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WFC3 slitless spectroscopy

WFC3 slitless spectroscopy. Harald Kuntschner Martin Kümmel , Jeremy Walsh (ST-ECF) Howard Bushouse ( STScI ) Grism Workshop, STScI November 15, 2010. WFC3. Filter wheels with WFC3 grisms. UV Channel. IR Channel. WFC3 Filters and Grisms.

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WFC3 slitless spectroscopy

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  1. WFC3 slitless spectroscopy Harald Kuntschner Martin Kümmel, Jeremy Walsh (ST-ECF) Howard Bushouse (STScI) Grism Workshop, STScI November 15, 2010

  2. WFC3 Filter wheels with WFC3 grisms UV Channel IR Channel

  3. WFC3 Filters and Grisms UV channel IR channel

  4. WFC3 grism parameters WFC3 Data Handbook version 2.1 (Cycle 18) IR FoV: 123” x 136” at 0.13”/pix UVIS FoV: 163” x 162” at 0.04”/pix

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

  6. 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

  7. G280 – Wavelength std WR14 The science spectra are extracted from the +1st order Which one is it actually? F300X G280 R≈70

  8. 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

  9. Extracting real WFC3 IR grismdata 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, in press; arXiv:1005.3071

  10. RegistrationTarget position – grism spectrum Superimposed direct + grism image • The position of the target (reference point) sets the full geometry of the spectral extraction • No shifts between direct and grism image! • Need for direct image when re-acquiring guide star Reference point: Xref, Yref

  11. WFC3 grism calibrations • Throughput of the instrument • Traces as function of 2D position • Wavelength solution as function of 2D position • Global background • … The calibrations are an integral part of the aXe software

  12. 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 +2nd order <=4% +2nd order <=7%

  13. 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

  14. 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

  15. G102 wavelength calibration PN Vy2-2

  16. 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

  17. 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.

  18. 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 See e.g. van Dokkum & Brammer 2010, ApJ, 718, 73 (two objects from ERS dataset) Atek et al. 2010, ApJ, 723, 104 (WISP survey) Straughn et al. 2010, AJ, in press; arXiv:1005.3071 (ERS dataset)

  19. See next talk 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 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

  20. 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/

  21. The Tutorial and the Cookbook Harald Kuntschner Martin Kümmel, Jeremy Walsh Grism Workshop, STScI November 15, 2010

  22. Aims • Practical example of a typical slitless data reduction with aXe • Challenge conception of “difficult slitless spectroscopy” • Explain main aXe concepts and applications • Provide some tips and tricks • Warning about common pitfalls

  23. The Cookbook • Did you install the software? • IRAF version 2.14 • STSDAS 3.12 with aXe • aXe 2.1 • aXeSIM 1.4 • aXe2web 1.2 • Step-by-step guide through a G141 grism data-reduction • Using the STSDAS PYRAF environment • aXe software package • Reduce (aXe) • Visualize (aXe2web) • Simulate (aXeSIM) • Data: WFC3 Early Release Science (ERS) II campaign (PID: 11359, PI: R. O’Connell) G141 observations in CDFS

  24. The data

  25. Extracting real WFC3 IR grismdata 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, in press; arXiv:1005.3071

  26. The reduction process

  27. Example instructions from Cookbook

  28. Cookbook Feedback • Please let us know what you • liked about the cookbook … • did not like … • Errors …? • What is missing …? • What else do you need to carry out your science? Tuesday, 9:00: Feedback session

  29. aXe source list co-added direct image direct images MultiDrizzle iolprep aXe data reduction Input Object Lists grism images direct images co-added direct image SExtractor axeprep Configuration & calibration files axecore source list drzprep manual modification axeprep aXe source list grism stamp images 1D spectra

  30. Visualization co-added direct image aXe source list grism stamp images 1D spectra aXe2web browsable html-pages

  31. ‘Helper’ slides for tutorial

  32. Object NamingBEAMS and spectral orders 0th order +1st order +2nd order +3rd order BEAM_1B BEAM_1A BEAM_1C BEAM_1D Object number from SExtractor catalogue e.g. BEAM_234A, BEAM_415A Configuration file: BEAM A

  33. Contamination Direct F140W image New Sources -1st order 0th order G141 Grism +2nd order +1st order +1st order +2nd order +3rd order

  34. aXe configuration and reference files

  35. aXe source list co-added direct image direct images MultiDrizzle iolprep aXe data reduction Input Object Lists grism images direct images co-added direct image SExtractor axeprep Configuration & calibration files axecore source list drzprep manual modification axeprep aXe source list grism stamp images 1D spectra

  36. Visualization co-added direct image aXe source list grism stamp images 1D spectra aXe2web browsable html-pages

  37. Output file types • .SPC.fits • Extracted 1-dim spectra • Multi extension binary FITS tables • .STP.fits • Extracted and rectified 2-dim spectra • Multi extension FITS images • Restricted usage and information • .MEF.fits • Extracted and fully rectified (drizzled) 2-dim spectra • Full spatial and wavelength coordinate system • Auxiliary info on errors, contamination …

  38. SPC.fits Multi ext. binary FITS tables 1-dim spectra

  39. STP.fits DS9 display of “BEAM_237A” Multi ext. FITS images 2-dim spectra Restricted use – only meant for quick display and visualization NOT FOR SCIENCE!

  40. MEF.fits SCI FITS images Fully calibrated 2-dim spectra CON “SIENCE READY”

  41. Predicted source traces • Contamination image ib6o23s0q_flt_2.CONT.fits • A typical 1000s exposure in G141 is “full” of traces • WFC3/G141 goes very deep!

  42. VisualizationHow to look at ~500 spectra? • aXe2web offers a convenient way to get an overview

  43. Simulations • Useful for • Proposal preparation (Phase 1) • Phase 2 submission • Post observation verification • Detection limits • Emission line limits

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