This article provides a summary of the calibration activities that are currently planed for the Roman Wide Field Instrument (WFI) during flight operations.

Introduction

Roman's WFI requires exquisite calibration to meet its requirements and some of these are at least an order of magnitude more stringent than what has been achieved with the Hubble Space Telescope and other space missions. While ground tests have achieved some baseline characterization (see the WFI Ground Testing Campaigns article), a robust program of calibration will need to take place in orbit to characterize the overall instrument sensitivity and response to the required accuracy. A notional on-orbit Calibration Plan has been devised; it currently estimates that about 6% of the mission time will be dedicated to calibration activities.

In broad terms, calibrations include several categories: 

• Internal calibrations will be carried out using the internal light sources in the Relative Calibration System (RCS) with the DARK optical element in place. These calibrations include the determination of the gain, dark current, pixel-level flat field, classic linearity, among others.
• External calibrations will be carried out via a combination of dedicated observations, including predefined reference fields (called Touchstone fields), and information extracted from regular science observations. These calibrations include flux calibration, point-spread-function (PSF) determination, large-scale flat fields, field color dependence, and geometric distortion. 
  
  • Touchstone Fields (S. Casertano et al. 2025, presentation) will be revisited regularly during the Mission, enabling high-quality characterization of the calibration stability. 
  • Regular Science Observations provide opportunities for many calibrations. As examples of the use of regular science observations to improve instrument calibration, the Galactic Bulge Time Domain Survey (GBTDS) will provide a rich source of information for PSF characterization, and the High Latitude Wide Area Survey (HLWAS) will provide useful input for large-scale flat field calibration via partially overlapping images. 
• Advanced Calibrations will be required to characterize some specific properties of the instrument. These include an ubercal-style calibration (e.g., following Padmanabhan et al. 2008) using science observations, needed to track the stability of the instrument response; a subpixel sensitivity campaign, needed to understand subtle variations in the instrument response to small shifts in source position; and the calibration for the count-rate dependent non-linearity. There may be other advanced calibration products beyond those listed here.

Both internal calibrations and external calibrations of reference fields (Touchstone fields) will require dedicated observing programs undertaken by the Science Operations Center (SOC) at STScI and the Roman Science Support Center (SSC) at IPAC. External calibrations from science surveys (GAS, CCS) will be be used for characterization activities by the SOC. The necessary calibration products will be generated by the SOC and applied to the data through the Roman WFI Data Pipelines. Advanced calibrations from science data will require accumulation of significant science data and will be generated from these data for use in data processing. This article provides an overview of internal and external calibration programs.

Summary of Planned Calibration Programs

The Table of Activities for the On-orbit Calibration Plan provides a summary of the Calibration Programs, their products, the calibration type, the cadence, and the estimated on-orbit observing time. The notional plan requires about 6% of the five-year prime mission (~113 days) to reach the calibration requirements that flow down from the science ones. This plan is the best-estimate plan, based on the current characterization of the performance and stability of the telescope and the instrument, as well as the current understanding of the likely science observations. The frequency and cadence for various types of detector calibrations will be adjusted based on the on-orbit performance and stability.

Calibration programs are grouped by their focus on one of three areas: Detector Calibrations, Imaging Calibrations, and Spectroscopic Calibrations. Many of the programs require specific data collections, where others can be performed on the science frames themselves or on large ensembles of science data. Some data collections can be used for multiple reference products. The current On-Orbit Calibration plan was developed with project partners and the Calibration Working Group. 

Calibration programs are grouped by their focus in one of three areas: Detector Calibrations, Imaging Calibrations, and Spectroscopic Calibrations. Future RDox articles will provide an in-depth discussion of each.

Internal Calibrations

Internal calibrations do not use light from outside of the WFI by placing the DARK element in the light path (see WFI Optical Elements). The internal calibration unit for the WFI is the Relative Calibration System (RCS), and the basic design of the RCS is given in the Description of the WFI article. 

The internal calibrations include:

• Imaging Darks
• Spectroscopic Darks
• Pixel-level flat field (p-flat)
• Sub-pixel level flat field
• Direct method for Count Rate Non-Linearity (CRNL)
• Read Noise and Read noise correlation
• Gain 
• Classic Non-Linearity (CNL) 
• Unstable Pixels
• Inter-Pixel Capacitance (IPC)
• Persistence
• Burn-in
• Inter-pixel non-linearities (brighter-fatter effect, BFE, and vertical trailing pixel effect, VTPE)

External Calibrations

External calibrations are taken without the DARK element in place. There are two methods of collection: (1) dedicated observations, including predefined reference fields called Touchstone fields or special exposure sequences using the RCS, and (2) calibrations derived from science surveys (GAS or CCS). Those calibrations derived from science surveys (GAS or CCS) are either derived on a per frame basis (like the ePSF or geometric distortion) or may require large ensembles of science data (like cross-survey calibration).

Touchstone Fields

Touchstone fields will be observed repeatedly over the course of the mission. The fields have been identified and the motivation for their selection is illustrated in the Roman WFI Calibration Touchstone Field Recommendations Document.

The following calibrations will use the touchstone fields:

• Small-scale Photometric Uniformity
• Bandpass Uniformity
• Spectrophotometric Flux Calibration
• Temporal Stability
• Cross-Survey Calibration
• Geometric Distortion
• PSF characterization and stability

Regular Science Observations

Some calibration requirements will need to be derived from science data, either in the science observations themselves or derived from large quantities of science data. 

The following calibrations will use regular science observations:

• PSF Characterization
• ePSF (for each science observation)
• Pointing Solution (for each science observation)
• LOLO method for Count Rate Non-Linearity (CRNL)

Advanced Calibrations

Advanced Calibrations will be required to characterize some specific properties of the instrument. Examples of advanced calibrations include an ubercal-style calibration (e.g., following Padmanabhan et al. 2008) using science observations, needed to track the stability of the instrument response; a subpixel sensitivity campaign, needed to understand subtle variations in the instrument response to small shifts in source position; and the calibration for the count-rate dependent non-linearity. Many of these will be performed in a self-calibration framework, while others will tie into other science datasets. Advanced calibrations are anticipated to require accumulations of large science datasets to be performed.

The advanced calibrations include, but are not limited to:

• Large Scale Photometric Uniformity
• Calibration of CRNL
• Subpixel sensitivity

For additional questions not answered in this article, please contact the Roman Help Desk.

References

Publications:

• "An Improved Photometric Calibration of the Sloan Digital Sky Survey Imaging Data", Padmanabhan et al. 2008
• "Will Gaia be precise enough to solve for the geometric distortion of the WFI?", Bellini 2018
• "2MASS/Gaia as an Absolute Astrometric Reference Frame", Bellini et al. 2017

Project Update Presentations and Reports:

• "Touchstone Field Report" presentation at the Roman Community Forum on February 26, 2025 by S. Casertano on behalf of the Calibration Working Group
• "Nancy Grace Roman Space Telescope: Wide Field Instrument Calibration Touchstone Field Recommendations" delivered March 31, 2025 by Williams et al. on behalf of the Calibration Working Group