NIRCam Known Issues

Known issues specific to NIRCam data processing in the JWST Science Calibration Pipeline are described in this article. This is not intended as a how-to guide or as full documentation of individual pipeline steps, but rather to give a scientist-level overview of issues that users should be aware of for their science. 

On this page

Specific artifacts are described in the Artifacts section below. Guidance on using the pipeline data products is provided in the Pipeline Notes section along with a summary of some common issues and workarounds in the summary section.

Information provided in this article affects all NIRCam observing modes. For mode-specific issues, see the coronagraphy, imaging, TSO, and WFSS issues pages. Please also refer to NIRCam Calibration Status.



Artifacts

Scattered light artifacts

Please see NIRCam Scattered Light Artifacts for further details.



Snowballs

Extreme cosmic ray impacts introduce large artifacts in near-IR detectors called snowballs that are not currently corrected by the pipeline. Most snowballs appear round. Some are elongated. Some have long tails or streaks. None of these features are properly flagged as cosmic rays by the current pipeline. Four or more dithered exposures enable the pipeline to reject these features as outliers.  See Shower and Snowball Artifacts for further details.



Persistence

Bright sources produce faint residual images that persist in subsequent integrations. This persistence declines exponentially in the detector pixels subjected to illumination.



Bad pixels

Ground testing revealed isolated bad pixels and some clumps up to 1.7" across, as seen in pre-launch NIRCam flat fields. Small high priority targets should avoid these regions. Primary dithers are recommended to fill in the large clumps of bad pixels in most exposures (any pattern is large enough).



Pipeline notes

The topics below affect all modes and reflect common questions about how to improve the quality of the data from the pipeline. 

1/f Noise

Horizontal striping across the data

The SIDECAR ASICs (detector readout electronics) generate significant 1/f noise during detector operations and signal digitization. This 1/f noise appears as horizontal banding that spans the entire width of the image, as seen in Figure 1. More discussion of 1/f noise is provided in NIRCam Detector Performance and NIRCam 1/f Noise Removal Methods.

Figure 1. NIRCam pupil and filter wheels 

Click on the figure for a larger view.

1/f noise is visible in this NIRCam image as horizontal bands that stretch across the entire detector.

The reference pixel correction step (refpix) in the stage 1 pipeline can mitigate 1/f noise. But, with the limited number of reference pixels available in NIRCam observations, the 1/f noise cannot be completely removed. 1/f correction will be even less effective in subarray observations that contain no or few reference pixels. There is currently no official calibration pipeline step designed solely to remove 1/f noise for NIRCam, but there are external software packages that can be used. See NIRCam 1/f Noise Removal Methods for comparisons between several of these packages.

Cosmic ray flagging

Noise flagged as jumps

If it appears too many pixels are being flagged as jumps in the data quality (DQ) maps, it may be that the noise in the image is being erroneously flagged. The jump step documentation lists arguments that users can provide when running the step. In a case where the jump detection appears to be too aggressive and is flagging noise, increasing the rejection_threshold parameter is one way to make the step less sensitive. Note that there are separate cosmic ray rejection thresholds for ramps that have 3 or 4 groups, each of which have a different default value that can be changed: three_group_rejection_threshold, four_group_rejection_threshold (default values are 6.0 and 5.0, respectively). For a similar case with overflagging of outliers when creating a mosaic image, see the Outlier Detection section.

Example demonstrating how to change jump step parameters
# call only the jump step
from jwst.jump import JumpStep
result = JumpStep.call('my_file.fits', rejection_threshold=5, save_results=True)
 
# call the entire pipeline while updating the jump step rejection_threshold
from jwst.pipeline.calwebb_detector1 import Detector1Pipeline
params = {'jump': {'rejection_threshold': 5}}
result = Detector1Pipeline.call('my_file_uncal.fits', steps=params, save_results=True)

Small number of groups

For integrations that have a small number of groups (e.g., 4 groups or less), default parameters for the jump detection step may not work well. The jump detection step uses the differences in signal between groups to determine whether there is a jump in the data. With very few groups in an integration, the jump step has only very limited data to work with. The set of parameters listed below may do a better job flagging jumps than the default values used by the pipeline.

Example demonstrating how to change jump step parameters
# call only the jump step
from jwst.jump import JumpStep
result = JumpStep.call('my_file.fits', rejection_threshold=3, only_use_ints = False, minimum_sigclip_groups = 20, save_results=True)
 
# call the entire pipeline while updating the jump step parameters
from jwst.pipeline.calwebb_detector1 import Detector1Pipeline
params = {'jump': {'rejection_threshold': 3,
                   'only_use_ints': False,
                   'minimum_sigclip_groups': 20
                   }
          }
result = Detector1Pipeline.call('my_file_uncal.fits', steps=params, save_results=True)

Outlier detection

Too many pixels flagged as outliers

When combining images to produce a final mosaic, the outlier detection step can sometimes flag too many pixels as outliers, even when a pixel may appear to be useable. There are several options in the outlier detection argument list that may be helpful to prevent the step from flagging good pixel values. The algorithm description gives information on how each of these input keywords are used. First, the equation in the fifth bullet point of step 6 describes how outliers are identified. By increasing the values in the snr parameter, you can specify that only pixels with larger deviations from the median are flagged as outliers. Another option is to change the value of the maskpt argument (see step 4 in the algorithm description). maskpt gives the fraction of the maximum weight to consider data to be good (default is 0.7). By lowering this value, lower weighted pixels will be included when resampling the data to create the median image. 

Example demonstrating how to change jump step parameters
# call only the outlier detection step
from jwst.outlier_detection import OutlierDetectionStep
result = OutlierDetectionStep.call('my_file.fits', snr='5.0 4.0', save_results=True)
 
# call the entire pipeline while updating the outlier_detection parameters
from jwst.pipeline.calwebb_image3 import Image3Pipeline
params = {'outlier_detection': {'snr': '5.0 4.0'}}
result = Image3Pipeline.call('my_asn.json', steps=params, save_results=True)

Saturation and Frame 0

Use of the zeroframe and first group

Sometimes data that uses one of the frame-averaged readout patterns (e.g., BRIGHT2) saturates after the first group on a few peak pixels, resulting in NaNs in the output calibrated files. NaNs indicate that no slope could be calculated. Similarly, bright objects in the data may be saturated even though the ETC predicted that they would not saturate "fully." For these cases, it's possible the zeroframe and first group may not be saturated and could be used to get valid slopes for those pixels, but they are not used by default in the Operations Pipeline.

The ramp-fitting step (ramp_fit) contains a parameter called suppress_one_group, which has a default setting of True. The name of this parameter can be misleading—it refers to the case where there is only one good data point for a ramp, regardless of whether that one good data point is from the first group or the zeroframe. When suppress_one_group=False, a slope should be computed for pixels that have either a good zeroframe or a good first group. If the first group is good, that will be used by itself to compute the slope. If the first group is saturated but the zeroframe is not, then the zeroframe will be used to compute a slope. 

Example demonstrating how to compute slopes for one group
# call only the ramp_fitting step
from jwst.ramp_fitting import RampFitStep
result = RampFitStep.call('my_file.fits', suppress_one_group=False, save_results=True)
 
# call the entire pipeline while updating the saturation step parameters
from jwst.pipeline.calwebb_detector1 import Detector1Pipeline
params = {'ramp_fit': {'suppress_one_group': False}}
result = Detector1Pipeline.call('my_file_uncal.fits', steps=params, save_results=True)

Snowballs

Empty cores and halos

Sometimes signal rate data contain sources with an empty core and surrounding halo with elevated signal. These features are residual signal from snowballs. Recent improvements to the jump step include an algorithm for better identification and removal of snowballs. Currently, this algorithm is turned off by default in the Operations Pipeline for NIRCam. But, you can turn it on by manually calling the jump step and setting expand_large_events=True. There are several keywords that affect the snowball detection algorithm. These are listed in the "Parameters that affect Near-IR Snowball Flagging" section of the jump step arguments documentation page.

Example demonstrating how to change jump step parameters
# call only the jump step
from jwst.jump import JumpStep
result = JumpStep.call('my_file.fits', expand_large_events=True, save_results=True)
 
# call the entire pipeline
from jwst.pipeline.calwebb_detector1 import Detector1Pipeline
params = {'jump': {'expand_large_events': True}}
result = Detector1Pipeline.call('my_file_uncal.fits', steps=params, save_results=True)



Summary of common issues and workarounds

The sections above provide detail on each of the known issues affecting NIRCam data; in this table we summarize some of the most likely issues for users to encounter along with any workarounds if available.  Note that greyed-out issues have been retired, and are fixed as of the indicated pipeline build


SymptomsCauseWorkaroundfix buildMitigation Plan
NC-G01: Bright objects in the field are saturated even though the ETC predicted that they would not saturate "fully."For most NIRCam readout patterns, there are many frames included in a groupBright objects may saturate by the end of a groupbut remain unsaturated in the first frame (frame0).

The calwebb_detector1 pipeline includes an option to correct an image using the unsaturated frame0 exposure, but this is turned off by default. To turn it on, rerun calwebb_detector1 with ramp_fit  parameter suppress_one_group set to False.

Note that frame0 is currently only used as an extra data point for saturated ramps.

 N/A

Updated issue

The NIRCam team will evaluate whether the frame0 correction should be turned on as the default for ramps affected by saturation; however, the investigation is still in progress due to higher priority issues.




Notable updates
Originally published