INTRODUCTION TO ULTRAVIOLET ASTRONOMY

• Earth's atmosphere is opaque below ~3200 Å.

• Interstellar medium is opaque below 912 Å (Lyman edge of H I)
  But optical depth ~ (freq)^-3, so ISM becomes transparent again below 400 Å

• Easily accessible "vacuum UV" band from space: 912-3200 Å
  • "Far-UV": 912-2000 Å
    
  • "Mid-UV": 2000-3200 Å

• With different technology: "Extreme UV" (70-912 Å)

• "Near-UV": 3200-4000 Å. Usually considered part of "optical" band, but often compromised by poor reflectivity or transmission of optical elements, atmospheric opacity.

B. MOTIVATION FOR UV OBSERVATIONS

1. Continuum sources
   • Stellar flux maximum (F_lam units) occurs at ~ 2900 Å/T₄.
     Energy distributions of hot stars (over 10,000 K) peak in UV.
     Most important: massive main sequence stars over 3 M_sun, responsible for most element synthesis, ionization, dissociation, and kinetic energy input to galaxies.
     For cooler stars (< 8000 K), the UV is in the Wien limit, implying high sensitivity to temperature (e.g. to measure main sequence turnoff in integrated light of stellar populations)

   • Hard nonthermal sources

   • "Big blue bump" in AGN = inner accretion disk?

2. Atomic & molecular spectral features
   • Many strong (often resonance) transitions of important species occur in UV:
     H, D, H₂, He, C, N, O, Mg, Si, S, Fe

   • Uniquely valuable:
     • Lyman series and metallic features in stars, ISM, IGM;
       
     • Atomic deuterium (offset from HI features);
       
     • O VI, C IV, N V (gas at 10^5-6K);
       
     • Carbon abundances from C III, C IV;
       
     • Lyman discontinuity (rest wavelength 912 Å) in high redshift galaxies;
       
     • Near-UV: [O II] and [Ne V] plasma diagnostics, Balmer jump stellar T,g diagnostic.

3. Low sky background
   Deep minimum in night sky background 1600-2500 Å, darkest in UV-optical-IR range (40x below best ground-based sky).

4. Sensitivity to dust
   • Dust extinction law is max in UV; local peak for Galactic dust at 2175 Å is an important & unique signature.

   • High & variable UV extinction is an advantage for studying grain types in different environments but a disadvantage for studying things behind grains.

5. Isolation of hot components in dominant cool sources: e.g. stellar chromospheres, hot stars/AGN in E galaxies

6. Restframe UV shifted to readily-observed optical window in high redshift (z > 1) galaxies and AGN. E.g. "Lyman-break galaxies"

• Stellar chromospheres, winds
• Mass exchange and accretion in binary systems (esp. WD, NS)
• Abundances in stars & chemical evolution of Galaxy
• Advanced stellar evolution (HB and beyond)
• Interstellar dust grains
• Hot ISM, galactic halos, fountains, winds
• Ages and abundances of stellar populations
• Massive star formation and histories of galaxies
• AGN (accretion disks, near-nuclear plasmas, reverb mapping)
• Deuterium abundances & Big Bang nucleosynthesis
• Proto/adolescent galaxies
• Cosmic star formation history, background light
• Evolution of the intergalactic medium
  Examples: click on thumbnails for larger view

C. MISSIONS

• Sounding rockets, balloons (e.g. SCAP/FOCA), manned missions (Apollo, Skylab). 1960's-early 90's.

• PI-class satellite missions: e.g. OAO-2, Copernicus.

• Space Shuttle sortie missions: e.g. Astro, ORFEUS, FAUST

• Satellite observatories:
  • International Ultraviolet Explorer (IUE)
    • 1978-1996 (a record to that time; later exceeded by HST)
    • Medium/high-dispersion spectroscopy 1200-3200 Å
    • Background information: GSFC/NSSDC; ESA, including science highlights.
    • Large data archive

  • Hubble Space Telescope (HST). Launched 1990.
    • 1200-4000 Å (plus optical and near-IR)
    • High resolution (0.05 arcsec) imaging (FOC, WFPC2, STIS, ACS/SBC, ACS/HRC)
    • Low-med-high resolution spectroscopy (GHRS, FOS, STIS)
    • High speed photometry (HSP)
    • Data archive

  • Extreme Ultraviolet Explorer (EUVE): spectroscopy 70-760 Å. Launched 1992.

  • Far Ultraviolet Spectroscopic Explorer (FUSE): High resolution spectroscopy 900-1200 Å. Launched 1999.

  • The X-ray/Gamma-ray satellites XMM and Swift both carry moderate-field support telescopes which can be used for multiband UV/optical imaging.

  • GALEX (the Galaxy Evolution Explorer Mission), launched in April 2003, is a 50-cm, wide-field (1.2^o) telescope imaging two broad UV bands centered at 1500 and 2300 Å. Microchannel plate detectors. Grism for low-resolution spectroscopy.

Historical emphasis in the UV was mainly on point-source spectroscopy, not imaging or extended faint sources (e.g. galaxies)
To 2003, only shallow all-sky surveys were available. FUV: TD-1, to 9th mag. EUV: ROSAT, EUVE, < 1000 bright sources. GALEX survey (now underway to AB ~ 21 mag) has remedied this situation.

D. UV INSTRUMENTATION

• Optical materials: limited choices for good transmissions or reflectivities; challenge greater for shorter wavelengths.

• Cleanliness, contamination control are critical because many materials likely to be deposited on optics are UV-opaque

• FUV, MUV designs similar to optical-band, but fewer surfaces preferred

• Filters: require good blocking of strong geocoronal, skyglow emission lines (esp. Lyman-alpha 1216 Å, O I 1302 Å). Also require excellent long-wave blocking (see next).

E. UV DETECTORS

• A major problem is the requirement for long-wavelength rejection. Visible photon rate is ~ 10⁶ higher than FUV rate in cool sources, like solar-type stars.

• Therefore require "solar blind" detection system.
  • Photoconductors like CCD's have broad bandwidths, extending to near-IR. Not good for UV unless excellent "red leak" rejecting filters are available. Filter rejection usually inadequate. Silicon also has large UV opacity. Some use of downconversion coatings (UV ==> visible photons), but relatively poor performance (e.g. WFPC1/2 on HST).

  • Photoemissive devices with large work functions preferred: photocathodes like CsI, CsTe, KrB have good UV QE, very low visible QE
    Main technical problem: must convert single emitted photoelectron into measurable signal, maintaining image quality in 2-D case.
    Photocathode UV QE's mostly well below visible QE's for CCD's (20-40% instead of up to 90%).

• 2-D UV detectors: typically hybrid designs
  • Stage 1: photocathode
    • Manufacture of large formats with uniformity, good MTF difficult

  • Stage 2: amplification/acceleration and image transfer
    • Image Tubes: high voltage (10-30 kV) photoelectron acceleration, electric or magnetic focussing
    • Microchannel Plates: electron multiplication in rigid channels, proximity focussing

  • Stage 3: detection/readout/storage
    • Phosphor plus film (e.g. Astro/UIT)
    • Phosphor plus CCD or diode arrays (e.g. Astro/HUT)
    • TV (scanning) readout (e.g. IUE, HST/FOC)
    • Electron bombarded diodes (e.g. HST/FOS, HST/GHRS)
    • Electron bombarded anode arrays (e.g. HST/STIS, FUSE, GALEX)

    • Key feature of most successful systems: (photoelectron-induced) event counting, with centroiding of x,y position.
      
• Current examples
  • STIS/ACS MAMA: Multi Anode Microchannel Array (at right)
  • FUSE detector: microchannel plate plus double delay line readout

• References:
  Timothy, PASP 95, 810, 1983;
  Joseph, UV Technology Overview, in "From X-rays to X-Band", an STScI Workshop, 2000.

F. SPACE ASTRONOMY: SPECIAL TECHNICAL REQUIREMENTS

• Launcher with finite payload
  • Space Shuttle (at right) probably most complex system ever built by humans. Total cost per launch about $400M (90% personnel).

  • (But Shuttle payload is 5x smaller (!) than the 1960's Saturn V).

• Power
  • Solar panels, batteries, radioactive sources, fuel cells
  • Explorer class experiment ~300 W; HST ~2KW

• Thermal control
  • Large temp gradients: e.g. Earth darkside: 273K from Earth, 3K on other side.
  • Control required for structural and electrical stability
  • Optics/electronics: typically maintain ~68 degrees by active heating, passive cooling to space
  • Detectors: usually require active cooling (thermoelectric, cryogens)
  • Issues: orbital dependence, materials (GEP), insulation, heat pipes, radiators

• Pointing control & stability
  • Gyros: provide 3D mechanical attitude reference
  • Auxiliary telescopes: star trackers, guiders: 1 arcsec-few arcmin
  • Focal plane trackers: e.g. HST: 0.007 arcsec RMS

• Scattered light rejection
  • Important to enable use of sunlit orbit
  • Sun, moon, bright Earth, bright stars, spacecraft structures, zodiacal light must all be considered
  • Complicated baffling, special coatings & materials needed

• Environmental protection
  • Radiation: high energy electrons, protons
    • "South Atlantic Anomaly" (R < 1000 mi)
    • Van Allen Radiation Belts (R < 25000 mi)
    • Shielding difficult; use rad-resistant equipment, special operations strategies instead
      Example serious consequence of radiation damage: reduced "charge transfer efficiency" in HST CCD cameras

  • Residual atmosphere (free oxygen), especially in ram direction
  • Orbital debris

• Communications & telemetry
  • Command & control
  • Housekeeping/health
  • Data
  • Relay satellites: TDRSS 200 MBPS, but limited access

• Launch protection/survival: "space qualification"
  • Temperature
  • Acoustic vibration
  • Acceleration
  • Cleanliness: contamination control before launch, outgassing suppression in space

• Reliability
  • Repair, upgrade impossible for all except HST (where it is costly)
  • Must predict and mitigate all possible failure modes
  • Is principal cost driver:
    • 95% reliable: $N
    • 98% reliable: $5N

  • Complete documentation essential: "PAPER IS OUR MOST IMPORTANT PRODUCT"

• ====> Costs!
  • Above list is reason that space experiments cost up to several 100x more than equivalent size ground-based facilities. Must weigh unique science return vs. cost.

  • "Cheaper, Faster, Better"? Pick two out of three.

• Exploring the Universe with the IUE Satellite, ed. Y. Kondo et al., (Dordrecht: Reidel), 1987.
• Astrophysics in the Extreme Ultraviolet, ed. S. Bowyer & R. F. Malina, (Dordrecht: Kluwer), 1995.
• The Ultraviolet Universe at Low and High Redshift, ed. W. Waller et al., (Woodbury, NY: AIP), 1997.
• Ultraviolet-Optical Space Astronomy Beyond HST, eds. J. A. Morse et al., (San Francisco, ASP), 1999.

• Ultraviolet Imaging Telescope/Astro-1 & Astro-2 Background & Pix

• List of all Orbital Astronomy Missions
  
• NASA Missions (all types)
  
• Hubble Space Telescope
  
• HST Multimission Data Archive (includes data for many UV missions)
  
• International Ultraviolet Explorer (IUE)
  
• Extreme Ultraviolet Explorer (EUVE)
  
• Far Ultraviolet Spectroscopic Explorer (FUSE)
  
• GALEX

• ASTR 511 Home Page

Last modified November 2009 by rwo