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

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

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

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

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

10. Registration Target 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 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 WFC
ST-ECF ISR WFC
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
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: (ERS dataset)

19. Extracting spectra with aXe
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

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

23. 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

24. Did you install the software?
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

25. The data

26. Extracting real WFC3 IR 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, in press; arXiv:

27. The reduction process

28. Example instructions from Cookbook

29. Tuesday, 9:00: Feedback session
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

30. aXe data reduction 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

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

33. ‘Helper’ slides for tutorial

34. Object Naming BEAMS and spectral orders
0th order st order nd order rd order
BEAM_1B BEAM_1A BEAM_1C BEAM_1D
Object number from SExtractor
catalogue
e.g. BEAM_234A, BEAM_415A
Configuration file:
BEAM A

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

36. aXe configuration and reference files
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

37. aXe data reduction 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

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

39. Output file types .SPC.fits .STP.fits .MEF.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 …

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

41. Restricted use – only meant for quick display and visualization
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!

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

43. 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!

44. Visualization How to look at ~500 spectra?
aXe2web offers a convenient way to get an overview

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