Understanding the Roman WFI Gain Reference File#
Kernel Information and Read-Only Status#
To run this notebook, please select “Roman Research Nexus {VERSION}” kernel at the top right of your window. For example “Roman Research Nexus 2026.2”.
This notebook is read-only. You can run cells and make edits, but you must save changes to a different location. We recommend saving the notebook within your home directory, or to a new folder within your home (e.g. file > save notebook as > my-nbs/nb.ipynb). Note that a directory must exist before you attempt to add a notebook to it.
Introduction#
The purpose of this notebook is to understand the content and purpose of the Gain (GAIN) reference file.
The GAIN reference file is used during the gain_scale step to convert from instrumental units (DN) to electrons. It is critical for proper noise propagation and variance estimation. The gain values are assumed to be in units of e/DN.
More details about this and other reference files can be found in the Reference File Information.
Local Run Settings#
If you want to run the notebook in your local machine, refer to the information in local installation instructions before proceeding with the notebook. The instructions provide important information about setting up your environment and installing dependencies.
Imports#
Libraries used:
astropy for image normalization
copy for making copies of Python objects
crds for access to calibration reference files
matplotlib and mpl_toolkits for plotting images
numpy for array manipulation
roman_datamodels for opening Roman WFI ASDF files
os for operating system functions
import os
from astropy.visualization import simple_norm
import copy
import matplotlib.pyplot as plt
from matplotlib import colors, colormaps as cm
from mpl_toolkits.axes_grid1 import make_axes_locatable
import numpy as np
import roman_datamodels as rdm
The Calibration Reference Data System (CRDS)#
The reference files, developed and validated by STScI’s Science Operations Center, are continually updated as new WFI data become available. For more information about how CRDS works and how it assigns the most appropriate reference file for each calibration step, refer to the notebook Understanding CRDS and How to Select Calibration Reference files.
IMPORTANT NOTE: Reference files are a work in progress and will be updated several times before Roman launch. If you notice irregularities or missing information, please understand that they may be a known issue. If you have questions, please contact the Roman Help Desk.
import crds
Now let’s dive into this reference file type.
Gain Reference File#
The GAIN reference file provides the conversion factor from Data Numbers (DN) to electrons for each pixel. It is used in the ramp fitting step for accurate variance calculation and noise modeling.
For more details, see the romancal documentation and Rdox documentation for the Ramp Fitting and Jump Detection.
Before proceeding, let’s check the environmental variables set for CRDS
print(f"CRDS server location: {os.environ.get('CRDS_SERVER_URL')}")
print(f"CRDS context file: {os.environ.get('CRDS_CONTEXT')}")
CRDS server location: https://roman-crds.stsci.edu
CRDS context file: roman-edit
If we want to change the context, we can do it in the next cell. In this case, we choose context roman_0058.pmap.
os.environ['CRDS_CONTEXT']='roman_0058.pmap'
Retrieving Reference Files#
As you run the exposure pipeline, the most up-to-date reference files will be automatically selected for each step. However, if you would like to use a specific reference file, retrieve it using the CRDS Python API and feed it to the Exposure Level Pipeline, see the notebook Understanding CRDS and How to Select Calibration Reference files for more details.
For the gain files in particular, the keywords that will identify the best reference file to use are:
ROMAN.META.INSTRUMENT.NAME
ROMAN.META.INSTRUMENT.DETECTOR
ROMAN.META.EXPOSURE.TYPE
ROMAN.META.EXPOSURE.START_TIME
These keywords may be combined into a single dictionary to find and download the file using crds.getreferences().
meta = {'ROMAN.META.INSTRUMENT.NAME': 'WFI',
'ROMAN.META.INSTRUMENT.DETECTOR': 'WFI01',
'ROMAN.META.EXPOSURE.TYPE': 'WFI_IMAGE',
'ROMAN.META.EXPOSURE.START_TIME': '2026-01-01 00:00:00'
}
ref_files = crds.getreferences(meta, reftypes=['gain'], observatory='roman')
ref_files
CRDS - INFO - Syncing 30 files
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_specpsf_0001.rmap 572 bytes (1 / 30 files) (0 / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_skycells_0002.rmap 712 bytes (2 / 30 files) (572 / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_sflat_0001.rmap 548 bytes (3 / 30 files) (1.3 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_saturation_0003.rmap 2.4 K bytes (4 / 30 files) (1.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_relflux_0001.rmap 555 bytes (5 / 30 files) (4.3 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_refpix_0003.rmap 2.3 K bytes (6 / 30 files) (4.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_readnoise_0006.rmap 3.4 K bytes (7 / 30 files) (7.1 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_photom_0004.rmap 2.3 K bytes (8 / 30 files) (10.5 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_optmodel_0001.rmap 586 bytes (9 / 30 files) (12.9 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_matable_0004.rmap 690 bytes (10 / 30 files) (13.4 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_mask_0003.rmap 2.3 K bytes (11 / 30 files) (14.1 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_linearity_0005.rmap 2.4 K bytes (12 / 30 files) (16.4 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_ipc_0003.rmap 3.3 K bytes (13 / 30 files) (18.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_inverselinearity_0005.rmap 2.6 K bytes (14 / 30 files) (22.1 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_integralnonlinearity_0002.rmap 2.8 K bytes (15 / 30 files) (24.7 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_gain_0003.rmap 2.3 K bytes (16 / 30 files) (27.5 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_flat_0006.rmap 8.5 K bytes (17 / 30 files) (29.7 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_etc_0002.rmap 2.3 K bytes (18 / 30 files) (38.2 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_epsf_0004.rmap 16.3 K bytes (19 / 30 files) (40.5 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_dustmap_0003.rmap 855 bytes (20 / 30 files) (56.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_distortion_0002.rmap 3.6 K bytes (21 / 30 files) (57.7 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_detectorstatus_0002.rmap 773 bytes (22 / 30 files) (61.2 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_darkdecaysignal_0002.rmap 783 bytes (23 / 30 files) (62.0 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_dark_0011.rmap 4.4 K bytes (24 / 30 files) (62.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_area_0002.rmap 2.3 K bytes (25 / 30 files) (67.2 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_apcorr_0003.rmap 2.3 K bytes (26 / 30 files) (69.5 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_abvegaoffset_0002.rmap 2.5 K bytes (27 / 30 files) (71.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_absflux_0001.rmap 555 bytes (28 / 30 files) (74.3 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_wfi_0056.imap 2.4 K bytes (29 / 30 files) (74.8 K / 77.6 K bytes)
CRDS - INFO - Fetching /home/runner/crds_cache/mappings/roman/roman_0058.pmap 344 bytes (30 / 30 files) (77.3 K / 77.6 K bytes)
CRDS - INFO - Syncing 1 files
CRDS - INFO - Fetching /home/runner/crds_cache/references/roman/wfi/roman_wfi_gain_0022.asdf 67.1 M bytes (1 / 1 files) (0 / 67.1 M bytes)
{'gain': '/home/runner/crds_cache/references/roman/wfi/roman_wfi_gain_0022.asdf'}
Examining Reference Files#
Reference files use roman_datamodels just like WFI science data products and can be accessed in the same way (see the tutorial Working with ASDF for more information). Let’s take a closer look at the files we retrieved from our crds.getreferences() example:
gain = rdm.open(ref_files['gain'])
gain.info()
root (AsdfObject)
├─asdf_library (Software)
│ ├─author (str): The ASDF Developers
│ ├─homepage (str): http://github.com/asdf-format/asdf
│ ├─name (str): asdf
│ └─version (str): 4.1.0
├─history (AsdfDictNode)
│ └─extensions (AsdfListNode)
│ ├─0 (ExtensionMetadata) ...
│ ├─1 (ExtensionMetadata) ...
│ └─2 (ExtensionMetadata) ...
└─roman (GainRef) # Gain Reference File Schema
├─meta (AsdfDictNode) # Common Reference File Metadata Properties
│ ├─author (str): Timothy Brandt # Author
│ ├─description (str): Derived from TVAC1 and FPS test flat fields. Updated to Build 17 versions of romanc (truncated)
│ ├─instrument (AsdfDictNode) ...
│ ├─origin (Origin): STSCI # Institution / Organization Name
│ ├─pedigree (str): GROUND # Pedigree
│ ├─reftype (str): GAIN
│ ├─telescope (Telescope): ROMAN # Telescope Name
│ └─useafter (Time): 2023-08-01T00:00:00.000 # Use After Date
└─data (NDArrayType) # Gain Data Array ...
Some nodes not shown.
We see that the gain reference file contains metadata plus the data, a 2D array with the gain value (electrons per DN) for each pixel.
Basic Statistics#
Now lets get some basic statistics on the cube (or a representative slice)
print("Gain array shape:", gain.data.shape)
print("\nGain statistics:")
print(f" Min: {gain.data.min():.4f} e-/DN")
print(f" Max: {gain.data.max():.4f} e-/DN")
print(f" Mean: {gain.data.mean():.4f} e-/DN")
print(f" Median: {np.median(gain.data):.4f} e-/DN")
print(f" Std: {gain.data.std():.4f} e-/DN")
Gain array shape: (4096, 4096)
Gain statistics:
Min: 1.0000 e-/DN
Max: 3.0000 e-/DN
Mean: 1.9948 e-/DN
Median: 1.9944 e-/DN
Std: 0.0984 e-/DN
Visualization#
Let’s check this reference file
fig, ax = plt.subplots(figsize=(10, 8))
my_cmap = copy.copy(cm.get_cmap('viridis'))
my_cmap.set_bad('black')
# Gain map
norm = simple_norm(gain.data, stretch='linear', percent=99.5)
im = ax.imshow(gain.data, cmap=my_cmap, norm=norm, origin='lower')
ax.set_title('Gain Map (electrons per DN)')
ax.set_xlabel('Science X (pixels)')
ax.set_ylabel('Science Y (pixels)')
# Colorbar
divider = make_axes_locatable(ax)
cax = divider.append_axes("right", size="5%", pad=0.05)
fig.colorbar(im, cax=cax, label='e-/DN')
plt.tight_layout()
plt.show()
About this Notebook#
Author: R. Diaz
Updated On: 2026-07-06
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