7.7 Count Rate Non-Linearity

Previous HgCdTe detectors on HST have suffered from a count-rate dependent non-linearity, motivating a search for a similar effect on the WFC3/IR detector. Utilizing a color term to account for differences in the observed bandpasses, statistically-significant non-linearity was detected in WFC3/IR photometry, a few times smaller than that observed in NICMOS. 

The direction (or sign convention) of the count-rate non-linearity is such that photometry of faint (i.e., sky dominated) sources calibrated with the published WFC3/IR zeropoints (derived from bright standard stars) will appear 0.04 +/-0.01 mag too faint. The calwf3 pipeline does not correct WFC3 IR data for count-rate non-linearity; an appropriate correction to stellar photometry will need to be made manually by the observer, if desired.

Combining all available measurements from direct images and those from the WFC3 grism, WFC3 ISR 2019-01 provides a consistent and improved characterization of WFC3 IR's count-rate dependent non-linearity of 0.0075 +/- 0.0006 per dex1, with no apparent wavelength dependence, measured across a dynamic range of more than a million in stellar apparent brightness. It compares the latest and best estimates to prior ones which also are summarized in the following paragraph. Prior results are consistent with but approximately four times less precise than the newer results, and hence we recommend the values in WFC3 ISR 2019-01.

The work prior to 2019 consisted of two studies. An initial measurement of the 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 (see WFC3 ISR 2010-07). Based on 235 stars in 47 Tuc observed with WFC3/IR in F110W and F160W and in similar bandpasses in NICMOS Camera 2, the non-linearity of WFC3/IR was measured at 0.011+/- 0.0023 and 0.010+/-0.0025 mag per dex, respectively, over a range of 10 magnitudes (4 dex). An independent measurement utilizing 1390 stars in NGC 3603 observed with ACS/WFC F850LP and WFC3/IR F098M yielded a very similar result, 0.010 +/- 0.0033 mag/dex. The consistency of the results from two different comparison detectors of different technology suggests the result is robust. Furthermore, an additional analysis using 24 bright stars observed by WFC3 and by 2MASS found that the count rate non linearity of the WFC3 IR detector is less than or equal to 0.01 mag per dex (WFC3 ISR 2011-15).

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