WFC3 In-Orbit Grism Performance

Harald Kuntschner (ST-ECF),Howard Bushouse (STScI),

Martin Kümmel (ST-ECF), Jeremy Walsh (ST-ECF)

STSCI 2010 Calibration WorkshopJuly 22, 2010

WFC3 grisms

• IR – G102 (805 - 1150 nm)• IR – G141 (1080 - 1690 nm)• UVIS – G280 (200 - 400nm)

SMOV + Cycle 17 calibration aims:• Establish 2-dim (in-orbit) trace, wavelength

and throughput calibrations• Characterization of stability, PSF etc.

SMOV +Cycle 17 calibr.

only Cycle 17 calibr.

G102 - Flux std GD153

lkjdsfklj

F098M

0th order +1st order +2nd order

G102R≈210

The science spectra are extractedfrom the +1st order

G141 - Flux std GD153

F140W

0th order +1st order +2nd order +3rd order

G141R≈130

The science spectra are extractedfrom the +1st order

WFC3 IR grism total throughput Peak 41% at 1100nm; >10% for 805 – 1150 nm Peak: 48% at 1450nm; >10% for 1080 -1690 nm

1st order

+2nd order <=4% +2nd order <=7%

1st order

Throughput compared to ground calibrations in TV3

0 - 10% more throughput consistent

with imaging data

TV3

G102

G141

G102 trace & wavelength calibration

• Target: Planetary Nebula HB12

• Many other (point) sources provide nice 2D trace coverage

GOAL: ~0.1 pix accuracy for all calibrations

Full WFC3 IR FoV

Field-dependent trace: G102Roughly linear traces

Significant variation ofoffset and slope with field position

Accuracy of trace: <0.2 pix

ST-ECF ISR WFC3-2009-17ST-ECF ISR WFC3-2009-18

Reference: Xref, Yref

G102 wavelength calibration

PN Vy2-2

G102: 2-dim dispersion calibration

• Roughly linear disp. solution; accuracy: <0.25 pixel• G102: Dispersion varies from 23.6 – 25.1 Å/pixel over FoV• G141: Dispersion varies from 45.0 – 47.7 Å/pixel over FoV

Wavelength Zeropoint Dispersion

Cross-dispersion PSFG102 G141

• IR grisms in focus and performing as good as in ground calibrations

• TV3SMOV

Imaging

Extracting ERS grism data in the CDFS

• 4 grism exposures – total of ~4200 seconds• ~ 500 spectra per grism can be extracted

F098M drizzled image G102 single grism

Persistence from preceding grism exposure Straughn et al. 2010, AJ submitted

See poster W9 by Martin Kümmel See persistence talk by Knox Long

Master sky background• High S/N master skies created from >100 publicly available WFC3 grism observ.• Average flux levels vary: G102 = 0.4 – 1.6 e/s; G141 = 0.9 – 2.4 e/s• Significant large scale structure as well as localized detector effects • Significant improvement of spectral extraction

G141 Master skySubtraction of scaled

global sky

Kümmel et al. , ISR in prep.

Before

After

See poster W9 by Martin Kümmel

Extracting spectra with aXe

Using a semi-automatic software (aXe)The software is already successfully being used for ACS + NICMOS grisms since 2003Direct image position is reference point (wavelength zero-point)Need for field dependent trace, dispersion and flat-field calibrationExtraction of source spectra and conversion to flux scale and uniform dispersion

See talk by Jeremy Walsh

Spectroscopic drizzling with aXe

Raw trace extraction

Single image drizzle (linear dispersion)

4 dithered images drizzled

GD153 with G102

WFC3 IR grism sensitivity

• Limiting magnitudes for 1h exposure, average background and S/N=5 in the continuum

• Emission line sources have been detected down to m(F140W)AB ≈ 24.5 in 2 orbits

WFC3 IR grism Magnitude

G102 JAB = 22.6

G141 HAB = 22.9

UVIS G280 grism – star WR14

+1st+2nd+3rd+4th

-1st-2nd

0th

Complex overlapping by many orders – very strong 0th orderTrace and dispersion solution show complex variation across FoV

G280 grism image

4096 pix

Conclusions

• IR grisms show high sensitivity and are well calibrated

• UVIS G280 shows complex overlapping and calibration for survey-use is very challenging

• aXe software provides semi-automatic means of extracting several 100 source spectra taking into account cross-contamination of sources

• All calibration and reference files published on the Web

http://www.stecf.org/instruments/WFC3grism/