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.
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