This page has the current version of Roman WFI and Observatory Performance. This is v1, June 2024.
Roman Performance Information
Filters
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
This update is based on the post-CDR optical design. The wavefront error model includes design residuals and Monte Carlo estimates of surface figure errors and alignment tolerances. We plan to issue a new release in 2025 once we have wavefront measurements of the integrated telescope and Wide-Field Instrument.
Point spread functions (PSFs) for the Nancy Grace Roman Space Telescope have been created using WebbPSF version 1.0, 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.
Imaging PSFs were calculated at the center of each SCA and also around the periphery of the focal plane; located at the center of a pixel and at the corner of a pixel.
*Note: PSF FWHM in arcseconds simulated for 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 detector (SCA 1) near the center of the WFI FOV. Wavelength distribution is that of a K0V star, sampled over each filter bandpass. N Eff Pix is number of effective pixels (aka noise pixels) under the PSF (1/sum of squares of pixel values). Fluxes normalized at telescope exit pupil. Images are ~7 arcsec square and typically contain 97%-99% of the incident flux. Two cases are provided: at the corner of 4 pixels, and at the center of a pixel.
FWHM (arcsec) of PSF is computed from 8-times oversampled PSF. Gaussian pointing jitter with FWHM = 8 mas is included. The number of noise pixels and maximum flux per pixel are computed on native detector pixels.
The Roman Effective area has been updated to reflect recalibration of the sensor ship assembly (SCA) quantum efficiency, and preliminary updates to the filter bandpasses. The tables have also been broken out by SCA to illustrate the small differences in QE and the shift in filter bandpasses with field angle. Download the Filter Effective Area tables [.ZIP]
The tables are in ECSV format, which is can be read via astropy.io.ascii.read() and any text editor.
The plot below illustrates both these effects by comparing the effective area for SCAs 1 and 9 for filter F158.
The bandpass shifts given here are representative models; these will be updated once the full set of data from the wide-field instrument thermal vacuum test has been obtained and analyzed.
The following figure shows the effective area for the full set of Roman filters and dispersers, for SCA #1.
Imaging Sensitivity
(Updated June 3, 2024)
The table below gives the 5-sigma AB magnitude limiting sensitivity, for twice the minimum zodiacal light background (roughly equivalent to that obtained at an ecliptic latitude of 25 degrees at a Solar elongation of 90 degrees), for 57 second and one-hour integrations, for point sources and a compact galaxy with half-light radius of 0.3 arcseconds.
S/N was computed for photometric apertures of radius 2 pixels for point sources and 6 pixels for the galaxies.
Filter
F062
F087
F106
F129
F158
F184
F213
F146
Wavelength (microns)
0.48-0.76
0.76-0.98
0.93-1.19
1.13-1.45
1.38-1.77
1.68-2.00
1.95-2.30
0.93-2.00
1 hr, Point
27.97
27.63
27.60
27.60
27.52
26.95
25.64
28.01
1 hr, r50=0.3”
26.70
26.38
26.37
26.37
26.37
25.95
24.71
26.84
57s, Point
24.77
24.46
24.46
24.43
24.36
23.72
23.14
25.37
57s, r50=0.3”
23.53
23.23
23.26
23.24
23.24
22.76
22.23
24.22
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 fν,limfor fixed SNR and integration time should be scaled from the 0.3” case approximately as fν,lim∝r50 or ΔABlim=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.
Zodiacal Light and Thermal Background
(Updated June 3, 2024)
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.
Count rate per pixel at minimum Zodiacal Light
F062
F087
F106
F129
F158
F184
F213
F146
0.25
0.251
0.277
0.267
0.244
0.141
0.118
0.781
Internal thermal backgrounds (count rate per pixel)
F062
F087
F106
F129
F158
F184
F213
F146
0.003
0.003
0.003
0.003
0.048
0.155
4.38
1.03
Imaging Sensitivity Calculator
(Note: data files have *not* yet been updated)
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.
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
Element name
Min (μm)
Max (μm)
Center (μm)
Width (μm)
R
G150
1.0/td>
1.93/td>
1.465/td>
0.930/td>
461λ(2pix)
P127
0.75
1.80
1.275
1.05
80-180 (2pix)
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.
The effective area as a function of wavelength for the Grism and Prism.
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.
Count rate per pixel at minimum zodiacal light
Grism
Prism
0.65
0.95
Grism Spectroscopy Sensitivity
(updated June 11, 2024)
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. Table gives 5-sigma detection limits for a one-hour exposure time with zodiacal light background at twice the minimum intensity. This is representative of an ecliptic latitude of 25 degrees and 90 degrees ecliptic longitude relative to the Sun (the middle of the object visibility window). Typical HLWAS backgrounds are ~30% lower.
For emission lines, the values are integrated line fluxes in units of 1e-17 ergs/cm^2/sec.
For continua, the values are the AB magnitude at which S/N=5 per pixel.
The values are averages over all 18 detectors; typical variations from one detector to another are 0.05 to 0.1 mag for the continuum cases and ~10% for the emission line limits.
5σ limits for Roman WFI grism in 1 hour on source, 2x minimum zodiacal background.
Emission line limits are in units of 10-17 erg cm-2 s-1, and continuum limits are in AB mags for 1 pixel
Wavelength (microns)
1.05
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
fline,17, r50=0; 1 hour
5.9
4.7
3.6
3.2
3.0
3.1
3.3
3.7
3.4
4.7
mAB, r50=0; 1 hour
21.3
21.5
21.6
21.6
21.5
21.3
21.2
21.0
20.8
20.4
fline,17, r50=0.3"; 1 hour
16.1
12.9
9.8
8.7
7.6
8.1
8.4
8.9
8.5
11.7
mAB(2 pix), r50=0.3",1 hour
20.5
20.6
20.8
20.7
20.6
20.5
20.3
20.2
20.0
19.6
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 b1/2, 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 flne ∝r50, and fcont ∝r501/2.
Prism Spectroscopy Sensitivity
(updated June 11, 2024)
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 Table below has AB at which S/N=5 per pixel (not per 2 pixels as earlier), at zodiacal light at twice minimum. This is representative of an ecliptic latitude of 25 degrees and 90 degrees ecliptic latitude relative to the Sun (middle of the object visibility window). Typical HLWAS backgrounds are ~30% lower.
These are averages over all 18 detectors; typical variations from one detector to another are 0.05 to 0.1 mag.
5σ limits for Roman WFI prism, 2x minimum zodiacal background
Wavelength
0.80
1.00
1.20
1.40
1.60
1.75
Δλ (for 1 pixel, in nm)
2.2
4.4
5.6
8.2
9.3
9.1
mAB(1 pix), r50=0; 1 hour
22.6
23.2
23.4
23.4
23.3
23.3
mAB(1 pix), r50=0.3"; 1 hour
22.0
22.6
22.8
22.8
22.8
22.7
mAB(1 pix), r50=0; 62 sec
19.9
20.4
20.6
20.7
20.6
20.5
mAB(1 pix), r50=0.3"; 62 sec
19.3
19.9
20.1
20.1
20.1
20.0
The same scalings that apply to grism spectroscopy can be used to scale other deep prism sensitivities from the 1-hour case.
Grism and Prism dispersion
The grism has constant dispersion and linearly increasing resolving power. The prism provides higher throughput and lower dispersion than the grism. The prism dispersion varies with wavelength and varies slightly with field angle.
Observations (Field of Regard, Slew/Settle Times, etc.)
Field of Regard and Optical Field Layout
The Roman field of regard and optical field layout are illustrated below. These figures provide information about the observing zones, and the angles between the detectors and the instruments.
Slew times
(Dec 2023)
Slew duration and the slew profile are determined by the slew length, the maximum allowed rate, and the maximum allowed acceleration. The maximum allowed rate is fixed (by observatory safing constraints). The maximum allowed acceleration is determined by the torque authority of the reaction wheels and the moment of inertia of the observatory. The torque is slightly different in the direction of the long axis of the WFI FoV than the short axis. For the purpose of estimating survey efficiency, slew times in the diagonal direction is a reasonable approximation.
Slews are acceleration limited if the rate does not reach the maximum value, and rate-limited for longer slews (>~1deg) where the maximum rate is achieved. Since the maximum allowed rate is fixed, improvements in torque authority primarily improve the performance of short slews. Roman's core community surveys are dominated by short slews.
Sharp changes in acceleration may excite structural or slosh oscillations that could adversely affect settling time. To avoid this, a shaped profile is computed onboard to avoid sharp acceleration discontinuities. More details can be found in Stoneking et al 2017.
The times for some example telescope slews are presented in the table below. These include settling time. These assume 1.566 Nm max torque, and MOI of 40,000 kg-m^2
Example visibility plots for two different target declinations are provided below. The Y-axis is the R.A, and the X-axis is the day of the year. The regions in green represent where/when the line of sight is in the Roman field-of-regard. A full set of visibility plots with declination increasing at intervals of 5 degrees is available here.
Image Caption: Visibility plots at Dec. = -30 deg (left) and -65 deg (right), with regions in green representing where/when the line of sight is in the field-of-regard.
The plot on the left corresponds roughly to the Galactic center if one were to draw a horizontal line at R.A = 266 deg. The spring and fall visibility windows are where that line intersects the green regions.
In the plot on the right, the ecliptic pole is at RA = 90 deg. This means that the bottom half of the plot is in the continuous viewing zone, which is why it is solid green.
The nominal roll angle for an arbitrary R.A and Dec., as a function of time throughout the year has been computed and can be found here.
This file contains two sets of columns for +/- values of Dec. = 1, 60, and 89 deg. The target R.A was fixed at +90 deg. By focusing on the month and day columns of the file, and the two pitch OK columns, a user may determine which days have the given R.A and Dec. in the field-of-regard. The roll columns give the S/C roll.
For example, if a user were to look at the low declination examples in the file (Dec. = 1 and -1 deg), they would observe that the roll angle doesn’t change much during the good days, because these lines of sight are still not too far from the ecliptic plane. However, if one were to look at the Dec. =-60 deg columns, the roll angle covers all of 0-360 deg. This is due to the fact that with target R.A = 90 deg, the target is in the continuous viewing zone.
