Understanding the Roman WFI Pixel Area Reference File

Understanding the Roman WFI Pixel Area 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 Pixel Area (AREA) reference file.

The AREA reference file provides the pixel area in steradians for each pixel. It is used during photometric calibration to correctly convert surface brightness to flux, especially important for extended sources.

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:

  • os for operating system functions

  • 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

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.

Pixel Area Reference File#

The AREA reference file contains the solid angle (in steradians) subtended by each pixel. It is used in the photometric calibration step to properly handle extended sources.

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 or Mosaic Pipeline, see the notebook Understanding CRDS and How to Select Calibration Reference files for more details.

For the area reference 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.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.START_TIME': '2026-01-01 00:00:00'
       }

ref_files = crds.getreferences(meta, reftypes=['area'], 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_area_0009.asdf   66.8 M bytes  (1 / 1 files) (0 / 66.8 M bytes)

{'area': '/home/runner/crds_cache/references/roman/wfi/roman_wfi_area_0009.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:

area = rdm.open(ref_files['area'])
area.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 (PixelareaRef) # Pixel Area Reference Schema
  ├─meta (AsdfDictNode)
  │ ├─author (str): Tyler Desjardins # Author
  │ ├─description (str): Roman WFI pixel area map. # Description
  │ ├─instrument (AsdfDictNode) ...
  │ ├─origin (Origin): STSCI/SOC # Institution / Organization Name
  │ ├─pedigree (str): GROUND # Pedigree
  │ ├─photometry (AsdfDictNode) ...
  │ ├─reftype (str): AREA
  │ ├─telescope (Telescope): ROMAN # Telescope Name
  │ └─useafter (Time): 2020-01-01 00:00:00 # Use After Date
  └─data (NDArrayType) # Pixel Area Array ...
Some nodes not shown.

The AREA reference file is typically a 2D array with pixel solid angle values in steradians.

Basic Statistics#

Now lets get some basic statistics on the cube (or a representative slice)

print("Area array shape:", area.data.shape)
print("\nPixel area statistics (steradians):")
print(f"  Min: {area.data.min():.2e}")
print(f"  Max: {area.data.max():.2e}")
print(f"  Mean: {area.data.mean():.2e}")
print(f"  Median: {np.median(area.data):.2e}")
print(f"  Std: {area.data.std():.2e}")

Area array shape: (4088, 4088)

Pixel area statistics (steradians):
  Min: 9.86e-01
  Max: 1.01e+00
  Mean: 1.00e+00
  Median: 1.00e+00
  Std: 6.18e-03

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')

norm = simple_norm(area.data, stretch='linear', percent=99.5)
im = ax.imshow(area.data, cmap=my_cmap, norm=norm, origin='lower')
ax.set_title('Pixel Area Map (steradians)')
ax.set_xlabel('Science X (pixels)')
ax.set_ylabel('Science Y (pixels)')

divider = make_axes_locatable(ax)
cax = divider.append_axes("right", size="5%", pad=0.05)
fig.colorbar(im, cax=cax, label='Pixel Area (sr)')

plt.tight_layout()
plt.show()
../../_images/7611877fde3b0c4b45b2e14673a54f0269aa4cde13e82347a7d6e42a31cb4b00.png

About this Notebook#

Author: R. Diaz

Updated On: 2026-07-06

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