STScI Calibration Workshop July 2010 Slitless Spectroscopy with HST Instruments Jeremy Walsh, Martin Kümmel & Harald Kuntschner, ST-ECF Former group contributors: Soeren Larsen, University of Utrecht Anna Pasquali, MPIA, Heidelberg Nor Pirzkal, STScI

HST slitless instrument modes - summary InstrumentOrbital lifeDisperser R λWavelength WFPC G200 grism G450 grism G800 grism 1800Å 4500Å 8000Å Å Å Å FOC FUVOP prism NUVOP prism 1200Å 2500Å Å Å STIS 1997-presentNUV prism All 1 st order gratings e. g. G750L grating 1200Å 8000Å Å Å NICMOS1997-presentG096 grism G141 grism G206 grism 1.0μm 1.7μm 2.0μm μm μm μm ACS2002-presentPR110L grism PR130L grism PR200L grism G800L grism 1500Å 1500Å 2500Å 8000Å Å Å Å Å WFC32009-presentG280 grism G102 grism G141 grism 3000Å 1.04μm 1.30μm Å μm μm

Elements of slitless spectroscopy No slit(s) – each dispersed object forms its own ‘virtual’ slit Effective spectral resolution depends on object ‘size’ in dispersion direction Multiple spectral orders (grism, not prism) Spectra can overlap → contamination Background integrated over whole disperser passband (with gradients in dispersion direction), different from filters Each slitless spectrum must have λ- calibration to be flat fielded WFC3 G141 WFC3 G141 median sky

Reduction strategy ElementReduction approach Object ≡ slitPositions, sizes and shapes of objects on companion direct image define slits → input object catalogue Object size ↔ spectral resolutionSlit width (height) defined by object size on dispersion (X-dispersion) axis. Convolve sensitivity with object size. Multiple spectral ordersDetermine positions (trace), wavelength calibrations and sensitivities for all orders relative to object reference position ContaminationFlag spectrum pixels overlapped by other spectra; estimate fractional contamination for all orders (model by 2D Gaussian or surface photometry) Background structureRemove background with an image formed from median sky Object-specific flat fieldingFrom the assigned wavelength of each pixel apply correction from a flat field cube F(x,y,λ)

Calibrations, calibrations Reliance on automatic spectral extraction process imposes strong demands on calibration (initially ground, amended by in-orbit). Field variations allowed for Position and trace of spectra from stars Wavelength calibration (WR stars and AGN [ACS], PN [NICMOS and WFC3]) over whole field Sensitivity from spectrophotometric standard stars (to <5%, aim 2%) Flat field coefficient cube F(x,y,λ) - established on- ground with monochromatic flats, supplement by in-orbit filter flats for large scale illumination. Applied as polynomial fit of variation of normalised flat field v. λ Grism (sky) background image from median combination of many grism images for global removal of background See talk by H. Kuntschner on WFC3 NIR grism calibration

See poster W9 by M. Kümmel on WFC3 NIR grism reduction Instrument independent package – all instrument parameters from set of files Configuration file specifying trace, dispersion and files: –Flat field cube –Background image –Sensitivity v. λ per order Originally developed for ACS, then applied to NICMOS (G141) and WFC3 HLA releases of NICMOS G141 and ACS G800L spectra Also applied to VLT FORS2 MXU data and Euclid (simulations) Slitless extraction software Simulations essential for software development and testing and as a proposal tool (esp. contamination mitigation strategies). Code derived from aXe contamination flagging Same configuration file as aXe provides instrument specificity Reduce simulations as real data Employ simulated slitless images for completeness estimation, S/N determination, etc.

Realities of slitless spectra Although several grism orders present, including – ve orders, no scientific exploitation other than +1 st order to date Zeroth order dispersed by the prism on which grating is ruled On account of large angular offset, higher orders are increasingly out of focus Multiple rolls are always helpful to ensure some uncontaminated spectra, but beware of combining spectra of extended objects at different rolls The ‘virtual’ slit for an extended object is not along the major axis of the target (see Freudling et al. 2008, for correction) Sensitivity established on point sources only, correction required for extended sources, otherwise 1D spectra have ‘ears’ Same target PAs differ by 122˚

Developments Use of drizzle software (with bad pixel rejection) to combine dithered spectra Cross correlate stellar spectra against templates to derive mean improvement to wavelength zero point per grism image (implemented in HLA ACS grism project) What to do about contamination? Contaminating spectra based only on photometry so spectrum crude. Subtracting the contamination may not be justified. Might consider iterative approach Slitless spectra of complex objects may look confusing; but restoration techniques bring gains. Extra information of object positions can be used to decompose point source(s) spectra from complex extended object spectra

Example is Lucy iterative decomposition of 10 strong lens knots in a field elliptical galaxy (ACS data, Blakeslee et al. 2004). Modification of IRAF task stecf.specinholucy for slitless spectra

Summary Slitless spectroscopy with HST is very sensitive – low background from space, high spatial resolution, compact PSF, efficient grisms Slitless spectroscopy not a ‘difficult’ technique. With care automatic extraction of thousands of spectra achievable HLA ACS G800L archive of spectra released – reduced with fully automatic pipeline PHLAG from archive download of raw data to science ready 1D and 2D spectra (and images) Look forward to a surge of slitless grism science from WFC3 NIR

Extra slides

Low-redshift star-burst galaxyM-star BAL QSO at z=2.81 Bright elliptical at z= ” Examples previews

Flux std star GD153 SMOV data G102 G141

Euclid (ESA Dark Energy Mission) Slitless Simulations with aXeSim End-to-end simulations of a spectroscopic slitless survey to measure DE equation of state parameters with Baryonic Acoustic Oscillations: Determination of limiting flux in Hα line and continuum Determination of redshift success rate (confusion) Determination of redshift accuracy Verify science requirements (cosmological parameters, additional science) ‣ Drive instrument requirements/design and survey strategy ‣ Identify critical areas (e.g. data reduction and analysis) Assess impact on science goals of slitless vs DMD-multi-slit approach (DMD optional mode subject to technological readiness) Results regularly reported to Euclid Study Science Team and presented in the Euclid yellow book (ESA Cosmic Vision M-class missions presentation event, Dec 1st 2009) ECF collaboration with Bologna, Milan, Durham (as part of NearIR Spectrograph consortium lead by A.Cimatti)