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. 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 (see WFC3 ISR 2010-07). Utilizing a color term to account for differences in the observed bandpasses, statistically-significant non-linearity was detected in WFC3/IR photometry in the same direction but a few times smaller than that observed in NICMOS. 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 is measured at 0.011+/- 0.0023 and 0.010+/-0.0025 mag per dex 1 , 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 yields a very similar result, 0.010 +/- 0.0033 mag/dex. The consistency of this measurement from two different comparison detectors of different technology suggests this result is robust. An additional analysis of the WFC3 IR count rate non linearity was carried out using observations of 24 brights stars that were also acquired by 2MASS( WFC3 ISR 2011-15).This work found that the count rate non linearity of the IR detector is less than 0.01 mag per dex, consistent with the earlier findings.

The impact of the count-rate non-linearity is 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.

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.