First On-orbit Calibration of WFC3-IR Count Rate -Dependent Non-Linearity

1. First On-orbit Calibration of WFC3-IR Count Rate-Dependent Non-Linearity cr(x,y)  f(x,y), cr is observed count rate f is the flux on pixel, source+sky  ≥ 1 (=1 for no effect) Adam Riess WFC3 ISR 2010-07 Count-rate non-linearity (a.k.a. the Bohlin Effect, reciprocity failure Is not the same effect as count non-linearity!

2. Rate Dependent Non-Linearity Problem: Zeropoints calibrated with standard stars at m=12. sky-dominated sources* (e.g., UDF) are ~5 dex (105) fainter NICMOS had a rate-dependent non-linearity of 0.063 mag/dex (F110W) and 0.029 mag/dex (F160W), so photometry errors of ~0.3 mag result if uncorrected! In 2006 NICMOS used “count-rate boosting” (i.e., lamps) to fully calibrate the effect. WFC3 cannot observe with lamps so will use the bright Earth limb (GO 11933 July 23rd) to do same. However, we can compare the WFC3-IR linearity now to NIC2/ACS using photometry of identical sources and comparable filters. *sky limits minimum count rate to m~23 for WFC3-IR, m~24 for NICMOS

3. NIC2/ACS to WFC3 Cross-calibration 47 Tuc (2 positions) NIC2(F110W/F160W) WFC3(F110W/F160W) NGC 3603 WFC3 (F098M) ACS(F850lp) ~1400 stars in common ~220 stars in common • All data processed using best current reference files (end 2009) as well as • contemporaneous P330 calibration star (all cameras, all filters). • Matched source list produced • NIC2 photometry already calibrated for rate-dep non-lin, ACS corrected for CTE

4. Stellar Photometry • All photometry from small, 0.2” apertures, measured relative to P330E • Color terms to transform, e.g.: NIC(F110W)-WFC3(F110W) NIC(F110W)-NIC(F160W) between P330E and cluster stars calibrated using appropriate Castelli and Kurucz (2004) models (for 47 tuc Z=-0.2, gravity=g50) and calspec of P330E with synphot • Solve for residual color dependence

5. Cross-calibration Model NIC or ACS (mag)=a0*WFC3 (mag) +Δa1[NIC or ACS Color (mag)] • a0 measures WFC3-IR non-linearity relative to NIC/ACS (from P330E to sources) cr=flux, =2-a0. Δa1 =residual color correction after synphot transform • Multi-linear regression, 2 free parameters. Clipped points C2 > 2.52 (~ 2%) % / dex 1.0 +/-0.2 1.1 +/-0.2 1.1 +/-0.3 Non-linearity detected >4 sigma. Factor of ~4x smaller than NICMOS. Result: Photometry of sky-dominated sources (m>23) expected to be measured too faint by 0.04 +/ 0.01 mag

6. 47 Tuc NIC2 vs WFC3-IR (example, F110W), ~220 stars Direct Comparison: NIC2 to WFC3, No color term Model Residuals F110W=23 (Vega), sky limit Residuals vs. color

7. ACS F850lp vs. WFC3-IR F098MNGC 3603, 1000 stars Straight difference, no color term Model Residuals Color residuals

8. What Next? Count-Rate Boosting • Bright Earth skimming observations with WFC3 • (like NICMOS lamp on/off), cleanest approach (model independent), should get strong result, scheduling July 23rd NICMOS NIC2 F110W lamp on - lamp off lamp off NICMOS ISR 2006-001

9. NICMOS Count-Rate Dependent Non-linearity Revisited Sky Vega Mag P330E Residual from 0.063 mag/dex calibration shown Stellar phot. WFC3 vs NIC2 lamp on vs off fractional residual 0.042 mag/dex (dashed) Original range Observed Count Rate (ADU/sec), NIC2 F110W NICMOS Count-rate Non-linearity model-independent calibration (F110W), 3 dex, to be 0.063 mag/dex (see ISR 2006-001, all cycles’s data now combined to extend 1 dex) Calibration extended by WFC3 (via ACS CCD) from 17 to 22 mag F110W (ISR 2010-007) non-linearity correction remains consistent to σ=0.03 mag at faint end (10 to 22rd mag)

10. Summary • We have measured the count-rate dependent non-linearity of WFC3-IR to be 1% +/- 0.2% per dex, insensitive to wavelength, by comparing stellar photometry over 3 dex down to the sky limit • We will re-measure this by boosting the count rate of a stellar field with bright Earth limb light later this month • Using WFC3-IR cross-calibrated via ACS, we affirm the calibration of the NIC2 count-rate dependent non-linearity down to near the sky (0.1 ADU/sec/pixel or F110W=22 mag) to 0.03 mag, good agreement with Bob Hill’s lab measurements of 3 different HgCdTe devices • Non-linearity splinter session at 3pm Friday, N310

11. Power Law Non-linearity in HgCdTe Sky Vega Mag P330E Stellar phot. WFC3 vs NIC2 lamp on vs off fractional residual Original range Observed Count Rate (ADU/sec), NIC2 F110W

12. WFC3 vs NICMOS Persistence Models (Smith et al 09) indicate persistence and cr non-linearity both result from trapping, degree should scale with trap density Persistence NICMOS WFC3 Our persistence (blue curve) is down by a factor of few from NIC, just like our non-linearity, which makes sense

13. Persistence and Non-Linearity:Catch and Release Count-rate Non-linearity and persistence appear to arise from the same phenomenon of charge trapping (Smith et al. 2007), former from capture, latter from the release. WFC3 persistence measured from stars in well dithered 47 Tuc Illumination Subsequent Image minus registered, prior image

14. Persistence and Non-Linearity (I have a tool to remove persistence using these functions) Persistence=P(tillum,f illum) Time decay function (double exp or power law) Illumination Dependence (Fermi-Dirac) Persistence Persistence Total prior illumination Time Since we see persistence in WFC3, we must have some degree of non-linearity

15. Next epoch

16. NICMOS Count-rate Non-linearity calibrated from 14 to 21st mag (F160W) =0.029 mag/dex 24-21=1.2 dex =0.035 mag extrapolation 25 (sky limited) =0.046 mag extrapolation 14 21.0 (Vega) =2 ADU in F160W

17. Synthetic Color Terms: NIC2 to WFC3-IR P330E color range of 47tuc vd8 I’d prefer more stars with colors 0.0-0.5 (rel P330E), but I propagate generous error of 0.05 Good agreement with use of 47 tuc colors to measure, but less precision