ASTR 511 [R. W. O'Connell]
INTRODUCTION TO ULTRAVIOLET ASTRONOMY
Astro-2 UV observatory in Shuttle payload bay.
A. THE UV BANDS
- 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
Highest density (bits per unit wavelength) of astrophysical
information on stars and gas
Key research areas:
- 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 Msun,
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?
- 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 105-6K);
- Carbon abundances from C III, C IV;
- Lyman discontinuity (rest wavelength 912 Å) in high redshift
- Near-UV: [O II] and [Ne V] plasma diagnostics, Balmer jump
stellar T,g diagnostic.
- Low sky background
- Sensitivity to dust
- Isolation of hot components in dominant cool sources: e.g. stellar
chromospheres, hot stars/AGN in E galaxies
- Restframe UV shifted to readily-observed optical window in high
redshift (z > 1) galaxies and AGN. E.g. "Lyman-break
- 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
- 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.
- Satellite observatories:
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.
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
- 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
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
- Microchannel Plates: electron multiplication in rigid channels,
- 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
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).
- 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,
- 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
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
- Relay satellites: TDRSS 200 MBPS, but limited access
- Launch protection/survival: "space qualification"
- Acoustic vibration
- Cleanliness: contamination control before launch, outgassing suppression
- 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.
November 2009 by rwo
Text copyright © 2000-2009 Robert W. O'Connell. Images in public
domain. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 511 at
the University of Virginia.