Observational Astronomy

Transcription

1 Observational Astronomy General considerations on observation...2 Optical observatories now and then...2 Basic characteristics of telescopes...4 Earth atmosphere...6 Physical and chemical structure...7 Constituents of atmosphere...9 Water vapor...9 Ozone...11 CO IONS...12 Effect of atmosphere on observation...13 Atmospheric extinction...13 Atmospheric emission...16 Atmospheric refraction...17 Differential diffraction...17 Seeing...18 Background sources...22 Observatory sites...26 Ground vs space...26 Ground-based sites (general requirements Optical/Infrared)...28 Ideal seeing conditions...30 Criteria for future large telescopes...33 Best observing sites on the ground...34 Conditions for San Pedro Mártir...36 Conditions at LMT site...38 Space orbits and the Moon...39 The James Webb Telescope (NGST)

2 General considerations on observation Optical observatories now and then Mount Palomar 5-meter: conceived 1930, completed m optical telescopes remain norm during 40 years o Technology reaches a plateau o Means of increasing sensitivity without increasing mirror size were available: 1. Observatory sites (Chile, Hawaii) a. seeing twice as good sensitivity equivalent to telescope twice as large 2. Dome and mirror seeing were eliminated sensitivity equivalent to best sites 3. Fast automatic guiding eliminate tilt component in the image blur 2

3 4. Photoelectric detectors with QE of 80% (~ 1% for eye, 4% for photographic plates) sensitivity equivalent to fourfold gain in telescope diameter 1980 o New barrier in sensitivity reached + advances in cosmology pressure to build 8-10-meter telescopes New technologies o New computer designs Faster + more powerful + cheaper o Faster improved optical figuring techniques Thinner deformable mirrors o Altitude-Azimuth mounting Reduce mass and cost of larger telescopes o Faster f-ratios Smaller domes and buildings 3

4 Basic characteristics of telescopes Sensitivity: the ability to detect faint sources 4

5 Angular resolution o Most important factor after sensitivity o Theoretically resolution is proportional to size o Improvements impeded by atmospheric turbulences New tendencies: o Adaptive optics eliminating seeing effect o Optical interferometry 2 10m Keck telescopes + 4 8m VLT o 6.5m optical telescope in space = NGST = James Webb telescope (launch 2013) o Giant mirror telescopes TMT 30m (construction 2009 operation 2015), OWL 40-60m ( ) and 100m (future) or more multiple active mirrors o ALMA Atacama Large Millimeter Array (operation 2012) 64 12m antenas = 10 15km baseline 10 milliarcsecond resolution 5

6 Earth atmosphere Global strategy of observational astronomy requires exact knowledge of properties of Earth s atmosphere Define potential or limits of ground based observation Best altitude as function of wavelength Effect of atmosphere on observation Opacity: capacity of absorbing radiation Scattering: prevent daytime observation in the visible and produce light pollution during the night Dispersion: deviation of apparent position of celestial objects depending on wavelength Thermal emission: affect IR and mm observations, day and night Turbulence: degradation of image + phase fluctuation Ionization: modifies propagation of radio waves All these phenomena varies in time and are strongly dependent on geographical location Different layers of atmosphere: 1. Troposphere: Contains approximately 75% of the mass of the atmosphere and almost all the water vapor and aerosols 2. Stratosphere: Stratified in temperature, with warmer layers higher up and cooler layers farther down 3. Mesosphere: Within this layer, ultraviolet radiation causes ionization 4. Thermosphere: International Space Station has a stable orbit within the upper part of the thermosphere, between 320 and 380 kilometers 5. Ionosphere: Ionized by solar radiation, plays an important part in atmospheric electricity and radio waves transmission 6. Exosphere: Last layer before space. The atmosphere in this layer is sufficiently rarefied for satellites to orbit the Earth, although they still receive some atmospheric drag The boundaries between these regions: tropopause, stratopause, mesopause, thermopause and exobase 6

7 Physical and chemical structure Atmosphere in radiative equilibrium with it surrounding balance between flux received from the Sun and radiated into space o Stationary distribution in T, P, ρ, etc. with time o Daily, annual and secular cycles are superposed Average structures as a function of altitude (fig2.1) T o Troposphere: < 0 z T o Stratosphere: > 0 z At all latitudes, significant deviations from average distribution are observed near ground level 7

8 Inversion layer: T z changes sign through certain vertical distance (more than km) o Stabilize layers closer to the ground prevent clouds layer from rising above the inversion layer - condition also exists for La Palma (Canary Islands), and coastal mountain ranges of Chile From 0 to 40km, composition of air is constant: P z = P H, RT Where scale height: H = m 7998 m 8 km Mg 0 M0 = 0.029kg : mean molecular mass of air T : mean temperature ( ) exp z 0 m R = 8.31 J K mole 1 1 T g CP CV Adiabatic Gradient of dry air: M m z = ad R CP Where CP and CV are the specific heats o Any gradient larger than this convective instability vertical currents 1 8

9 Constituents of atmosphere Principals: O 2(~21%) and N 2 (~78%) relative abundances stay constant between 0 and 100 km Minor constituents: Argon (0.93%), Carbon (0.03%), Neon (0.0018%), Helium ( %) o Important role in maintenance of physical conditions at Earth s surface - Radiative balance + UV flux o Affect directly astronomical observation: water vapor (cloud cover + NIR + mm), CO 2 (NIR + Optical), O 3(UV) o Affected by Human activity CO 2 + dust + depletion in O 3 Water vapor Mixing ratio or fractional content (defined locally): Varies between 0 and r ( ) S T characteristic of saturation Function of altitude + strongly dependent on latitude + time mass of HO mass of air 2 r = 3 ( gram m) 3 ( kg m) 9

10 Quantity of precipitable water above altitude 0 Where NHOis the number of molecules per unit volume 2 w z = N dz z : ( ) 2 0 z HO o For pressure + temperature: N m = r( z) HO 2 Column of precipitable water: [ cm] ( ) Where ρ 0 is the density of air at altitude z 0 zh ρ 0 z0 P P h = r z e dz HO 2 T T 0 0 Global mean column of precipitable water ( MODIS/Terra MOD07_ L2 - Atmosphere Profile Product) Source: Rapid variation of r( z) with height H smaller than for dry air 3km in troposphere Observatories at high altitude (several km) improves the quality of observation, especially in the NIR and in the mm 10

11 Ozone The distribution of atmospheric ozone in partial pressure as a function of altitude Source: Responsible for absorption in UV 11

12 CO2 Responsible for MIR absorption green house effect o Vertical distribution follows O 2 and N 2 o Mixing ratio independent of altitude o Increase with human activity IONS Above 60km photochemical reactions: + O + hv O + e o 2 2 o 2 O + hv O + O+ e + Varies with solar irradiation (circadian cycle) + activity Several ionic layers local maximum of Ne Absorb very long radio waves (ionosphere) 12

13 Effect of atmosphere on observation The atmosphere affects observations in several ways: 1. Extinction reduce the flux 2. Line and thermal emission unwanted background (infrared) 3. Refraction alter apparent position; disperses image spectroscopically 4. Turbulence (seeing) blurs image of observed object Atmospheric extinction Absorption and scattering of photons by collision with air molecules or particles In absorption process photon is destroyed and its energy transferred to the molecule, leading to subsequent emission o Primary absorbers: HO, 2 CO 2, O 2, O 3 In scattering process direction and energy of photon are changed o Rayleigh scattering Scattering by air molecules with typical size much smaller than λ o Mie scattering Scattering by small particles with sizes close to λ Extinction: o Depends on zenith angle Z (path through atmosphere increases with that angle) o Air mass: ratio of quantity of air along the observed direction to that towards zenith For zenith angle Z< 60 sec Z air mass proportional to ( ) 13

14 Atmospheric windows : spectral regions where observation is possible from surface of the earth o Extinction in optical 10-15% o Atmosphere becomes opaque below 300nm (due to ozone layer at altitude of km) o NIR ( µm) partial absorption due to water vapor and oxygen o Beyond 1.3 µm absorption bands o Beyond 25 µm atmosphere is completely opaque up to λ of 1mm 14

15 Particle number density for absorbers falls exponentially with altitudes Scale height of 2-3 km for HO 2 o Mauna Kea (4200m) above 95% of atmospheric water vapor column depth ~ 1.5mm (equivalent thickness of a layer containing all precipitable water) o Antartica plateau - column depth mm o Above 10 km low amount of precipitable water advantage of balloons and airplanes observatories 15

16 Atmospheric emission Daytime: scattering of sunlight prevents observations in visible and near infrared Nighttime: scattering of moonlight + fluorescence (airglow); emission of spectral line in NIR due to radiative de-excitation of atoms and radicals ( OH ) in the upper atmosphere (~ 100 km) spatial and temporal fluctuations limits photometric accuracy in NIR Beyond 2.3 µm, atmospheric radiation is dominated by thermal emission; effective temperature K (gaseous nature radiated less than Black Body); emission approach black body peaking at 12 µm Beyond 2 µm, thermal emission from telescope also becomes important 16

17 Atmospheric refraction Bending of light due to variable atmospheric density along light path. The source appears higher than actually is Function of zenith angle: o Zero at zenith to 0.5 degree at horizon o Variation with altitude, humidity and wavelength Correction in pointing control system, but induce field rotation for wide fields (differential diffraction) Differential diffraction Variation of index of refraction with wavelengths shorter wavelengths diffracted more than longer ones (several arc seconds at large zenith distance) Correction: introducing dispersing elements in the instruments (photometry) or aligning slit along the parallactic angle (spectroscopy) 17

18 Seeing Variation in intensity and direction of the amount of light reaching the aperture of a telescope Due to atmospheric turbulence Strongly dependent on temperature fluctuations o Variation of index of refraction density fluctuation temperature fluctuation Temperature fluctuations result from turbulent mixing of air layers due to: o convection (ground layer and troposphere below inversion layer) o mechanical turbulence (weakly stratified troposphere- region of high wind shear just above and below the jet streams o Stratosphere above troposphere is much more stable o Turbulence occurs in very thin layers just a few meters deep 2 Structure coefficient for the index of refraction C n : Describes the variation of index of refraction of a turbulent fluid as a function of temperature, pressure and wavelength 18

19 Characterization of seeing: Fried Length r 0 (coherence length): Diameter of bundle of rays issuing from a source at infinity which travel together through the various turbulent atmospheric layers and arrive still parallel and in phase at the telescope entrance: o In the visible, r 0 varies from typical value of 10cm to 30cm at best sites Coherence time τ 0 : Transit time of the statistical coherence region of diameter r 0 over the line of sight r0 determined by wind speed v (first order): τ0 v Isoplanic angle θ 0: Angle on the sky over which the incoming beam remains coherent 0 θ0 0.6 r h, where h is the altitude of the main turbulence layer above the telescope 19

20 Degradation of images by seeing: In general, the seeing degrades an image in two ways: image motion and image blur The apparent direction of an observed object is determined by the average direction of the wave front o Small aperture telescopes experience greater image motion than large ones, because wave front distortions tend to have larger slope changes over small scales The reverse is true for image blur o Large telescopes suffer from a larger image spread than smaller ones A telescope with an aperture equal to r 0 would primarily suffer from image motion To reach diffraction-limited performance, 0 r must be somewhat larger (~ 1.6 times) than the telescope diameter 20

21 For a telescope with large aperture compared to r 0, the FWHM of the image is: λ FWHM = 0.98 ; r 0 Since r0 65 λ the seeing varies as λ 15 o In the visible seeing varies between 1 and 0.35 o In the NIR seeing varies between 0.75 and 0.25 Scintillation: Variation in intensity of the image, due to the curvature of the wave front over the surface of the aperture Tends to focus and defocus the image Affect small telescopes, with aperture size r 0 or less 21

22 Background sources Noises that affect observation but do not originate in the source o It includes natural sources in the sky, atmospheric emission, thermal emission from telescope (NIR), side effects in the detector Galactic background: faint stars and dust (galactic cirrus highly irregular patches of emission) 22

23 Zodiacal light: produced by dust grains orbiting the Sun concentrated in the ecliptic plane o Scattering of sunlight (spectrum close to the Sun) and thermal emission (spectrum of black body ~265K) o Maximum toward the Sun and at short wavelength directly away from it (backscattering) o Minimum away from the Sun at ~ 60 of ecliptic latitude (minimum thickness of zodiacal light and cooler temperature) o Studied by DIRBE instrument on COBE Cosmic rays (CR) as background noise Atomic nuclei (protons) and electrons accelerated at high energies (velocity ~ 0.9c and energies ev) o Origin from the Sun but mostly in our Galaxy o Trapped by magnetic field of the Earth (Van Allen radiation belt) o Attenuated by the atmosphere, but reactions produce secondary particles (muons) at ground level o Rates depend on latitude and altitude: 50 per cm 2 per hour at sea level Twice that at 4000m in space 1 per cm 2 second CRs produce spurious charges in detectors (CCD) Affecting single pixels or several ones (grazing CR) Also, spurious counts can be generated within instruments by electronics or Cerenkov radiation Can be eliminated by splitting observations into sub-exposures then combining them using median South Atlantic Anomaly (SAA) A dip in the lower Van Allen belt caused by reduced magnetic field above Brazil Space telescopes need to avoid SSA or shut down their instruments 23

24 Atmospheric background Three regimes: o 1) Optical regime, about 1 µm dominate by moon scatter Dark time = Moon is less than a quarter full -- reserved for most demanding observations in the visible Bright time = Moon is more than half full -- used for high resolution spectroscopy or infrared observations Gray time = in between o 2) Non-thermal infrared regime from 1 to 2.5 µm (narrow OH emission lines) o 3) Thermal regime from 3 µm and above Stray light Two origins: o Light from celestial sources outside the field of view Eliminated using baffles and stop o Thermal emission from telescope and instruments Eliminated by termal insulation + Lyot stops Detector noise o Unwanted photoelectrons generated by detector or readout process Eliminated by reducing (mathematical algorithm) process 24

25 Coping with atmospheric and thermal emission (very important for NIR observation) o Thermal flux peak at 10 µm to observe sources which can be several orders of magnitude per square arcsecond fainter than background this background must be subtracted o Dithering if object small compared to FOV one can dither around the source and use the field surrounding the source to subtract the background Draw back: useful FOV will be reduced o Chopping at frequency of 3 to 10 Hz, one switch between object and sky (the secondary mirror is modulated) Draw back: telescope need to be repointed as a whole (beam-switching or nodding) each 60 seconds reduce useful integration time 25

26 Observatory sites Location is most important impacts on scientific performance, design fabrication and operations at the highest level Ground vs space Ground Construction well within modern capabilities Man access allows for correction of initial difficulties, tuning-ups, repairs and upgrades Adaptive optics promise similar results as space telescope (but with smaller FOV ~ a few arcsecond) Size of mirrors and instruments are theoretically unlimited Lifetime can be quite long (~ 40 years) Performance limited by site seeing and atmosphere (observations through windows only) Gravity + wind buffeting made it difficult to build very large telescopes Space Image quality limited by optics on FOV of several arcminutes Access to wide wavelength regions completely unimpeded (OH lines between 1 and 2µm avoided cosmological window ~ 3.5µm opened) Sensitivity enhanced in the IR (reduced background emission from atmosphere and telescope; cryogenic temperatures possible without frost) Excellent instrument stability little variation in thermal and dynamics conditions (optical alignment, PSF, throughput and detector characteristics remain unchanged for months to years stable observation and calibration properties) Engineering: low levels of mechanical and thermal disturbance Lifetime is short (~10 years) and instruments cannot be upgraded (HST exception, but at high costs) Costs: 10 to 100 times higher than cost of groundbased telescope of comparable size 26

27 Conclusions: 1) Performance of ground telescope observatories approaching that of space telescope 2) Costs of space observatories converge with those of largest ground-based facilities To tailor the respective scientific and operational goals to minimized the potential overlap Space missions are only justified if it is essentially impossible to obtain the desired scientific data from a ground based observatory A space telescope should be at least two orders of magnitude more powerful than its ground based competition (at the time the space mission is proposed) Ex. Advantage of space for IR observations is difficult to match on Earth Point source sensitivity with integration time calculated for S/N = 5; Left = broadband and right = spectroscopic; the shaded areas show the relative optical depth (left) and approximate regions of good transparency (right) 27

28 Ground-based sites (general requirements Optical/Infrared) Minimal cloud cover: o Partial cloud cover affects photometry o Clouds increase background in IR o Cease to be a problem above tropopause (8km Artic 18 Km tropics) o High altitude clouds = ice (little opacity) Source: Low water vapor: primary absorber and emitter (10µm) Low temperatures: o NIR background radiation emitted by atmosphere and telescope dominates scattered light and thermal emission from zodiacal light o Local background decreases as exponential function of the temperature Low atmosphere pressure: o Opacity and emissivity is determined by pressure broadening of spectral lines o The highest is the altitude and the lowest is the opacity 28

29 Dark sky: low level of city light pollution o The intensity of artificial illumination of the night sky due to city lights varies as the inverse 2.5 power of the distance from the city; o Amount of light is proportional to population; Low optical turbulence: good seeing maximize sensitivity and resolution Low nighttime wind velocity: small wind buffeting Low nighttime temperature variation: minimize dome seeing Low nighttime relative humidity: minimize risk of frost on optical elements Low level of radio wave and microwave radiation: avoids detectors and electronic equipment interference Low dust pollution; minimize contamination of the optics Low enough latitude : maximize sky coverage and access to significant celestial regions (galactic center; Large Magellanic Cloud) Good site accessibility; reduce operation costs Low seismicity: reduce structural constraints 29

30 Ideal seeing conditions Low optical turbulence is the important factor of a good observing site Seeing is directly related to the microthermal activity high-frequency temperature fluctuations associated with atmospheric turbulence 4 main atmospheric layers: 1. Surface layer (ground layer): Turbulence generated by wind shear due to frictional and topographic effects at ground surface Influenced by geometry and large scale roughness Increases with boulder, crags and trees Best sites ~ a few meters ex. Paranal = 5m, Mauna Kea = 6-10m Enclosure slit + telescope + primary mirror must be located above the surface layer 30

31 Ideal shape is isolated conical peak impinging airflows divide and flow around the peak on either side Slope greater than 7 but less than 18 avoid up-slope motion Flat top observatory as close as feasible to the ridge so as to sit in unperturbed flow When several telescopes are clustered near each other (Ex. VLT) laid out in the direction perpendicular to prevailing wind mountain ridges (Ex 2m SPM) not good sites because they disturb the airflow (mixes free air with cool air in contact with mountain side variation of refractive index 31

32 2. Planetary boundary layer: layer where frictional dissipation due to Earth s surface is significant a continuous transfer of air mass due to diurnal cycle Upper limit of planetary layer is the inversion layer ~ 1000m in altitude 3. Atmospheric boundary layer: above the planetary layer Still affected by ground both mechanically and thermally due to mountain ranges that emerge above the planetary boundary or gravity waves To avoid gravity waves it is best to be near the sea In La Silla this layer is the principal source of seeing, extending 500m above the site (generate 80% of the seeing) 4. Free atmosphere: unaffected by the ground activity is synoptic (i.e. of very largescale origin such as trade winds in the tropics, westerly winds in the mid-latitudes and jet streams in the higher layers); microthermal activity results from windinduced mechanical turbulence in zone with temperature gradients (shear zones); Jet streams velocity is maximal in mid latitudes seeing due to free atmosphere is better in the tropics and near the poles; seeing on average 0.4 arcseconds; 32

33 Figure: 16 years statistics at altitude 200-millibar atmospheric pressure; vertical bars variation in longitude (solid lines 0, dashed lines 180 ); small triangles = La Silla, dots = Paranal, squares = Mauna Kea; Criteria for future large telescopes Not the same as those applied for present 4m and 10m telescopes switch from site with excellent seeing to those with low wind speed and seeing characteristics that are easy to correct Atmospheric compensation techniques have the potential to correct for mediocre seeing -- what counts is not seeing but how well it can be corrected: o Depends on τ 0 (atmospheric coherence time) small values more difficult the correction o τ 0 is linked to wind velocity in upper atmosphere best sites are at low latitudes or at the poles Image degradation is also due to wind buffeting preferred sites have low average wind speed 33

34 Best observing sites on the ground Locations: o Areas with less than two octas of clouds cover, 50% of the time annually (unshaded areas) o Cold ocean currents (arrows) o Approximate boundaries of the regions of stable tropical maritime air (dotted circles) o 40 latitudes north and south indicated by dashed lines Island sites: finest observing conditions o tropical regions low cloud cover and wind velocity o far from land mass unperturbed airflow o one single peak above inversion layer good conditions regardless of wind direction o low light pollution o very few sites: Mauna Kea Hawaii and Canary Islands Coastal sites: conditions similar to island sites if prevailing winds come from the direction of the sea; o Cold sea is beneficial lowers the inversion layer o Baja California (California current) and Chilean coastal ranges (Humboldt-Peru currents) 34

35 Inland sites: need to be high (3000m or higher) and face into unperturbed airflows o Mountains ranges downhill from a large flat plain or desert Antartica: exceptional conditions o Global circulation induced by Earth s rotation has a singularity resulting in very calm upper atmosphere o Atmosphere thermally stable and stratified, especially during winter nights, thanks to cold surface and absence of solar heating o Low mechanical turbulence and thermal stability produce very good seeing o Low level of precipitable water (IR observation) o Low temperature of ground surface reduce thermal emission of optics o Sites at high altitudes on the continent appears most promising Photometric nights: more than 6 continuous hours with no clouds over 18 above the horizon Wind velocity: median velocity measured 10m above the ground Seeing: mean optical seeing Precipitable water: occurring at night time only 35

36 Conditions for San Pedro Mártir (Baja California) (source: Tapia, M. 2003, RMAA, 19, 75) Yearly fraction of nights with photometric and spectroscopic conditions for period o Average fraction of photographic nights 0.63 o Average fraction of spectroscopic nights 0.81 o Number of photographic night as increased from 52% to 75% - not clear if this depends on real climate variation or change in night quality evaluation technique? o Largest fraction of clear nights from observatory in North Hemisphere 36

37 Seeing and AO conditions (Source: Avila et al. 2004, PASP, 116, 682) Isoplanic angle: 1.87± 0.04arcsecond Very stable turbulence conditions above 8km SPM one of best site for Next Generation Large Optical Telescopes, using AO (source: Lawrence et al. 2004, Nature, 431, 278) Seeing (arcseconds) vs Isoplanic angle (Arcseconds) vs coherence time (milliseconds) 37

38 Conditions at LMT site Sierra Negra 4600m (left) and Citlaltepetl or Pico de Orizaba (rigt background) the highest peak in Mexico 38

39 Space orbits and the Moon Choice of orbit strongly tied to available launch systems -- to place a payload in high orbit requires 100 times the payload mass in fuel, sophisticated engines and staging systems high costs and risks of developing new launch systems result in limited menu of launch and orbit options Near equatorial low Earth orbits: most astronomical satellite o Perfect transparency of space and cosmic ray shielding by Earth s magnetic field o Requires minimal launch capability Higher orbits: energetically more expensive to reach o Geosynchronous orbits accessible within hours of travel o Second Lagrangian point, 3 months away A) Low inclination low Earth orbit: 300 to 1000km in altitude with inclination up to about 30 Below 300km impossible because of atmospheric drag Above 1000km not desirable because of high density of particles in Van Allen belts Low inclinations gain in launch velocity contributed by Earth s rotation Orbit inclination typically that of launch site (28 for NASA Cape Kennedy, 5 for ESA Kourou, French Guiana) 2π 3 Period: P a = where G = N m kg and GM For HST, with altitude of 600 km 98 minutes; 24 M = kg Specific feature = plane of orbit rotates with time (due to oblate shape of the Earth - center of gravity not coincident with center of mass 39

40 Advantages of low orbits Mass that can be placed in orbit is greater Facility can be maintained (Space Shuttle) Cosmic ray level is low Disadvantages of low orbits Observing efficiency is poor because of frequent target occultation by Earth For HST within 96 min, for example, almost half the time (40 min) is lost Long exposures requires that target and guide stars be reacquired at every pass Large temperature swings optical misalignment, image degradation and pointing errors B) Sun-Synchronous orbit By selecting a near-polar orbit of appropriate altitude the precession rate equal 1 day per year eastward, keeping the orbital plane in a fixed direction relative to the Sun If orbital plane is the Earth s terminator, Sun remains in same half One must wait one year to map the whole sky (IRAS) Require specific launch sites (Vadenberg Air Force Base in California) Less efficient in launch velocity 40

41 C) Geostationary (GEO) and geosynchronous orbits: Circular, equatorial, direct (westward) orbit with a period exactly equal to a sidereal day appears stationary from Earth Altitude km or 5.6 Earth radii Great utility for communication and weather satellites permits continuous contact with a single ground station, allowing real-time ground-observatory-like operations Relatively far form Earth portion of the sky block by Earth is minimized Located in Van Allen belts, increasing detector background and risk of permanent damage to electronic Geosynchronous orbit noncircular and non-equatorial Describes a 8, ex. IUE (International Ultraviolet Explorer) Not ideal for IR because of Van Allen belts D) High Earth orbits (HEO): Avoid the effect of Van Allen belts Circular orbit ( km) with period of about 4 days energetically costly reducing possible payload mass Highly elliptical (1000km x km) energetically less demanding and allows weeks of observing time from Earth, but satellite cross radiation belts twice; 41

42 E) Sun-Earth Lagrangian point 2: Special case of three body problems where one of the mass is negligible and one of the two larger mass is in circular orbit around the other Exist five positions in orbital plane (Lagrangian points L1 to L5), where the small-mass object will move in circular orbit, locked-in relative to the two other objects At these points the gravitational pull from the two main bodies is balanced by the orbital centrifugal force Two points are stable L4 and L5 and the other are metastable Each pair of large masses has such points (ex. Earth-Moon and Sun-Earth) By periodic station-keeping a satellite can maintain is metastable Lagragian point (in essence the spacecraft orbit the Lagrangian point halo orbits) 42

43 L2 point of Sun-Earth is ideal for astronomical viewing since the Sun, Earth and Moon are always on one side of the telescope maximizing sky coverage o Ideal for passively cooling the telescope a single shield can protect the telescope from the Sun o Constant distance from the Sun (1 AU) provides a stable thermal environment and continuous solar illumination for generating power o Distance from Earth is small for wide-band radio communication without the need for large ground antennas (MAP, NGST, Spitzer) o Relatively benign influence of meteoroid, compared to orbital debris in low Earth orbit F) Moon Offers a stable platform on which large and potentially widely separated instruments (interferometers) can be installed o Assuming manned lunar base, the possibility of repairing and upgrading telescopes and instruments No atmosphere no seeing or opacity problems At night Moon is relatively cold Main problem: alternation of day and night (1 lunar day ~ 27.3 Earth days) temperature of 400K during day Shielding impossible Cannot observed during ½ of time Moon not ideal for infrared astronomy because temperature during night is only 100K compared to 30K in space Although gravity is only 1/6, still have important effects on large telescope No atmosphere Intense UV, X-ray + continued bombardment by micro meteorites:100 (0.5mm) /yr/ sqm) Automatic deployment on the Moon is extremely difficult Observatories will be affordable only after manned bases are installed 43

44 G) Sun-Jupiter Lagragian point 2: the best astronomical site in the entire solar system! Stay in the total shadow of Jupiter total darkness, low temperature (7K equilibrium temperature in the solar system) Unimpeded by Sun avoidance constraints, the sky coverage would be close to 100% No need for sun shield, telescopes or interferometers with huge apertures or with huge base lines could be located there Drawback = power would have to be generated by thermonuclear means 44

45 The James Webb Telescope (NGST) Source: Key elements: 6.5m primary mirror (18 sections), lightweight, deployable (launch date 2013?) 45

46 Large sunshield enables passive cooling of telescope and instruments Second Lagrange point (L2) orbit, with deployment during orbit insertion Diffraction-limited imaging quality (Strehl = 0.8) for lambda = 2 micron micron wavelength range with zodiacal-light-limited imaging performance below 10 micron Imaging and spectroscopic instrumentation over this wavelength range 5 years required lifetime, 10 years goal Risk mitigation by extensive testing 46

47 Predicted encircled energy fraction as function of radius at 1 micron - encircled energy within 0.15 arcsec radius, determined by sub-segment errors, is 84% at 1 micron - diffraction limited at 2 microns (Strehl ratio of 0.84, dominated by large scale errors) The log-scale image of the Point Spread Function (PSF) shows the effects of the hexagon shaped mirror at the low level fluxes of the diffraction rings Highly effective sunshield and semi-rigid mirror segments on a thermally robust backplane provides a very stable PSF. Variations in the encircled energy at 1 micron are expected to be less than 0.5% even after a worst case (hot-to-cold) thermal slew There will be no need for thermal settling and wave front corrections after large slews, significantly increasing the observatory efficiency 47

48 Second Lagrange point (L2), approximately 1.5 million km from Earth, outside the orbit of the Moon. The region about L2 is a gravitational saddle point, where spacecraft may remain at roughly constant distance from the Earth throughout the year by small station-keeping maneuvers Large halo orbit in a plane slightly out of the ecliptic plane. This orbit avoids Earth and Moon eclipses of the Sun. The halo orbit period is about 6 months. Nominal station keeping maneuvers will be performed every half orbit (3 months) Benign and essentially unchanging environment - no significant gravitational torques and thermal influence from the Earth and Moon are greatly reduced. The main operational influence to consider is the torque created by the Solar wind on the sunshield 48

49 Instruments: NIRCam Near-IR and visible camera; Sensitive over the micron wavelength range; Two broad- and intermediate-band imaging modules, each with a 2.2 x 2.2 arcmin field of view; Each imaging module has two channels, with light split by a dichroic at ~2.35 micron; Short wavelength channel " pixels, long wavelength channel " pixels Each module has coronagraphic capabilities NIRSpec Multi-object dispersive spectrograph (MOS); Capable of observing more than 100 objects simultaneously Sensitive over the 1-5 micron wavelength range; 3.4' x 3.4' field of view ~0.1" pixels o R=1000 MOS Mode, 3 gratings cover micron; o R=3000 Integral Field Unit or Long-slit Mode; o R=100 Prism, mm in one exposure; MIRI Mid-IR camera and Integral Field Unit (IFU) and long-slit spectrograph; Sensitive over the 5-28 micron wavelength range; 1.88' x 1.27' field of view imaging; 12 filters; 3" x 3" IFU R=3000 spectrograph, in 5-10 and micron channels; R=100 long-slit 5-10 micron spectrograph Coronagraphic capabilities FGS Fine Guidance System; Enable stable pointing at the milli-arcsecond level Sensitivity and field of view to allow guiding with 95% probability at any point on the sky (i.e. 95% at the galactic poles, better at most other places); 3 fields-of-view, one of which has R~100 tunable filter capability 49

50 Scientific goals The End of the Dark Ages First Light and Reionization seeks to identify the first bright objects that formed in the early Universe, and follow the ionization history 50

51 Assembly of Galaxies To determine how galaxies including gas, stars, metals, physical structures (like spiral arms) and active nuclei evolved to the present day These images show the merger UGC06471 and UGC06472 at different redshifts. The first image is the original HST/WFPC2 F300W image of this z = 0.01 galaxy pair. The middle image is the simulated NGST 1.76 micron image if this same galaxy pair was at z = 5.0. The right image is for z = 12.0 and at a wavelength of 3.81 micron A 24"x 24" simulated NGST image labeled with redshifts. The image has 0.06 arcsecond resolution in all three bands (i.e. diffraction-limited at 2 microns). This represents only 1% of the total NIRCam field of view 51

52 The full 2'x2' image 52

53 The Birth of Stars and Protoplanetary Systems This project focuses on the birth and early development of stars and the formation of planets Planetary Systems and the Origins of Life Studies the physical and chemical properties of solar systems (including our own) and where the building blocks of life may be present 53