The optimal number of groups and integrations for time-series observations are defined by instrument-specific strategies & signal limits — the so-called saturation limits.
When preparing time-series observations (TSOs), the concept of saturation is one of the central ones when trying to optimize the number of groups and integrations. This, in turn, helps define not only which targets are actually feasible scientific cases with the instruments onboard JWST, but also strategies to optimize the efficiency of the observations (i.e., the photon-collecting time).
In this article, we give an overview of the concept of saturation in the context of JWST for TSOs: what it means, how important it is for the different instruments and strategies to cope with its impact for scientific observations.
The information presented here is based on pre-flight predictions of observatory, telescope and/or instrument behavior; or based on experience with other missions (e.g. Hubble, Spitzer). It is subject to significant uncertainty until in-flight data is available to study the various sources of systematic noise and their impact on the photometric precision achievable with JWST instrumentation.
What is Saturation?
Saturation in the JWST context
The strict physical definition of saturation for a pixel in a detector is related to its full well capacity: the signal level at which any additional photon reaching a given pixel can no longer be counted. For JWST, proposers will get information on saturation from the Exposure Time Calculator - the tool they are expected to use to optimize the groups and integrations of observations.
Interestingly, the Exposure Time Calculator - unless stated otherwise - does not use the above definition of saturation. Although the full well capacity of the pixels has been directly measured for all the detectors onboard JWST, there is at this (pre-launch) stage still some uncertainty on the overall throughput of the observatory and therefore on the flux arriving at the instrument detectors for a given astrophysical source. On top of this, the Exposure Time Calculator considers a single representative limit flux value for all the pixels, and therefore, the defined maximum signal level has to be chosen wisely. Because of this, the saturation in the JWST Exposure Time Calculator is really a signal limit set to ensure that data can be adequately calibrated and thus represents our best pre-launch knowledge of the observatory limits. Depending on the instrument, this is around ~70% to ~80% of the detectors' full well capacity for TSOs.
For the Cycle 1 Call for Proposals, it is expected that users use this signal limit as a guide and follow the instrument-specific guidelines to interpret those limits. These will be much better characterized during the JWST commissioning phase.
Comparison to other saturation definitions
Definitions of saturation or signal limits in other space-based instruments like, e.g., the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) or Spitzer's Infrared Array Camera (IRAC), have defined observational strategies for TSOs in the past. The detectors used by WFC3 and IRAC are similar to the ones used by JWST's near-infrared instruments and MIRI, respectively; but the detectors on board JWST have benefited from more recent technology development. Users should therefore be cautious when translating their HST or Spitzer experience to JWST. Here we compare how the concept of saturation differs between these generations of instruments.
For HST/WFC3, a commonly adopted definition of saturation involved measuring each pixel's deviation from linear behavior: when the non-linearity correction for a pixel is above 5%, the pixel is considered "saturated" (Hilbert 2008; signals above this limit are simply not corrected for non-linearities in HST/WFC3 data per contractual specifications). Initial considerations for planning observations with HST/WFC3 for TSOs (typically for transit spectroscopy) suggested some observations could benefit from targeting counts much lower than this saturation level to avoid the non-linear regimes of the detector (McCullough & MacKenty, 2012). However, recent methods for dealing with lightcurve systematics (which come mainly from persistence/charge trapping; see e.g., Stevenson & Eck 2019 and references therein) have mostly lifted this constraint for HST/WFC3. Observations targeting around ~70% of the "saturation" level were still able to deliver (differential) high-precision spectrophotometry during TSOs at the highest fluences (Stevenson & Fowler, 2019). This suggests that near photon-limited precision is achievable even in possible non-linear regimes of the detector for that instrument configuration. Spitzer/IRAC uses the "traditional" definition of saturation (i.e., the signal level at which full well is achieved). Recommended signal limits are also based on the level of non-linearity of the detector: correction at the 1% level is only possible up to ~90% of full well, after which the signals cannot be linearized.
Translating the above HST and Spitzer experience to JWST is, however, not straightforward. If the same saturation definition for HST/WFC3 was applied to the analogous JWST near-infrared detectors (i.e., the ones used by the NIRCam, NIRISS and NIRSpec instruments), they would cut the dynamical range of the detectors by about 75%, as the JWST detectors present higher non-linear responses. Similarly, the same statement applies to the MIRI detectors when compared to the Spitzer/IRAC ones. Ground-testing and new calibration methods, however, should allow for non-linearity corrections which are more precise than the ones performed for Spitzer/IRAC and HST/WFC3 (see, e.g., the case for NIRCam; Canipe et al. 2017). This suggests that the limitation for most TSOs — at least for differential measurements — will not be the instrument non-linearity behavior, but most likely various other sources of systematic noise. The contributions of various systematics for TSOs will also be different between JWST and its predecessors, as they depend on numerous design and performance parameters, such as pixel scale and sampling, pointing stability, and flat field stability.
Testing these complex and often correlated issues in a physically representative way is exceptionally challenging on the ground due to the required measurement precision. Full characterization can only be performed once JWST is in orbit.
Target signal limits for TSOs
Given the uncertainties related to ground-testing of the instruments onboard JWST, defining tight signal limits to target during proposal preparation is hard to do for the Cycle 1 Call for Proposals. For proposals that can afford it, targeting from 50% saturation (in the JWST context, see above) up to the saturation limit should be on the safe side, with the former being perhaps the safest choice for Cycle 1. Note that in most cases, this range of signal limits won't have an impact on the precision but only on the observation efficiency (i.e., the photon-collecting time).
When defining a number of groups per integration considering these signal limits, it is important to be aware as well that, due to the non-destructive reading of the JWST detectors, the JWST Pipeline can flag and disregard saturated groups, allowing users to use the remaining groups in the ramp to construct the final ramp. This adds an additional level of freedom in that if a number of groups saturate during the ramp, this does not imply the whole integration is lost. This is useful, e.g., for MIRI observations, where saturating in the last group can actually be helpful.
Users are encouraged to check the instrument-specific strategies that have been written by the instrument teams, to which we link to below.
TSOs of very bright targets
If the observations being planned with JWST involve a bright object which is at the limit of what is observable with the observatory according to the signal limits defined by the Exposure Time Calculator, it might be wise to first revise the observational strategy being proposed and be absolutely sure that there is no other possible mode to observe the target in order to lower the maximum fluence attained by the observations. For example:
- If performing spectrophotometry: is it possible to use an instrument mode with a higher resolving power? This might help to spread the light of the object in more pixels.
- Is it possible to use smaller subarrays? These can help decrease the read times.
- Is it possible to use a faster readout mode for the detector?
- Is saturation happening in the spectral region of interest? If not, mild saturation might be an option; note, however, charge diffusion can occur for neighboring pixels (see, e.g., Plazas et al. 2018).
In general, if after revising all these points the target still saturates (i.e., the object achieves saturation in less than 3 groups), then observations could possibly enter a regime of risk in which the quality of the to-be-acquired data is uncertain, and recommendations to follow are strongly instrument-specific. Guidelines for these cases are provided in the instrument-specific pages, which are linked below so users can explore options in this direction.
Below we link to the recommended strategies relevant for TSOs of each of the instruments onboard JWST. These contain important information that must be reviewed by proposers when deciding to use a given instrument during proposal planning and submission.
- MIRI TSO Recommended Strategies
- NIRISS SOSS Recommended Strategies
- NIRCam TSO Recommended Strategies
- NIRSpec Detector Recommended Strategies
Canipe A., Robberto M., Hilbert B. 2017, JWST-STScI-005167, SM-12
A New Non-Linearity Correction Method for NIRCam
Hilbert B. 2008, Instrument Science Report WFC3 2008-39
WFC3 TV3 Testing: IR Channel Nonlinearity Correction
McCullough P. and MacKenty J., 2012, Instrument Science Report WFC3 2012-08
Considerations for Using Spatial Scans with WFC3
Plazas A. A., Shapiro C., Smith R., et al. 2018, PASP, 130, 065004
Laboratory Measurements of the Brighter-fatter Effect in an H2RG Infrared Detector
Stevenson K. B. and Eck W. 2019, Instrument Science Report WFC3 2019-13
Pre-Flashing WFC3/IR Time Series, Spatial Scan Observations
Stevenson K. B. and Fowler J. 2019, Instrument Science Report WFC3 2019-12
Analyzing Eight Years of Transiting Exoplanet Observations Using WFC3's Spatial Scan Monitor