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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. - PowerPoint PPT Presentation
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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
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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
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