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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 Workshop July 22, 2010. WFC3 grisms. IR – G102 (805 - 1150 nm) IR – G141 (1080 - 1690 nm) UVIS – G280 (200 - 400nm)

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WFC3 In-Orbit Grism Performance

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  1. WFC3 In-Orbit Grism Performance Harald Kuntschner (ST-ECF), Howard Bushouse (STScI), Martin Kümmel (ST-ECF), Jeremy Walsh (ST-ECF) STSCI 2010 Calibration Workshop July 22, 2010

  2. 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.

  3. G102 - Flux std GD153 • lkjdsfklj 0th order +1st order +2nd order The science spectra are extracted from the +1st order F098M G102 R≈210

  4. G141 - Flux std GD153 0th order +1st order +2nd order +3rd order The science spectra are extracted from the +1st order F140W G141 R≈130

  5. 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 1st order +2ndorder <=4% +2ndorder <=7%

  6. Throughput compared to ground calibrations in TV3 G141 G102 0 - 10% more throughput consistent with imaging data TV3

  7. 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

  8. Field-dependent trace: G102 Roughly linear traces Significant variation of offset and slope with field position Accuracy of trace: <0.2 pix ST-ECF ISR WFC3-2009-17 ST-ECF ISR WFC3-2009-18 Reference: Xref, Yref

  9. G102 wavelength calibration PN Vy2-2

  10. G102: 2-dim dispersion calibration Wavelength Zeropoint Dispersion • 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

  11. Cross-dispersion PSF G102 G141 • TV3 • SMOV Imaging • IR grisms in focus and performing as good as in ground calibrations

  12. See persistence talk by Knox Long See poster W9 by Martin Kümmel 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 Straughn et al. 2010, AJ submitted Persistence from preceding grism exposure

  13. See poster W9 by Martin Kümmel 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 sky Before Subtraction of scaled global sky After Kümmel et al. , ISR in prep.

  14. See talk by Jeremy Walsh Extracting spectra with aXe • Using a semi-automatic software (aXe) • The software is already successfully being used for ACS + NICMOS grisms since 2003 • Direct image position is reference point (wavelength zero-point) • Need for field dependent trace, dispersion and flat-field calibration • Extraction of source spectra and conversion to flux scale and uniform dispersion

  15. Spectroscopic drizzling with aXe GD153 with G102 • Raw trace extraction • Single image drizzle (linear dispersion) • 4 dithered images drizzled

  16. 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

  17. UVIS G280 grism – star WR14 4096 pix G280 grism image +4th +3rd 0th +2nd -1st +1st -2nd Complex overlapping by many orders – very strong 0th order Trace and dispersion solution show complex variation across FoV

  18. 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-contaminationofsources • All calibration and reference files published on the Web http://www.stecf.org/instruments/WFC3grism/

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