WFC3 In-Orbit Grism Performance
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Transcript of WFC3 In-Orbit Grism Performance
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
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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/