Introduction
Subtraction of background signal can take several different forms depending on the observing mode and the available data. Here we give an overview of the different methods that are available, when they can be used, and where they occur in the processing flow. Imaging and spectroscopic observations share one method for background subtraction, while others are unique to spectroscopic data only. See the documentation for the individual steps mentioned here for complete details on how each of them function.
Imaging Mode
Background subtraction for imaging data is currently available in several places within the calibration pipeline stages.
Image-from-image subtraction can be performed by the background step during calwebb_image2 processing. The background images come from observations of a dedicated background target.
Background matching and subtraction can be performed within an ensemble of images by the skymatch step during calwebb_image3 processing.
Local background subtraction for individual sources can be performed by the source_catalog step within the calwebb_image3 pipeline.
Spectroscopic Modes
Spectroscopic observations allow for some additional ways of performing background subtraction. The list of options includes:
Image-from-image subtraction can be performed by the background step during calwebb_spec2 processing. The background images can come from:
Observations of a dedicated background target
Nodded observations of a point-like science target
Subtraction of a “master” background spectrum, where the master background spectrum can come from:
Observations of a dedicated background target
Nodded observations of a point-like science target
Dedicated background slitlets in a NIRSpec MOS exposure
A user-supplied spectrum
Local background subtraction for individual spectral can be performed by the extract_1d step when doing 1D spectral extraction.
The following table shows the list of image-from-image and master background subtraction methods available for various spectroscopic observation modes, and indicates the pipeline and step in which the subtraction operation occurs. The table also shows which method is applied by default in the operational pipeline when the available data support multiple methods.
Note
Master background subtraction is applied in the calwebb_spec3 pipeline for most spectroscopic modes, but for NIRSpec MOS mode it is applied during calwebb_spec2 processing.
Mode |
calwebb_spec2 background |
calwebb_spec3 master_background |
calwebb_spec2 master_background_nrs_slits |
|---|---|---|---|
NIRSpec Fixed Slit: |
|||
Dedicated background |
Default |
Optional |
|
Nodded point source |
Default |
Optional |
|
User supplied |
Default |
||
NIRSpec IFU: |
|||
Dedicated background |
Default |
Optional |
|
Nodded point source |
Default |
Optional |
|
User supplied |
Default |
||
NIRSpec MOS: |
|||
Background slitlets |
Default |
||
Nodded point source |
Default |
||
User supplied |
Default |
||
MIRI LRS Fixed Slit: |
|||
Dedicated background |
Default |
Optional |
|
Nodded point source |
Default |
Optional |
|
User supplied |
Default |
||
MIRI MRS: |
|||
Dedicated background |
Default |
Optional |
|
Nodded point source |
Default |
Optional |
|
User supplied |
Default |
These background subtraction methods are only available for the observing modes listed in the table. Other spectroscopic modes, including NIRCam and NIRISS Wide Field Slitless Spectroscopy (WFSS), NIRCam Time Series Grism, NIRISS Single Object Slitless Spectroscopy (SOSS), and MIRI LRS slitless, use other ways of handling background.
Image-from-Image Subtraction
As explained in the documentation for the background step, this process combines one or more exposures to be treated as backgrounds into a sigma-clipped mean background image, which is then directly subtracted, in detector space, from an exposure being processed in the calwebb_image2 or calwebb_spec2 pipelines for imaging or spectroscopic data, respectively. For imaging mode observations this is only possible when observations of a designated background target have been obtained. For spectroscopic modes this is possible either through observations of a designated background target or when nodded exposures of a point-like target are obtained (e.g. using the MIRI LRS “ALONG-SLIT-NOD” dither pattern for an LRS fixed slit observation). Exposures from one nod position can be used as background for exposures at the other nod position, assuming the source is point-like.
In either instance, the exposures to be used as background are included in the
image2 or spec2 ASN file used to process the science target exposures, where
the background exposures are labeled with an ASN member type of “background”.
Spectroscopic observations that have designated background target exposures or nodded exposures can use either the image-from-image or master background subtraction methods. In the operational pipeline the image-from-image subtraction method is applied by default and the master background subtraction is skipped. A user has the option to reprocess the data and apply the other method, if desired.
Master Background Subtraction
In general, the master background subtraction method works by taking a 1D background spectrum, interpolating it back into the 2D space of a science image, and then subtracting it. This allows for higher SNR background data to be used than what might be obtainable by doing direct image-from-image subtraction using only one or a few background images. The 1D master background spectrum can either be constructed on-the-fly by the calibration pipeline from available background data or supplied by the user. See the documentation for the master background subtraction step for full details.
As with image-from-image subtraction, there are different ways of obtaining the data necessary for constructing a master background spectrum, depending on the observing mode:
Observations of a designated background target
Nodded observations of a point-like source
Dedicated background slitlets in a NIRSpec MOS exposure
User-supplied master background spectrum
All of these scenarios apply the master background subtraction during calwebb_spec3 processing, except for NIRSpec MOS observations. Master background subtraction for NIRSpec MOS, using either data from background slitlets contained in each MOS exposure or a user-supplied master background spectrum, is applied during calwebb_spec2, due to unique methods that must be used for MOS exposures.
For scenarios that apply master background subtraction during calwebb_spec3 processing, the fully-calibrated 1D spectra (“x1d” products) from either dedicated background target exposures or nodded science exposures are used by the master_background step to construct the 1D master background spectrum. These are the x1d products created during the last step of the preceding calwebb_spec2 pipeline when it is used to process each exposure. Again, see the documentation for the master background subtraction step for full details of the source of the background data for these scenarios.
If the user supplies a 1D master background spectrum, the construction of the master background spectrum in the pipeline is skipped and the user-supplied spectrum is used in its place. This applies to all modes, including NIRSpec MOS.
As mentioned above, NIRSpec MOS observations require special handling to correctly apply master background subtraction. If a MOS observation uses an MSA configuration that includes one or more slitlets containing only background signal, the background slitlets are fully calibrated and extracted to produce one or more 1D background spectra. The background spectra are combined into a 1D master background spectrum, which is then interpolated back into the 2D space of all slitlets and subtracted. If the user supplies a master background spectrum for a MOS observation, that spectrum is used to do the subtraction. Again note that for NIRSpec MOS mode these operations take place during calwebb_spec2 pipeline processing, not calwebb_spec3 like all other modes.