Data Levels and Products
Wide Field Instrument (WFI) science data products are described by data levels that indicate the level of calibration and types of products contained within. This article offers a comprehensive overview and details regarding the composition of WFI data products across each data level.
Overview of Data Levels
Science data products from the Roman WFI instrument made available to users via the Roman Archive (see Accessing WFI Data for more information). The WFI files are described by five data levels 1 – 5 (often abbreviated L1, L2, etc.). Note that a sixth level (L0) refers to the raw, packetized data received from the WFI instrument; however, L0 data are generally unavailable to most Archive users. Level 1 – 4 products are generated by the Roman science centers, while L5 products are contributed by the community.
Changes to the technical details and other specifications presented here are expected as part of the development of the Roman data management system. In addition, some details may be omitted while topics are in active development; information on these topics will be added in future RDox releases.
High-Level Summary of WFI Data Products
| Data Processing Levels | File Suffix | Description |
|---|---|---|
| Level 0 | TBD | Raw, packetized data from the telescope. |
| Level 1 |
| Uncalibrated detector ramps. |
| Level 2 |
| Calibrated detector rate images. |
| Level 3 | TBD | Re-pixellated data including, e.g., co-additions and mosaics. |
| Level 4 | TBD | Information extracted from pixel data including, e.g., source catalogs, 1-D spectra, and light curves. |
| Level 5 | TBD | Products created by the Roman user community. |
Information
All L1 – 5 products are equally accessible to Roman Archive users, while L0 data are restricted. See Accessing WFI Data for more information.
Detailed Descriptions of WFI Data Products
In addition to the descriptions below, the schema detailing the contents of the WFI science files may be found in the Roman Attribute Dictionary (RAD) repository on GitHub.
Level 1 - Uncalibrated Data
Science Ramps
WFI L1 files are reconstructed from the packetized L0 data. During this reconstruction, the data are reoriented from the detector to the science coordinate frame (see pySIAF for Roman for more information on WFI coordinate frames), and basic metadata are populated. The L1 file contains the science data as a three-dimensional data cube that represents a single, uncalibrated ramp exposure. Each detector is mapped to an individual L1 file, therefore an exposure of the WFI instrument produces 18 L1 files (one for each detector). The science data cube in the L1 file is shaped as an array with (N resultants, 4096 rows, 4096 columns). The L1 file also contains an additional data cube shaped as an array of (N resultants, 4096 rows, 128 columns) corresponding to samples of the 33rd amplifier virtual reference pixels (see Description of WFI for more information). The number of resultants N along the z-axis of the cube is dictated by the settings of the multi-accumulation table used for the exposure.
L1 Science Data Specifications
| Array | Description | Units | Type | Dimensions |
|---|---|---|---|---|
| data | Science data, including the border reference pixels. | Digital Number (DN) | uint16 | (N resultants, 4096 rows, 4096 columns) |
| amp33 | Amp 33 reference pixel data. | DN | uint16 | (N resultants, 4096 rows, 128 columns) |
Guide Window Data
Guide window data, which are composed of high-frequency readouts of small windows on the WFI detectors, are used onboard for target acquisition and fine attitude control adjustments during exposures. These guide window data are stored as L1 files separate from the science data, and are not processed to higher data levels.
L1 Guide Window Specifications
| Array | Description | Units | Type | Dimensions |
|---|---|---|---|---|
| signal_frames | Reconstituted and oriented signal frames. | DN | uint16 | (I frames, J combined resultants, K reads, Y rows, X columns) |
| pedestal_frames | Reconstituted and oriented pedestal frame GW images. | DN | uint16 | (I frames, J combined resultants, K reads, Y rows, X columns) |
| amp33 | Amp 33 reference pixel data. | DN | uint16 | (I frames, J combined resultants, K reads, Y rows, X columns) |
Level 2 - Calibrated Exposures
The L2 WFI data are calibrated rate images in instrumental units of (photo)electrons / second (e– / sec). L2 files are generated from L1 inputs using the Exposure Pipeline in romancal , which applies detector-level calibrations (e.g., bad pixel masking, classic non-linearity correction, dark current subtraction), aligns the image to Gaia astrometry, and populates additional metadata such as conversion information to physical flux units (see Roman STScI Data Pipelines for more information). As with L1 science data files, the L2 files contain one detector per file, therefore a complete exposure of the WFI contains 18 L2 files.
Data taken in the WFI spectroscopic mode have not been flat fielded at L2, nor do they have photometric calibration information in the metadata. A wavelength-dependent flat field and flux calibration will be applied during 1-D spectral extraction in the Science Support Center (SSC) spectroscopic pipeline.
In the L2 files, the science data arrays have dimensions of (4088 rows, 4088 columns) after the removal of the 4-pixel wide border of reference pixels present in L1 files (refer to the Description of WFI for additional information on reference pixels). During processing, the reference pixel information from the L1 files is copied into new arrays in the L2 files. This data facilitates user access to the values of reference pixels used to compute the reference pixel correction applied by the Exposure Pipeline.
Data quality information is added to the L2 products by the Exposure Pipeline. The data quality array results from the bitwise addition of data quality bit flags. In this context, the value of a pixel is the unique sum of powers of 2, where each power corresponds to a specific data quality flag. Further details on data quality flags will be provided in future updates.
L2 Science Data Specifications
| Array | Description | Units | Type | Dimensions |
|---|---|---|---|---|
| data | Science data, excluding border reference pixels. | e– / sec | float32 | (4088 rows, 4088 columns) |
| err | The total error array. | e– / sec | float32 | (4088 rows, 4088 columns) |
| dq | The data quality array. | N/A | unit32 | (4088 rows, 4088 columns) |
| var_flat | The variance array associated with the flat field. | e– / sec | float32 | (4088 rows, 4088 columns) |
| var_poisson | The variance array associated with the Poisson noise. | e– / sec | float32 | (4088 rows, 4088 columns) |
| var_rnoise | The variance array associated with the read noise. | e– / sec | float32 | (4088 rows, 4088 columns) |
| amp33 | Amp 33 reference pixel data. | DN | uint16 | (4096 rows, 128 columns) |
| border_ref_pix_left | Original border reference pixels (left). | DN | uint16 | (4096 rows, 4 columns) |
| border_ref_pix_right | Original border reference pixels (right). | DN | uint16 | (4096 rows, 4 columns) |
| border_ref_pix_top | Original border reference pixels, (top). | DN | uint16 | (4 rows, 4096 columns) |
| border_ref_pix_bottom | Original border reference pixels, (bottom). | DN | uint16 | (4 rows, 4096 columns) |
| dq_border_ref_pix_left | Data quality for border reference pixels (left). | N/A | uint32 | (N resultants, 4096 rows, 4 columns) |
| dq_border_ref_pix_right | Data quality for border reference pixels (right). | N/A | uint32 | (N resultants, 4096 rows, 4 columns) |
| dq_border_ref_pix_top | Data quality for border reference pixels (top). | N/A | uint32 | (N resultants, 4 rows, 4096 columns) |
| dq_border_ref_pix_bottom | Data quality for border reference pixels (bottom). | N/A | uint32 | (N resultants, 4 rows, 4096 columns) |
Level 3 - Mosaics
L3 products are the co-additions or mosaics of L2 files. A single L3 product may be based on the input of one or more L2 products. During L3 product generation, the data are corrected for geometric distortion and are converted from instrumental units to physical surface brightness units of MegaJanskys per steradian (MJy / sr). Data quality information from the L2 file(s) is used to screen out various undesirable detector effects from the final L3 product. The flux from multiple input pixels are combined in the output pixel grid using the drizzle algorithm. The size and shape of the L3 products depends on the settings used to create the final product. For more information on the L3 science data pipeline, see Roman STScI Data Pipelines.
Information regarding specific L3 products produced by the Roman science centers will be added in a future RDox release.
Level 4 - Extracted Data
L4 products contain information that has been extracted from pixellated L2 and L3 data. These products may include source catalogs, 1-D spectra, and light curves.
Information regarding specific L4 products produced by the Roman science centers will be added in a future RDox release.
Level 5 - User Contributed Products
L5 data products are any products created by Roman users and made available to the community via the Roman Archive. As these products are heterogenous in nature, documentation on collections of L5 products will be made available via MAST. See Accessing WFI Data for more information on L5 products.
For additional questions not answered in this article, please contact the Roman Help Desk at STScI.