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Optical Astronomy: Towards the HST, VLT and Keck Era

Optical Astronomy: Towards the HST, VLT and Keck Era. Introduction & Overview Chris O’Dea. Acknowledgements: Marc Postman, Jeff Valenti, & Bernard Rauscher. Aims for this lecture. Historical overview A brief history of optical astronomy trends in aperture and detector size CCD Detection

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Optical Astronomy: Towards the HST, VLT and Keck Era

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  1. Optical Astronomy: Towards the HST, VLT and Keck Era Introduction & Overview Chris O’Dea Acknowledgements: Marc Postman, Jeff Valenti, & Bernard Rauscher

  2. Aims for this lecture • Historical overview • A brief history of optical astronomy • trends in aperture and detector size • CCD Detection • Observing Issues • Effect of the Atmosphere • Effect of the Space Environment

  3. Aims for this lecture II. • Optical Science • Pretty Pictures • HST • VLT • The synergy between optical and radio (real astrophysics) • The radio loud/quiet quasar transition • Time scales for fueling and activity in radio galaxies • Current `big’ issues in optical astronomy

  4. Atmospheric Transmission (300-1100 nm)

  5. History • Pre-history: mismatch between solar and lunar cycles required astronomical observations to calibrate calendars and predict times for natural and agricultural events • Newgrange, Ireland 3500 BC • Stonehenge, England 3000 BC • First millenium BC – Greeks search for • Systematics of planetary motion • Geometric model for planetary motion • Ptolemy’s Almagest (AD 145) presented robust geometric model of planetary motion • 12th century Islam- Need for more accurate measurements of positions led to first “observatories” – dedicated structures housing large, fixed instruments.

  6. History • 1575 Tycho Brahe’s Uraniborg – prototype of modern observatory • 1609 Galileo uses telescope for astronomy • Features on the moon • Sattelites of Jupiter • Stars remained unresolved • Development of reflecting telescopes (enables larger collecting areas) • Gregory 1663, Newton 1668, Cassegrain 1672 • Spectroscopy • 1817 Fraunhofer combines narrow slit, prism and telescope to make first spectrograph and discovers spectrum of the sun • 1859 Kirchoff shows that the solar spectrum reveals the chemical composition

  7. History • Photography • 1845 daguerreotype of sun – Focault & Fizeau • 1870’s - Improvements led to photography of faint stars and nebulae • 1872 – Draper obtained photographic spectrum of Vega • 1875-1900 Combination of Photography and Spectroscopy led to a shift of astronomy from positional measurements to astrophysics

  8. History • 1970’s 4-m class telescopes become common • 1980’s CCDs are developed • 1990 HST launched • 1990’s 10-m class telescopes become available

  9. Newgrange Megalithic Passage Tomb • Passage is illuminated for 17 min after dawn Dec 19-23 • Built ~3500 BC in County Meath, Ireland • On winter solstice sun shines down roof box and illuminates central 62-ft passage.

  10. Tycho Brahe’s Uraniborg • Built 1576-1580 • Prototype of “modern” observatory • First “Big Science” – required 1% of Danish national budget! • Dedicated to precision positional measurements (one arcmin) – made possible advances by Copernicus and Kepler

  11. Telescopes in Time 1858: Lassell 48” First “Large” Reflector 1859: Clark 18.5” 1609 Galileo 1.75” 1672 Newton 1.5” 1897 Yerkes 40” Largest Refractor 1948 Hale 200” 1917 Hooker100”

  12. Hubble & Humason 1931, ApJ, 74, 43 Edwin Hubble H~560 km/sec/Mpc

  13. Aperture vs Time Keck Galileo Newton

  14. The Biggest Telescopes Today Size Distribution of the 46 largest optical telescopes HST

  15. CCD Camera Development for Ground Applications: Luppino, 1998 DMT38k2 WFHRI36k2 18k x 18k CFH_MEGA18k2 MMT_MEGA18k2 OMEGA16k2 SDSS10kx12k 8k x 8k UW12kx16k CFH8kx12k UH8K2 Macho 8k2 NOAO8k2 DEIMOS8k2 QUEST8k2 MDM8k2 MAGNUM8k2 CTIO8k2 ESO8k2 EROS8k2 4k x 4k NOAO4k2 BTC4k2 UH4k2 MOCAM4k2 8kx8k 4kx4k 2k2 2k x 2k 2kx2k

  16. CCD Camera Development for Space Applications: SNAP 250x2k2 18k x 18k GEST 60 3kx6k GAIA136x2k2 Fame 24 2kx4k 8k x 8k Kepler 21x2k2 4k x 4k ACS 4kx4k WF3 4kx4k 2k x 2k WFPC2 4x0.8k2 WFPC1 4x0.8k2 STIS 1kx1k

  17. Astronomy at the end of the 20th Century • Questions about the universe have become progressively more sophisticated • From “Are there other galaxies? (ca. 1920)” to “What is the origin of structure in the universe?” • From “How many planets in our solar system? (Pluto discovered 1930)” to “How many extra-solar planetary systems lie within 100 light years of the sun?” … and are any inhabited? • The basics of cosmology (age & density of universe), detailed maps of the nearby galaxy dist’n, a basic theory of stellar evolution, and a census of the stars in the solar neighborhood exist (or will exist within 5 years). • Astronomers today rely heavily on joint observations from ground & space and data spanning large regions of the electromagnetic spectrum.

  18. CCD Detection

  19. Metal Electrode Silicon Dioxide + Depletion Region Silicon Substrate MOS Capacitor: • CCDs are arrays of Metal Oxide Semiconductor (MOS) capacitors separated by channel stops (implanted potential barriers). • Application of positive voltage repels majority carriers (holes) from region underneath oxide layer, forming a potential well for electrons. • A photon produces an electron-hole pair: the hole is swept out of depletion region and electron is attracted to the positive electrode. • Photoexcited charge collects in “depletion region” at PN junction. • Collected charge is shifted to amplifier (CCD) or sensed in situ (IR).

  20. Structure of a 3-Phase CCD • Consider a 3-phase CCD. • Columns are separated by non-conducting channel stops. • Rows are defined by electrostatic potential. • Charge is physically moved within the detector during readout.

  21. CCD Vertical Structure • In the vertical direction, one sees a PN junction and control electrodes. • Depletion regions form under both the metal gate and at the PN junction. • Charge is collected where these depletion regions overlap.

  22. Charge moves in a CCD • By changing electrode voltages, charge can be moved to the output amplifier. • This process is called charge transfer. • In an IR array, this does not happen. Charge is sensed in place.

  23. CCD Readout Amplifier CCD Readout Amplifier: Packet of Q electrons is transferred through the output gate onto a storage capacitor, producing a voltage V=Q/C.

  24. The Atmosphere

  25. Atmospheric absorption versus airmass • The amount of absorbed radiation depends upon the number of absorbers along the line of sight AM=1 AM=2 Atmosphere

  26. Atmospheric absorption versus altitude • Particle number densities (n) for most absorbers fall off rapidly with increasing altitude. • x0,H20~ 2 km, x0,CO2~ 7 km, x0,O3~ 15-30 km • So, 95% of atmospheric water vapor is below the altitude of Mauna Kea.

  27. Atmospheric Turbulence • A diffraction-limited point spread function (PSF) has a full-width at half-maximum (FWHM) of: • In reality, atmospheric turbulence smears the image: • At Mauna Kea, r0=0.2 m at 0.5 mm. • “Isoplanatic patch” is area on sky over which phase is relatively constant.

  28. Atmospheric Turbulence 1.4Oseeing 0.5Oseeing noseeing! Lick 3-m Figer 1995PhD Thesis Keck I 10-m Serabyn, Shupe, & FigerNature 1998, 394, 448 HST/NICMOS 2.4-m Figer et al. 1999ApJ. 525, 750

  29. Adaptive Optics: “Eye Glasses” for Ground-based Telescopes Laser Guide Star Atmosphere Wave Front Sensor Adjust Mirror Shape

  30. Adaptive Optics: “Eye Glasses” for Ground-based Telescopes

  31. Where does NGST win? • NGST should perform better than current 10m class ground-based telescopes. • In the mid-IR range (wavelengths  3 ), NGST will produce better quality (higher S/N) images and spectra than a 50m AO corrected ground-based telescope. • For surveying large fields of view – AO only works over a small field of view. • Sky is much darker in space in NGST’s wavelength range – better faint object detection.

  32. Observing in Space

  33. HSTFacts • Deployed 25 Apr 1990 • Mass: 11600 kg • Length: 13.1 m • Primary diameter: 2.4 m • Secondary: 0.34 m • f/24 Ritchey-Chrétien • 28 arcmin field-of-view • 0.11 mm < l < 3 mm • 0.043 arcsec FWHM at 5000 Å

  34. HST Orbit: • Height = 590 km • Orbital period = 96.6 minutes • Precessional period = 56 days • Inclination = 28.5° • Continuous viewing zones (CVZ) at  = ±61.5°

  35. Space Environment:

  36. Magnetic Flux Tubes:

  37. ACS CCD 10 year dose CCD Radiation Damage: • Radiation damage limits the science lifetime of a CCD • Ionization damage - flat band shifts • Bulk damage • Displacement of Si atoms in lattice produces traps • Hot pixels created by electrons from silicon valence band jump to trapping centers and generate high dark current • Annealing once a month to mitigate hot pixel accumulation. • WFPC2 is warmed to +20o C • STIS CCD is warmed -15o C • 80% of new hot pixels (>0.1 electron sec –1 pix –1 ) fixed

  38. Losses Transferring Charge SITe 1024  1024 CCD thinned backside NGC 6752, 8  20s, ‘D’ amp at the top Courtesy R. Gilliland (STScI)

  39. Parallel Degradation of Charge Transfer Efficiency Serial

  40. Optical Science • Pretty Pictures • Astrophysics

  41. Wide Field & Planetary Camera 2

  42. Hubble Deep Field

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