This article provides a brief summary of the calibration activities that are currently planed for the Roman WFI during flight operations.

Introduction

Roman's WFI requires exquisite calibration to meet its requirements and some of these are an order of magnitude more stringent than what has been achieved with the Hubble Space Telescope and other space missions. Ground tests notwithstanding (see the WFI Ground Testing Campaigns article), a robust program of calibration will need to take place in orbit, to characterize the 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 RCS with the dark 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, PSF determination, large-scale flat fields, field color dependence, and geometric distortion. 
  
  • Touchstone fields 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 at STScI (SOC) and the Roman Science Support Center (SSC) at IPAC. External calibrations from regular science exposures 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 STScI 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 will provide 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 prime mission time to reach mission calibration requirements that flow down from science requirements. 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 on-orbit performance and stability.

Each row in the table represents a unique calibration program from which more than one calibration product can be derived. Many of the programs require specific data collections, where as others can be performed on the science frames themselves or on large ensembles of science data. The current On-Orbit Calibration plan was developed with project partners and the Calibration Working Group. 

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 WFI is the 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
• Gain 
• Classic Non-Linearity (CNL) 
• Unstable Pixels
• Inter-Pixel Capacitance (IPC)

External Calibrations

External calibrations are taken without the DARK element in place. There are two forms: (1) dedicated observations, including predefined reference fields called Touchstone fields, and (2) calibrations derived from regular science observations, which could be done on a per frame basis or could use large ensembles of science data.

Touchstone Fields

Touchstone fields are fields that will be observed repeatedly over the course of the mission. As of writing, candidate fields have been identified, but final selections of the Touchstone fields have not been made. Some considerations for the field include the stellar density across the WFI field-of-view and the availability of the field at different times in the mission. 

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 at STScI.

References

• "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