactic latitudes, the Zodi intensity is typically ~1.5x the minimum. For observation into the galactic bulge the Zodi intensity is typically 2.5-7x the minimum.

Grism Spectroscopy Sensitivity

The Roman WFI slitless grism has a spectral range of 1.00-1.93 microns and a dispersion of about 1.1 nm/pixel, essentially independent of wavelength, yielding a 2-pixel resolving power of R = λ / δλ = 460 λ / μm for a point source. The table below gives representative 1-hour sensitivities for both line and continuum. The continuum limit assumes coaddition of 2 pixels (2.2 nm) in the dispersion direction.

Sensitivities for other integration times (between a few minutes and tens of hours) and zodiacal backgrounds can be scaled from the above using f _lim∝t^-1/2 b^1/2, where t is integration time and b the zodiacal background level. Because this is slitless spectroscopy, the scaling of sensitivity with size for r∝r₅₀ > 0.3" differs between line and continuum sensitivity, with limiting behaviors of f_lne ∝r₅₀, and f_cont ∝r₅₀^1/2.

Prism Spectroscopy Sensitivity

The Roman WFI slitless prism has a spectral range of 0.75-1.80 microns and a resolution that is strongly wavelength dependent, with 80 < λ / δλ < 180. The highest resolution is at the blue end of the prism wavelength coverage. In addition to its lower dispersion, the prism has higher throughput than the grism, making it more sensitive to continuum.

The same scalings that apply to grism spectroscopy can be used to scale other deep prism sensitivities from the 1 hour case.

More information on the grism and prism sensitivity is available in tabular format on the Anticipated Performance Tables page

The WFI detectors are Teledyne H4RG-10 models with 10 micron pixel pitch. Each of the 18 detectors has 4096 x 4096 pixels and 32 readout channels. The detectors have very low dark current at their 95 K operating temperature and low noise. Detailed information on the design and properties of the Roman detectors is available in Mosby et al. 2020. The key parameters are summarized below.

Detector measured quantities

 

Quantity	Specification Value	Typical actual	Units
Dark CurrentDark current is the leakage current associated with the detector material when they are not being exposed to external radiation.	<0.1	<0.005	e-/sec/pixel
CDS Read NoiseCDS noise is the RMS noise from the difference of successive detector readouts.	<20	15-16	e- RMS
Total Noise in 180sTotal noise is the uncertainty on the slope fit for a sample-up-the-ramp sequence for an exposure time of 180 sec.		5-6	e- RMS
Quantum EfficiencyQuantum efficiency quantifies the fraction of incident photons that get detected as signal. Quantum efficiency varies as function of wavelength due to the specific detector material, coatings, and pixel design.	>80%	~95%	(avg 0.8-2.1μm)
Quantum EfficiencyQuantum efficiency quantifies the fraction of incident photons that get detected as signal. Quantum efficiency varies as function of wavelength due to the specific detector material, coatings, and pixel design.	>60%	75-80%	(avg 0.6-0.8μm)
Pixel operabilityPixel operability refers to the number of detector pixels meeting all specifications.	>95%	>99%	N/A
PersistencePersistence is the observed release of photogenerated charge from a previous readout in a subsequent readout. It appears as a memory effect in the detector.		~0.1	e-/s/pixel