NIRSpec Background Recommended Strategies
Strategies for removing the sky contamination from JWST NIRSpec observations are provided for three observing modes: multi-object spectroscopy, integral field spectroscopy, and fixed slits spectroscopy.
A large fraction of NIRSpec observations are detector noise-limited, while spectroscopy with the prism, as well as verification and confirmation images, may be dominated by the background noise at certain wavelengths.
The main background components that affect NIRSpec observations are:
- The in-field sky radiance, contributed by the zodiacal cloud of the Solar System and the Milky Way.
- The straylight from the out-of-field sky.
These contributions are extensively described in the JWST Background Model article.
Other signal components may affect NIRSpec observations in IFU and MOS mode, such as light through individual MSA failed open shutters, and the cumulative ″leakage″ signal caused by the closed MSA shutters which are not completely opaque to sky illumination. A description of the MSA flux leakage problem is presented in the article NIRSpec Micro-Shutter Assembly and a procedure on how to perform a correction for an IFU observation is described in the NIRSpec MSA Leakage Correction for IFU Observations article.
Background removal strategies
Not all NIRspec observations require background subtraction. Bright objects may have surface brightness in the NIRSpec bands that is significantly brighter than the surface brightness in the JWST Background Model at 1 - 5 μm. In that case, if the background is clearly negligible, it may not be necessary to subtract it. Note that pixel-to-pixel background subtraction (nodding) adds noise.
Two background subtraction strategies are possible for NIRSpec observations; pixel-to-pixel subtraction and master background subtraction.
The first background subtraction method is the pixel-to-pixel subtraction at count rate level. This method will always be used for in-scene nods. For sources that are point-like or compact, an in-scene nodding strategy maximizes the on-source exposure time, and thus the signal to noise ratio (SNR). Several in-scene nodding strategies are implemented within the available dither patterns for each observing mode, and the recommended ones are highlighted in Table 1.
For extended sources observed in fixed slit and IFU modes, observing the ″blank sky″ signal at a dedicated off-scene position is recommended. Pixel-to-pixel background subtraction processing will be performed only if the grating wheel has not moved between the target and off-scene associated background exposures. If the grating wheel moved between the target and background exposures (as would be the case if they were in different visits), pipeline processing will follow a more involved "master background" subtraction method described below.
Table 1. Recommended strategies for background subtraction of compact sources
|Fixed Slit spectroscopy|
2, 3, 5 (nodding) points2
|Integral Field Spectroscopy|
2, 4 points3
Multi Object Spectroscopy
2 shutters 2 points4
3 shutters 3 points4
5 shutters 3 points4
5 shutters 5 points4
(1) Bold options are the recommended ones.
(2) These choices map to the options for Primary Dither Positions*in the FS APT template.
(3) These choices map to the options for Dither Type = 2-POINT-NOD, and 4-POINT-NOD in the IFU APT Template.
(4) These choices are obtained using the checkbox Nod in Slitlet in the Dither Setup section of the MPT Planner. The number of dithers is set by the default slitlet shape. For the 5-shutter slitlet, there is an option for the number of exposures per configuration. Three or five may be selected, corresponding to the central shutter and either the two extreme shutters in the slitlet, or all the shutters, respectively.
*Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.
Master background subtraction
The second background subtraction method is based on an independent flux-calibrated 1D background spectrum. For the MOS mode, this master background can be obtained as part of the science observation by designing the MSA configuration to include ″blank sky″ shutters. During pipeline processing, spectra from these background shutters will be extracted and combined into a single background spectrum to be subtracted from the target spectrum. Master background shutters can be added to an existing MSA configuration, or when designing a new MSA configuration, using the Manual Planner tool in MPT. In the case of high spectral resolution observations, the number and distribution of background shutters needed will depend on the infield background uniformity and the desired combined spectral coverage. In this case, the IRS2 readout mode is highly recommended, to minimize the impact of the 1/f noise in the MSA master background.
For IFU or fixed slit mode observations of point sources, a master background may also be constructed using off-source spaxels. Pipeline processing will likely not be available for this option, but users will be able to extract and create a master background spectrum for self-correction of their target exposures on their own. Additionally, the off-scene nod option discussed above for the IFU and implemented in APT using a Target Group (see NIRSpec Dithering Recommended Strategies), requires this method if the grating wheel moves between the target and background exposures. This is because the grating wheel positioning is not exactly repeatable when returning to the same disperser. However, if the disperser used for the target exposures remains in place for the background exposures, the pixel-to-pixel background subtraction method is performed in the pipeline by default.
How does the SNR scale with the background subtraction strategy?
The background subtraction step with multiple nods is not completely built into the JWST Exposure Time Calculator (ETC).
In the JWST Exposure Time Calculator (ETC) in IFU mode, the SNR is automatically calculated for the "IFU Nod Off Scene", and the 2-point "IFU Nod In Scene" strategies. For the case of background-limited observations, Table 2 helps the user to evaluate the impact that the different pixel-to-pixel background subtraction strategies have in the SNR scaling of the background-subtracted signal (Column 5, SNRBS-signal ). Column 3 contains the number of exposures required by the ETC to accordingly scale the source signal, and thus yield the total exposure time to match the associated strategy in APT. The value SNR represents the signal-to-noise ratio that the ETC calculates for the case of two exposures with an off-scene nod. The SNR will scale as shown considering the background subtraction strategies of Aperture Spectral Extraction (FS or MOS) or MSA Full Shutter Extraction (MOS only).
Table 2. Signal-to-noise ratio scaling with nod strategy for background-limited observations
Off-scene nodding (IFU Only)
IFU Nod Off Scenea, c
(1 point on source, 1 point off-source)
2 point a,d
2 point f
4 point e
(a) an ETC built-in strategy.
(b) This is the number of exposures one would enter in the ETC Detector Setup tab "exposures per specification" parameter.
(c) The "IFU Nod Off Scene" strategy in the ETC Strategy tab corresponds to the off-scene nod option described in NIRSpec Dithering Recommended Strategies, which is implemented in APT using a Target Group.
(d) The 2-POINT-NOD option in the APT IFU template corresponds to the nod strategy "IFU Nod in Scene" in the ETC Strategy tab, with 1 exposure specified in the ETC Detector Setup tab.
(e) The 4-POINT-NOD option in the APT IFU template corresponds to the nod strategy "IFU Nod in Scene" in the ETC Strategy tab, with 2 exposures specified in the ETC Detector Setup tab.
(f) The two-, three-, and five-point Primary Dither Positions in the APT Fixed Slit Template are represented in the ETC by specifying 2, 3, or 5 exposures in the ETC Detector Setup tab. Likewise, the two-, three-, and five-point nods specified in the MOS mode of APT using the MPT automatic Planner Dither Setup are represented in the ETC by specifying 2, 3, or 5 exposures in the ETC Detector Setup tab. For manually-designed MOS observations, nods can be specified using the Manual Planner Cross-Dispersion Offset parameter.
To summarize, NIRSpec in-scene nodding constitutes an optimized strategy to reliably measure the background signal, while maximizing the on-source exposure time. It is important to consider that the noise for the background estimate is scaled by the number of background measurements as √(n-1), with n being the number of points.
Dedicated background observations
For a given position on the sky, the background signal is time-variable throughout the year, driven by the seasonal variability of the zodiacal cloud.
To reliably measure the background signal with dedicated sky exposures in the case of extended objects, it is recommended to link observations or visits in a NON-INTERRUPTIBLE manner using APT TIMING special requirements.
For background-limited observations, independently of the background subtraction strategy, it is important to evaluate how the background varies throughout the year, potentially affecting when that observation is being best scheduled to minimize the background noise.
The Background-Limited JWST Observations article provides a technique to assess the impact of seasonal variations at a given sky position using the ETC. The user can ensure low background levels by using the APT BACKGROUND-LIMITED special requirement.