Testing and Validation of the Updated Pixel-Based Non-Linearity Calibration File for WFC3/IR

Abstract

The WFC3\IR channel has an innate non-linear response to incident photons, which is corrected for in the calwf3 pipeline with the NLINFILE reference file. The 2009 solution is based on an average polynomial correction for each IR quadrant and is found to be poorly constrained at high fluence levels (e-) approaching the saturation limit. Using a variety of image types, sources, and sample sequences, we test a new pixel-based linearity correction developed by Shenoy et al. (2025). In nearly all cases, the new correction improves the linearity at fluence levels higher than 50,000 e-, with improvements up to 7% for pixels with fluences approaching the saturation limit (80,000 e-) in the last ima reads. The pixel-based solution also significantly decreases the number of cosmic rays erroneously flagged (due to non-linearity correction errors) during ramp fitting in calwf3, leading to improved photometric accuracy in the calibrated flt data and higher signal-to-noise ratios, particularly in Quad 1 (upper-left detector quadrant). Because the new solution tends to make sources brighter, we recalibrate the five HST flux standards used to compute the IR zeropoints and find a negligible impact (0.1-0.2%) on the published values by Calamida et al. (2024), smaller than the RMS dispersion (0.5%) in the observed to synthetic flux ratios for all five flux standards. The new NLINFILE 9au15283i lin.fits was delivered to CRDS in October 2025 and will be used to reprocess all WFC3/IR imaging and grism observations in the MAST archive. An updated reference file a2412448i lin.fits was delivered in February 2026, improving the results at the highest fluence levels by a few tenths of a percent. Please consult the Addendum for details.

Figures (22)

• Figure 1: CRHIT DQ flags in the last ima read, e.g. [DQ, 1], for internal flat fields (INTFLATS) acquired with different sample sequences: SPARS50 (ibvl06snq) and SPARS25 (ibmg01iwq). For each INTFLAT, the left panel is calibrated with the 2009 quad-based NLINFILE and the right panel with the new pixel-based NLINFILE. Yellow points show the position of cosmic ray flags (DQ $\geq$ 8192) populated by calwf3 during the up-the-ramp fit. Each panel reports the percentage of detector pixels flagged, with the new solution flagging two times fewer pixels in the SPARS50 INTFLAT, and three times fewer pixels in the SPARS25 INTFLAT. With the new solution, the number of flagged pixels is now similar in all four quadrants.
• Figure 2: Mean ratio between the ima instantaneous count rate and the flt count rate vs. the mean fluence level ($e^-$) for a set of INTFLATs taken in SPARS50 sample sequence. Dashed lines are INTFLATS calibrated with the 2009 NLINFILE, while solid lines show ramps calibrated with the new NLINFILE. The last four reads are saturated and excluded from the plot. The new correction reduces the peak-to-peak range from $\sim3\%$ to $\sim1\%$ across all fluence levels.
• Figure 3: Same as Figure \ref{['fig:intflat_spars50']}, but for a set of SPARS25 INTFLATS. The last three reads are saturated and excluded from the plot. The new correction reduces the peak-to-peak range from $\sim$6--7$\%$ to $\sim2\%$, representing the largest improvement observed across all sample sequences tested.
• Figure 4: Same as Figure \ref{['fig:intflat_spars50']}, but for a set of SPARS10 INTFLATS that sample low fluence levels. The peak-to-peak range is 1̃% across all fluence levels for both the old and the new corrections.
• Figure 5: Same as Figure \ref{['fig:intflat_spars50']}, but for a set of STEP25 INTFLATS. The peak-to-peak range over all fluence levels is reduced from 5% to 2% when using the new linearity correction.