WFC3 Data Handbook V. 4.0

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7.6	Count Rate Non-Linearity

Previous HgCdTe detectors on HST have suffered from a count-rate dependent non-linearity, motivating an investigation of a similar effect on the WFC3/IR detector. An initial measurement of this effect was made by comparing the photometry of star clusters observed over a wide dynamic range and at overlapping wavelengths in WFC3/IR and NICMOS and/or ACS-WFC. Utilizing a color term to account for differences in the observed bandpasses, we find a significant detection of a non-linearity in WFC3/IR photometry which is in the same direction but a few times smaller than that of NICMOS. From 235 stars in 47 Tuc observed with WFC3/IR in F110W and F160W and in similar bandpasses in NICMOS Camera 2, we measure a non-linearity of WFC3/IR of 0.011+/- 0.0023 and 0.010+/-0.0025 mag per dex¹, respectively, over a range of 10 magnitudes (4 dex). An independent measurement utilizes 1390 stars in NGC 3603 observed with ACS/WFC F850LP and WFC3/IR F098M and yields a very similar result, 0.010 +/- 0.0033 mag/dex. The consistency of this measurement from two different comparison detectors of different technology indicates this result is robust. The impact of this non-linearity is that photometry of faint (i.e., sky dominated) sources calibrated with WFC3/IR zeropoints will appear 0.04 +/-0.01 mag too faint.

An analysis of the WFC3 IR count rate non linearity was carried out using observations of 24 brights stars that were also acquired by 2MASS. This work found that the count rate non linearity of the IR detector is less than 0.01 mag per dex (see WFC3 ISR 2011-15 for the details)

1

One dex is a factor of 10. Thus 1 mag per dex means a difference of 1 magnitude per factor of 10 in brightness.