Science Capabilities of the Cosmic Origins Spectrograph

							The Cosmic Origins Spectrograph




                   James C. Green
                University of Colorado
                With a little help from:
•   COS Core Team                  •   Government and Industry
     – Cynthia Froning                  – Hsiao Smith
          • Project Scientist           – Francis Cepollina
     – Steven Osterman                  – Dave Leckrone
          • Instrument Scientist        – Preston Burch
     – J. Michael Shull                 – Malcolm Needner
     – John Stocke                      – Don Hood
     – Theodore Snow                    – Rick Higgins
     – Jeffrey Linsky                   – Brian Osborne
     – Dennis Ebbets                    – Tom Delker
     – Oswald Siegmund                  – Mark Erikson
     – Barry Welsh                      – Mark LaPole
     – Jason McPhate                    – Ed Shade
     – Stephane Beland                  – Jean Flammand
     – Steven Penton                    – Francis Bonne-Mason
     – Kevin France                     – Bruno Touzet
     – Eric Burgh
     – Charles Danforth            •   Special Thanks to:
     – Brian Keeney                     – Jon Morse
     – Lisa Winter                      – Erik Wilkinson
     – Yangsen Yao                      – John Andrews
     – David Sahnow                     – Ken Brownsberger
   COS Performance Philosophy
• Maximum sensitivity with adequate
  spectral resolution
• Sensitivity depends on both:
  –Large signal (large effective area)
  –Low noise (low scatter gratings, low
   background detectors )
                            COS Optical Layout
 Calibration                                    OSM2: G185M, G225M,      NUV MAMA
  Platform                                      G285M, G230L, TA1          Detector
                                                                         (STIS spare)

 FUV XDL
  Detector



Aperture Mechanism:
Primary Science Aperture,
Bright Object Aperture
                                                                      OSM1: G130M,
                                                                      G160M, G140L,
    • COS has 2 channels to provide low and medium                    NCM1

      resolution UV spectroscopy                                       Optical bench
                                                                        (not shown):
           – FUV: 1150-1775Å, NUV: 1700-3200Å                         re-use of GHRS
                                                                           bench
    • FUV gratings: G130M, G160M, G140L
    • NUV gratings: G185M, G225M, G285M, G230L
           – M gratings have spectral resolution of R ~ 20,000
      COS Sensitivity Advantages
• Effective Area gains: 10-20 X STIS (more signal)
• Background signal ~ 10% of STIS (depends on
  source brightness)
• Bandpass comparison:
  – STIS Echelle: 600 Å | COS FUV: 300 Å
• Net Sensitivity Gain – 10 -100 X
• Note: STIS Echelle Modes have much higher
  spectral resolution
    The Power of COS for IGM Studies
As of mid-July:
87 IGM sightlines
observed, 300+ hrs
(GO, GTO, ERO)
• Total Ly
pathlength
z = 22.93

COS has already
10x the pathlength
and 15x the number
of absorbers of all
previous
GHRS+STIS
studies.
He II Reionization: Shull, et al, 2010
 COS has performance below 1150 Å
• Effective area comparable to one channel of
  FUSE at lower spectral resolution
• See poster by Osterman
NUV Imaging
     Time Resolved Spectroscopy




• A flare occurred
  during the
  observation of a
  late type (naked)
  T Tauri Star
Sufficient S/N to see changes in spectral shape and
strength of emission features during event and
during “low state”
             Data Analysis Issues
• Pulse Height Screening
   – Each photon carries pulse gain information (5 bit)
   – Never pas through pulse height 0 data. Typical screening
     value is 4 (varies with position)
• Grid wire shadows – locations are well known and
  easily removed
• Co-adding of spectra from different wavelength
  positions
   – Software tool available from CU website cos.colorado.edu
    On-Orbit Performance Issues
• Spectral resolution:
  – The spectral resolution of the FUV channel drops
    to 18,000 at 1150 Å due to the convolution of the
    HST OTA point spread function with the COS line
    spread function. The wide aperture of COS allows
    the wings of the OTA PSF to enter the instrument.
    (This effect is also seen in the 2” slit on STIS). The
    effect mitigates slightly as the wavelength
    increases.
 On-Orbit Performance Issues

– This results in a non-Gaussian LSF.
  The 18,000 resolution is a calculation based on
  the modulation transfer function with an imposed
  Rayleigh criterion, as opposed to a FWHM
  calculation based on a Gaussian fit to a known
  non-Gaussian function (which yields R = 16,000)
    On-Orbit Performance Issues
• Loss of effective area:
  – The FUV channels are losing effective area at
    approximately 5% /year at all wavelengths. (This
    result is based on a limited number of samplings.
    The stability and long term trends of the
    degradation rate are currently unknown.)
  – The physical cause of the drop is unknown but
    atomic oxygen attack on the photocathode is the
    current leading candidate. If true, this effect may
    accelerate during solar maximum.
                Conclusions
• Despite the unexpected drop in effective area
  and resolution, COS remains a stunningly
  effective scientific instrument that is enabling
  previously impossible observations of multiple
  phenomena.
• Please look over the many COS posters to
  appreciate the significant diagnostic capability
  that has been provided to the community
  with COS.