Science with the new HST after SM4

WFC3 slitless spectroscopy

Harald Kuntschner

Martin Kümmel, Jeremy R. Walsh

(ST-ECF)

WFC3-team at STScI and NASA

Science with the new HST after SM4

WFC3 Grism Parameters

Disperser Channel Wavelength Resolution* Å/pixel TiltX Length range (Å) (deg.) (pix)

G280 UVIS 2000-4000 70@3000 Å 13 -2.8 250

G102 IR 7800-10700+ 210@10000 Å 25 +0.6 210

G141 IR 10500-17000+ 130@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

Science with the new HST after SM4

WFC3-IR G141Ground calibration; simulated single point source

1014 pixel (full size)~130 arcsec

detector defects

Combined white light + direct image

Target positionon direct image

0th order +1st orderScience1.1 -1.7mu

+2nd order +3rd order

Science with the new HST after SM4

WFC3-IR G141 trace and dispersion calibration

• Linear trace solutions (but 2-dim variations)

• Linear dispersion solutions with rms < 5 Å

• Dispersion: 46.9 Å/pixel (130 pixel length)

• 2-dim solution varies from 45 to 48 Å/pixel

* Data points omitted in fit due to wavelength shift caused by steep sensitivity decline

disp

ersi

on

Science with the new HST after SM4

WFC3-IR G141 Throughput

• +1st order– Peak efficiency of ~40% at

1420 - 1640nm

– Good sensitivity range: 1120 - 1660nm

• +2nd order– Efficiency < 6%

• 0th order– Efficiency < 1.5%

0th +1st +2nd +3rd

Science with the new HST after SM4

WFC3-IR G102Ground calibration; simulated point source

Target positionon direct image

0th order +1st orderScience0.8 - 1.1mu

+2nd order

1014 pixel (full size)~130 arcsec

detector defectsCombined white light + direct image

Science with the new HST after SM4

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

Science with the new HST after SM4

WFC3-IR G102 Throughput

• +1st order– Peak efficiency of ~30% at

960 - 1120nm

– Good sensitivity range: 840 - 1140nm

• +2nd order– Efficiency < 5%

• 0th order– Efficiency < 1.5%

0th +1st +2nd

Science with the new HST after SM4

WFC3-UV G280

0th order

+1st -1st

4096 pixel (full size)160 arcsec

+2nd -2nd

Combined monochromatorsteps

+1st+2nd

+3rdOrder overlap

Higher ordersHigher orders

200nm330nm

0th

Science with the new HST after SM4

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 > +2

orders

Trace

Science with the new HST after SM4

WFC3-UV G280 Throughput

• 1st order– Peak efficiency of 24% at

240nm

– 'Good' sensitivity range: 200 - 330nm

• 2nd order– Efficiency < 2%

• 0th order– Efficiency equal to first

order at 330nm and rising

+2nd +1st 0th -1st -2nd

0th order dominates

Science with the new HST after SM4

Limiting magnitudes

V-band detection limits for point source, 1-hour exposure and S/N=5

See Instrument HandbookSee also posters by Kümmel et al. and Pirzkal et al.

Science with the new HST after SM4

How to design observations?F150W and IR G141 with M51 image, galaxy images

and Gaussians (for HII regions and stars)

Simulated direct image Simulated dispersed image

Science with the new HST after SM4

See poster by Kümmel et al.

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.)

Science with the new HST after SM4

UDF simulationsDirect image Dispersed image

F160W < 23.0, “noise free” simulation to show 2-dim distribution

UV G280 - subarray, not to scale

Science with the new HST after SM4

Extraction software

• 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

http://www.stecf.org/

Science with the new HST after SM4

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