WFC3 slitless spectroscopy

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WFC3 slitless spectroscopy

• Harald Kuntschner
• Martin Kümmel, Jeremy R. Walsh
• (ST-ECF)
• WFC3-team at STScI and NASA

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WFC3 Grism Parameters
Disperser Channel Wavelength Resolution
Å/pixel TiltX Length
range (Å)
(deg.) (pix)
G280 UVIS 2000-4000 70_at_3000
Å 13 -2.8 250 G102 IR
7800-10700 210_at_10000 Å 25
0.6 210 G141 IR
10500-17000 130_at_14000 Å 47 0.3
130 Resolution based on FWHM of Gaussian
core in cross-dispersion direction XTilt to
positive X axis Limits at 2 transmission
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WFC3-IR G141Ground calibration simulated single
point source
detector defects
Combined white light direct image
Target position on direct image
0th order
1st order Science 1.1 -1.7mu
2nd order
3rd order
1014 pixel (full size) 130 arcsec
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WFC3-IR G141 trace and dispersion calibration

• Linear trace solutions (but 2-dim variations)
• Linear dispersion solutions with rms lt 5 Å
• Dispersion 46.9 Å/pixel (130 pixel length)
• 2-dim solution varies from 45 to 48 Å/pixel

dispersion
Data points omitted in fit due to wavelength
shift caused by steep sensitivity decline
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WFC3-IR G141 Throughput

• 1st order
• Peak efficiency of 40 at 1420 - 1640nm
• Good sensitivity range 1120 - 1660nm
• 2nd order
• Efficiency lt 6
• 0th order
• Efficiency lt 1.5

0th 1st 2nd 3rd
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WFC3-IR G102Ground calibration simulated point
source
detector defects
Combined white light direct image
Target position on direct image
0th order
1st order Science 0.8 - 1.1mu
2nd order
1014 pixel (full size) 130 arcsec
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WFC3 G102 Trace and Dispersion

• Similar to G141 linear trace and dispersion
  relations well reproduced by a simple 2-dim
  solution
• Dispersion 24.6 Å/pixel
• (210 pixel length in 1st order)
• 2-dim solution ranging from 24.0 - 25.5 Å/pixel

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WFC3-IR G102 Throughput

• 1st order
• Peak efficiency of 30 at 960 - 1120nm
• Good sensitivity range 840 - 1140nm
• 2nd order
• Efficiency lt 5
• 0th order
• Efficiency lt 1.5

0th 1st 2nd
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WFC3-UV G280
Order overlap
0th
3rd
200nm
Combined monochromator steps
330nm
2nd
1st
0th order
1st
-1st
Higher orders
2nd
Higher orders
-2nd
4096 pixel (full size) 160 arcsec
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WFC3-UV G280

• Trace 5th order polynomial fits
• Dispersion 4th order polynomial needed to
  achieve good fit
• (rms 0.2 pixel)
• 13 Å per pixel
• At least 8 to -8 orders visible
• Order overlap between 1st and 2nd beyond 380nm
• Heavy order overlap for gt 2 orders

Trace
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WFC3-UV G280 Throughput

• 1st order
• Peak efficiency of 24 at 240nm
• 'Good' sensitivity range 200 - 330nm
• 2nd order
• Efficiency lt 2
• 0th order
• Efficiency equal to first order at 330nm and
  rising

0th order dominates
!
2nd 1st 0th -1st -2nd
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Limiting magnitudes

• V-band detection limits for point source, 1-hour
  exposure and S/N5
• See Instrument Handbook
• See also posters by Kümmel et al. and Pirzkal et
  al.

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How to design observations?

• F150W and IR G141 with M51 image, galaxy images
  and Gaussians (for HII regions and stars)

Simulated dispersed image
Simulated direct image
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The simulation package aXeSIM

• PYRAF package (ST-ECF Webpage, STSDAS in Jul
  08) designed and supported at ST-ECF
• One command to run full multi-object simulation
  simdata
• Simple object shape and spectra as built-in
  defaults
• Produces associated direct image (opt.)
• Performs default extraction of simulated spectra
  (opt.)
• Adds noise (opt.)

See poster by Kümmel et al.
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UDF simulations
Direct image
Dispersed image
F160W lt 23.0, noise free simulation to show
2-dim distribution
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Extraction software
http//www.stecf.org/

• ST-ECF offers a semi-automatic extraction
  software (aXe) to extract fully calibrated 1-dim
  spectra from WFC3 grism observations including
  contamination estimates
• The software is already successfully being used
  for ACS grisms since 2003
• Available as part of STSDAS and from ST-ECF Web
  pages

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Summary

• Highly efficient and well behaved IR grisms
• Open up wavelength not accessible from the
  ground!
• Challenging UV grism (order overlap, bent traces,
  strong 0th order)
• Software support with 2-dim simulation and data
  extraction packages, as well as general user
  support and calibration reference files available
  from ST-ECF (http//www.stecf.org/)
• Existing, good experience with ACS grisms and
  NICMOS HLA project