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
Click on the figures for a larger view.
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
Click on the figures for a larger view.
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 labeled 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.
- 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.
How do I mitigate giant planet impingement on the NIRSpec MSAs?
Users can check the position of the giant planet with respect to the NIRSpec MSAs using the Jupyter notebook provided below. The user will need to provide the name of the planet, the name of the satellite, the date range for the upcoming cycle and a time step (default of 1 day) for the purposes of determining when the target is in the leading or trailing FOR, and a specific date to plot the scene. This notebook outputs the date ranges when the planet is in the leading and trailing FOR and shows the position of the giant planet (at the correct angular diameter) with respect to the satellite when it is centered in the NIRSpec IFU. The correctly oriented NIRSpec FOV for the given date is also shown.
Download the NIRSpec IFU Satellite Checker Jupyter Notebook.
The position of a giant planet within the MSA footprint determines the position of the giant planet 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. Specifically, giant planet satellite observations in the NIRSpec IFU must have both a BETWEEN special requirement and a Central Meridian Longitude Observing Window or Orbital Longitude Observing Window.
Most satellites where giant planet impingement is a potential issue will synchronously rotate around their host planet, meaning observations at greatest elongation will correspond to either the leading (~90°) or trailing hemisphere (~270°). Many science cases require observations of either the leading or trailing hemisphere. This constraint is applied via the Central Meridian Longitude Observing Window or Orbital Longitude Observing Window. This ensures that the satellite will be observed near greatest elongation, making giant planet impingement easier to avoid. For programs without a strong driver for leading or trailing hemisphere observations, users are encouraged to select the leading hemisphere constraints in order to place observations in the trailing field of regard (FOR), which does not share any overlap with the micrometeoroid avoidance zone (MAZ).
To avoid giant planet satellite impingement on the NIRSpec MSAs, NIRSpec IFU observations of a satellite's leading hemisphere should occur in the trailing FOR and observations of a satellite's trailing hemisphere should occur in the leading FOR (Figure 4).
The Jupyter notebook provided above determines the date ranges when a specified satellite is within the leading and trailing FOR. The date range for the appropriate FOR should be added to APT as a BETWEEN special requirement. Running the APT Visit Planner will generate viable windows when both the BETWEEN and Central Meridian Longitude Observing Window or Orbital Longitude Observing Window are simultaneously satisfied. Choosing a specific date and time in the Jupyter notebook will show the viewing geometry of the giant planet, satellite, and MSA footprints, allowing confirmation that the proper combination of constraints was specified.
If using JPL Horizons for planning observations, note that quantity 23 ("Sun-Observer-Target ELONG angle" under "Table Settings") provides a "leading/trailing flag" that is the opposite of the actual FOR. Thus, when Horizons reports the "T" flag, this corresponds to the leading FOR, and the "L" flag corresponds to the trailing FOR.
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, e.g., 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, but observations that require execution in the leading FOR will likely still be accommodated.