Moving Target Scattered Light Considerations

Scattered light from bright, extended sources, such as the planets Mars, Jupiter, and Saturn, can greatly affect observations of fainter nearby satellites. In this article we discuss the primary issues and strategies for mitigating scattered light contamination.

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Scattered light and general mitigation

Light that reaches the detectors via paths other than the direct re-imaging of external sources on the detectors is considered "scattered" or "stray" light. Scattered light excludes the diffracted light of the JWST point spread function (PSF). Sources of scattered light include glints off optical mounts (including the secondary mirror struts) and diffuse scattering caused by particulate contamination on the optics. While scattered light has been observed, the good news for JWST is that it is much weaker than the PSF itself. As such, scattered light is primarily observed between the diffraction spikes of the PSF. The brightest, most extended objects observable by JWST are the planets Mars, Jupiter, and Saturn, and the satellites of Jupiter and Saturn, in particular, are key targets for study within bright scattered and diffracted light backgrounds (e.g., Figures 1 & 2). Additional information on scattered light is provided for the MIRI, NIRCam, NIRISS, and NIRSpec instruments.

Figure 1. Examples of Jupiter scattered light in the NIR

NIRCam F140M (top) and F356W (bottom) images showing Jupiter (saturated) and the positions of a few of the interior satellites and Europa; a diffraction spike from Io is visible in module B. These images qualitatively show the strength of Jupiter's diffraction spikes, the bright scattered light between the spikes, and the wavelength-dependence of the scattered light.
Figure 2. Scattered light contours

NIRCam F140M (top) and F356W (bottom) images showing Jupiter (saturated) and contours of scattered light. Contours mark multiples of typical background flux from 2× to 1024× in the F140M image and 0.5× to 512× in the F356W image; the contour representing 8× the background level is marked in each image. The contours representing very high flux are compressed close to the limb of Jupiter. These images quantitatively show the strength of Jupiter's diffraction spikes, the bright scattered light between the spikes, and the wavelength-dependence of the scattered light.
Figures 1 & 2 demonstrate the 2 major pitfalls to observing giant planet satellites in close proximity to their planet: contamination from diffraction spikes and scattered light. Some general strategies to mitigate this contamination include:

  • If a particular rotational longitude is not required, plan your observations near greatest elongation. This will reduce flux from both the diffraction spike and scattered light.

  • Specify a roll angle to move the satellites out of a diffraction spike. The range of allowable roll angles is likely to be small, but this could help move coplanar satellites out of a strong diffraction spike.

  • Specify a dedicated background observation in one of 2 ways:
    • At the same radial distance as the target but at a different position angle. Make sure the background is either on or off a diffraction spike to match the location of the target.
    • For targets with a special requirement to observe them near greatest elongation, a background observation can be set up using the TORUS Level 2 target specification. This allows a fixed position to be specified with respect to the center of the giant planet and can be used to define a background position 180° in orbital longitude from the target. The properties of the scattered light and any diffraction spikes should be comparable between the two positions.

FGS impingement and keep-out radius

Observations of giant planet satellites in, e.g., the NIRSpec IFU or NIRCam imager could result in the giant planet itself falling on either the FGS1 or FGS2 detector. Such impingements can increase the background flux enough to affect guide star acquisition. Tests were performed during commissioning (PID 1022) to incrementally move Jupiter away from NIRCam and towards the gap between the 2 FGS FOVs to evaluate the maximum distance where guide star acquisition failed. These tests confirmed a previously determined "keep-out radius" to ensure effective guide star acquisition of 110, 141, and 120 arcseconds from each FGS center for Mars, Jupiter, and Saturn, respectively. Every approved observation (fixed or moving targets) is tested for the proximity of these 3 planets to the FGS FOVs.

FGS impingement is not run as part of the APT visit planner in order to avoid excessive run time, but is instead determined as part of the scheduling process at STScI. The vast majority of observations, including moving target observations, will be unaffected. Only observations of targets near Mars, Jupiter, and Saturn are likely to have their visibility windows affected by keep-out radius enforcement. Situations where neither FGS detector can be used for guiding are exceedingly rare.

NIRSpec impingement and mitigation

During commissioning, Jupiter was placed at various positions around and within each science instrument aperture. All apertures obviously suffered from scattered light when Jupiter was in close proximity, but only the NIRSpec IFU was subject to additional contamination when Jupiter was not near the aperture. Specifically, all NIRSpec modes share the same detectors and light leakage through the micro-shutter arrays (MSAs) can contaminate the IFU slices. Attenuation is ~10,000× but for an object as bright as Jupiter this can result in significant leakage, even through closed shutters (see Figure 3), and this can affect IFU observations of faint satellites.

Figure 3. Jupiter flux contamination in the NIRSpec MSAs

Examples of MSA contamination in NIRSpec IFU slices from commissioning (PID 1022). For the test, Jupiter was placed in the center of each NIRSpec MSA footprint; in the image above, each MSA is accurately positioned relative to the others and labeled. The position of the Jupiter spectra on the IFU slices depends on its position in the MSA footprint, making it difficult to easily remove this contamination.
The position of Jupiter within the MSA footprint determines the position of the Jupiter spectra within the IFU slices on the detectors, making post-observation corrections extremely challenging. Therefore, pre-observation mitigation strategies should be applied. NIRSpec IFU observations of giant planet satellites should be carefully planned to ensure that the planet does not fall on or near any of the NIRSpec MSAs. This is most easily done for observations that have no timing constraints (i.e., any longitude and any date are acceptable):

  • Determine the visibility windows of the target using JPL Horizons. Targets are observable when they fall between elongation angles of 85° and 135° (the JWST field of regard).

  • Quantity 23 ("Sun-Observer-Target ELONG angle" under "Table Settings") provides a leading/trailing flag, corresponding to the leading field of regard (FOR) and trailing FOR. Observations of solar system objects near the ecliptic will have one of 2 focal plane orientations (see Figure 4).

  • The micrometeoroid avoidance zone (MAZ) is defined to cover the entirety of the leading FOR for targets within 45° of the ecliptic, which means the majority of Solar System observations will be executed in the trailing FOR, unless an observation is time-critical and must be executed in the leading FOR. Note that the remainder of these recommendations and Figure 4 are generalized for leading and trailing FOR observations, but in the majority of cases, the trailing FOR should be assumed.

  • Ensure the satellite has a rotation phase, central meridian longitude (CML), or orbital longitude special requirement to constrain the observation to occur when the satellite is near greatest elongation. A specific greatest elongation (or hemisphere) should be chosen to match the leading/trailing field of regard during one of the previously determined observability windows. A matching Between Dates timing special requirement should be applied for the chosen date range to ensure the eventual execution dates of the observation are appropriate.

Despite this strategy, observations of satellites with small semi-major axes may still suffer from giant planet contamination in the MSAs, due to the small angular separations involved. Observations of the planets themselves, especially Jupiter, are likely to suffer from MSA contamination as well, but this is less of a concern because of the significant attenuation through the MSA shutters; for instance, comparing the flux from Jupiter to 1/10,000th of Jupiter is less of an issue than comparing the flux of 1/10,000th of Jupiter to the flux of Amalthea.

Figure 4. Leading vs. trailing FOR

Only 2 orientations of the JWST focal plane are possible for solar system objects near the ecliptic, and these are dependent on the position of the target within the JWST field of regard (FOR). The 2 situations, Leading FOR (JWST approaching the target) and Trailing FOR (JWST receding from the target), are shown above for the "leading" and "trailing" fields of regard. The inset with Saturn (not to-scale) shows that giant planet satellite observations in the trailing FOR should be made of the satellite's leading hemisphere (if rotating synchronously) or when the satellite is near greatest eastern elongation to keep the giant planet off the NIRSpec MSAs. Conversely, observations in the leading FOR should be made of the satellite's trailing hemisphere or when the satellite is near greatest western elongation. Keep in mind that the micrometeoroid avoidance zone (MAZ) will dictate that the majority of observations occur in the trailing FOR.

Extended PSFs with added scattered light

To determine the approximate scattered light contamination for a satellite target of interest, one can use the following resources:

  • The Simulated JWST Observations page contains extended PSF models (4' × 4') with added scattered light created using WebbPSF and spaced logarithmically in wavelength. A readme file is provided with additional information on the creation of these PSF models. Note that the PSFs and scattered light information are pre-commissioning.

  • Giant planet spectral models: Solar System Object Spectra. These spectra can be used to determine the flux levels at wavelengths of interest. They are the same spectra available in the Exposure Time Calculator (ETC). Note that Mars is not included in this spectral library.

Convolving the extended PSF model with a uniform disk of the same size as a giant planet, normalizing the scene, and multiplying by the flux at the desired wavelength provides a reasonable approximation of the flux distribution in the scene.

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