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

Adam RiessWFC3 ISR 2010-07

Count-rate non-linearity (a.k.a. the Bohlin Effect, reciprocity failureIs not the same effect as count 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)

Rate Dependent Non-LinearityProblem: 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

NIC2/ACS to WFC3 Cross-calibration

•All data processed using best current reference files (end 2009) as well ascontemporaneous P330 calibration star (all cameras, all filters). • Matched source list produced•NIC2 photometry already calibrated for rate-dep non-lin, ACS corrected for CTE

47 Tuc (2 positions)NIC2(F110W/F160W) WFC3(F110W/F160W)

NGC 3603WFC3 (F098M) ACS(F850lp)

~220 stars in common~1400 stars in common

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

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 (~ 2%)

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

% / dex

1.0 +/-0.2

1.1 +/-0.2

1.1 +/-0.3

47 Tuc NIC2 vs WFC3-IR (example, F110W), ~220 stars

Direct Comparison: NIC2 to WFC3, No color term

Model Residuals

Residuals vs. colorF110W=23 (Vega), sky limit

ACS F850lp vs. WFC3-IR F098MNGC 3603, 1000 stars

Straight difference, no color term

Model Residuals

Color residuals

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

lamp off lamp on - lamp off

NICMOS NIC2 F110W

NICMOS ISR 2006-001

0.042 mag/dex (dashed)

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)

NICMOS Count-Rate Dependent Non-linearity Revisited

Observed Count Rate (ADU/sec), NIC2 F110W

Original range

Sky Vega Mag P330E

lamp on vs offStellar phot.WFC3 vs NIC2

frac

tiona

l res

idua

l

Residualfrom 0.063mag/dexcalibrationshown

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

Power Law Non-linearity in HgCdTe

Observed Count Rate (ADU/sec), NIC2 F110W

Original range

Sky Vega Mag P330E

lamp on vs offStellar phot.WFC3 vs NIC2

frac

tiona

l res

idua

l

WFC3 vs NICMOS Persistence

NICMOS

WFC3

Our persistence (blue curve) is down by a factor of few from NIC, just like our non-linearity, which makes sense

Pers

iste

nce

Models (Smith et al 09) indicate persistence and cr non-linearity both result from trapping, degree should scale with trap density

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

Persistence and Non-LinearityPersistence=P(tillum,f illum)

Total prior illumination

Pers

iste

nce

Pers

iste

nce

Time

Since we see persistence in WFC3, we must have some degree of non-linearity

Time decay function (double exp or power law) Illumination Dependence (Fermi-Dirac)

(I have a tool to remove persistence usingthese functions)

Next epoch

21.0 (Vega)=2 ADU in F160W

=0.029 mag/dex

24-21=1.2 dex=0.035 mag extrapolation

25 (sky limited)=0.046 mag extrapolation

14

NICMOS Count-rate Non-linearity calibrated from 14 to 21st mag (F160W)

Synthetic Color Terms: NIC2 to WFC3-IR

I’d prefer more stars with colors 0.0-0.5 (rel P330E), but I propagate generous error of 0.05Good agreement with use of 47 tuc colors to measure, but less precision

P330E

color range of 47tucvd8