WFC3 slitless spectroscopy

Harald KuntschnerMartin Kümmel, Jeremy Walsh (ST-ECF)

Howard Bushouse (STScI)

Grism Workshop, STScINovember 15, 2010

WFC3

UV Channel

IR Channel

Filter wheels with WFC3 grisms

WFC3 Filters and Grisms

UV channel IR channel

WFC3 grism parameters

WFC3 Data Handbook version 2.1 (Cycle 18)

IR FoV: 123” x 136” at 0.13”/pixUVIS FoV: 163” x 162” at 0.04”/pix

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

G280 – Wavelength std WR14

F300X G280R≈70

The science spectra are extractedfrom the +1st order

Which one is it actually?

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

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

RegistrationTarget position – grism spectrum

Reference point: Xref, Yref

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

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 integralpart of the aXe software

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

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

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

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

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)

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 next talk by Martin Kümmel

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/

The Tutorial and the Cookbook

Harald KuntschnerMartin Kümmel, Jeremy Walsh

Grism Workshop, STScINovember 15, 2010

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

• 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

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

The data

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

The reduction process

Example instructions from Cookbook

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

direct images

MultiDrizzle

co-added direct image

SExtractor

source list

manual modification

aXe source list

aXe source list co-added direct image

iolprep

Input Object Lists grism images direct images

axeprep

Configuration &calibration files

axecore

drzprep

axeprep

grism stamp images 1D spectra

aXe data reduction

co-added direct image grism stamp images 1D spectraaXe source list

aXe2web

browsable html-pages

Visualization

‘Helper’ slides for tutorial

Object NamingBEAMS and spectral orders

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

BEAM_1B BEAM_1A BEAM_1C BEAM_1D

Configuration file:BEAM A

Object number from SExtractorcatalogue

e.g. BEAM_234A, BEAM_415A

Contamination

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

New Sources

Direct F140W image

G141 Grism

0th o

rder

-1st order

+1st order

+2 nd order

aXe configuration and reference files

File Type

WFC3.IR.G141.V1.0.conf Configuration file (instrument setup)

WFC3.IR.G141.1st.sens.1.fits Throughput for +1st order

WFC3.IR.G141.1st.sens.2.fits Throughput for +2nd order

… …

WFC3.IR.G141.flat.fits Flat-field cube

WFC3.IR.G141.sky.V1.0.fits Master sky-background

direct images

MultiDrizzle

co-added direct image

SExtractor

source list

manual modification

aXe source list

aXe source list co-added direct image

iolprep

Input Object Lists grism images direct images

axeprep

Configuration &calibration files

axecore

drzprep

axeprep

grism stamp images 1D spectra

aXe data reduction

co-added direct image grism stamp images 1D spectraaXe source list

aXe2web

browsable html-pages

Visualization

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 …

SPC.fits

Multi ext. binary FITS tables

1-dim spectra

STP.fits

Multi ext. FITS images

2-dim spectra

DS9 display of “BEAM_237A”

Restricted use – only meant for quick display and visualizationNOT FOR SCIENCE!

MEF.fits

FITS images

Fully calibrated2-dim spectra

SCI

CON

“SIENCE READY”

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!

VisualizationHow to look at ~500 spectra?

• aXe2web offers a convenient way to get an overview

Simulations

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