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Space-based Astronomy Instruments

Space-based Astronomy Instruments. David Lumb Science & Robotic Exploration European Space Agency. Momentous Week for Space Astronomy. Why Space ?. The atmosphere. Absorption

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Space-based Astronomy Instruments

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  1. Space-based Astronomy Instruments David Lumb Science & Robotic Exploration European Space Agency CERN Summer School May 2009

  2. Momentous Week for Space Astronomy CERN Summer School May 2009

  3. Why Space ? CERN Summer School May 2009

  4. The atmosphere • Absorption Most radiation incident on the Earth’s upper atmosphere does not reach the ground. The atmosphere opaque to all but some radio wavebands and light in the optical window. (390 nm – 780 nm) plus the near UV, near IR and some far-IR wavebands. Much of the infrared (21 mm to 1 mm) suffers absorption from H2O and CO2 whilst UV absorbed by O3. Gas atoms and molecules absorb X-rays and γ-rays . • Scattering Scattering of light is strongest when the λ of the light is ~ diameter of the scattering particles. Visible light more readily scattered by dust and mist than IR. ( car headlights in fog ) Try experimenting with IR TV remote control with water bag or chalk clouds • Seeing Variations in density of the atmosphere in a line of sight with an object cause intensity fluctuations. The variations in the refractive index of a cell of air above a telescope will alter the apparent position of an object, normally over a range of a few arcseconds. (distinguish between a star and a planet merely by observing Stars “twinkle” more than the bright planets) CERN Summer School May 2009

  5. HST History • Lyman Spitzer developed the concept of a telescope in space. In 1946 — more than a decade before the launch of the first satellite • In 1962, the USA's National Academy of Sciences recommends building a large space telescope. • In 1977, Congress votes to fund the project and construction of Hubble Space Telescope begins. • Launch in 1990 – discovery of the spherical aberration • 1993 – first Servicing Mission • 1995 – first Hubble Deep Field data CERN Summer School May 2009

  6. Early Instrument Concepts • Earliest concepts considered astronauts periodically retrieving film! • CCDs eventually prevailed over vidicons after a period of “invention push & market pull” driven by NASA and CCD manufacturers CERN Summer School May 2009

  7. Space Radiation Environment CERN Summer School May 2009

  8. CCDs and Cosmic Rays • Ground based – muons with rate varying with altitude • In orbit – order magnitude higher in LEO, several times more again in interplanetary space CERN Summer School May 2009

  9. CCDs and Radiation Damage • Early tests of ~krads Co60 showed no major ionising damage problems • Environment specific tests for neutrons and protons then showed that bulk damage very important for dark spikes and charge transfer efficiency at low signal levels • Shielding and mitigation methods critical, and still remains the Achilles heel for space-borne use of CCDs CERN Summer School May 2009

  10. HST Operations The HST is in a low earth orbit (LEO) with an orbital period of approximately 96.5 minutes. This introduces a variety of observing constraints Depending on target declination, viewing times range from approximately 52 —96.5 minutes per orbit. Reflected and scattered light also restrict the science instruments, and the pointing control system (PCS), from observing targets within 20 degrees of the bright earth limb and 7.6 degrees of the dark earth limb. CERN Summer School May 2009

  11. HST Operations #2 The poles of the HST orbit lie 28.5 degrees from Earth's equatorial pole and combine with the HST earth limb avoidance to define a variable circle of roughly 4 — 16 degrees in radius centered on the orbit pole near declination. Orbital precession causes these northern and southern continuous viewing zones (CVZ) to sweep completely around the equatorial pole in right ascension A 50-degree cone of avoidance around the sun and a 9-degree cone around the moon define limits within which observations are not normally planned or executed. This prevents stray light contamination and damage to the inside of the telescope from those bright sources. Spacecraft Roll Restrictions HST's solar panels rotate around only one axis. The resulting single degree of freedom translates into roll limits of the spacecraft about the target-telescope line of sight. CERN Summer School May 2009

  12. HST Operations#3 Guide Star Availability Observations require one or two guide stars to enable HST to maintain stable pointing during long exposures. The use of one guide star only provides X and Y axis control and allows the telescope to drift in roll about the guide star position at a rate of about one milliarcsecond per second of time. The magnitude of the target's motion in the science aperture increases with the exposure time and the length of the moment arm between the guide star and target position. Therefore, most observations require two guide stars. Three Fine Guidance Sensors (FGS) use areas lying at the outer edge of the primary field of view to locate and lock onto the necessary guide stars. guide stars move in and out of the FGS fields of view over time and may not be available at all times for a given target. South Atlantic Anomaly (SAA): Earth's offset magnetic dipole allows solar and extrasolar particle radiation to penetrate closer to its surface in the region and produces a many orders of magnitude increase in the ionizing radiation density at HST'S altitude. Data contamination increases for all detectors and presents safety concerns for some instruments. HST passes through some part of the SAA inabout 60% of the physical orbits each day, Unplanned events, instrument safing sub-system failures (gyros computers) CERN Summer School May 2009

  13. Space vs Ground • Resolution advantage of HST over ground-based telescopes will decrease as adaptive optics systems become more widespread. One advantage of the larger ground-based telescopes is their sensitivity as their primary mirrors are much larger. • Space-based telescopes essential for most IR, UV, X-ray and γ-ray observations. Developments over decades have seen improvements in resolution, sensitivity and operational life spans. But … CERN Summer School May 2009

  14. Space vs Ground • Cost. More expensive to design, build, launch and operate a telescope in space. HST cost about US$2 billion to build & launch. (&>$700M/service) The Gemini project of two 8m telescopes cost US$184m. The VLT in Chile cost $600m for 4 x 8.2m telescopes • Lifespan. The lifetime of most space telescopes is limited by the amount of onboard fuel they can carry for corrections and orbital adjustment. Also rely on gyroscopes for control and pointing. Difficult or impossible to control the telescope if fail. • Risk. Some space astronomy missions have failed to catastrophic failure of the launch vehicle. Others, such as Hipparcos have had to be curtailed or modified due to incorrect orbit insertion. • Size. For an ESA mission to L2 €200,000 per kilogram. Also restrictions on the size of satellites. The primary mirror for the HST was limited to fit into the cargo bay of a Shuttle. James Webb Space Telescope will use a 6.5m segmented mirror made of beryllium that will be folded up for launch on an Ariane 5 launch vehicle. • Upgrades and MaintenanceGround-based telescopes can be upgraded relatively easily. Telescopes such as the AAT, Gemini and the VLT are improved throughout their lives by the addition of new instruments and sometimes improved optics. Mechanical and software problems can be fixed by engineers and scientists. CERN Summer School May 2009

  15. Herschel Instruments • PACS is a camera and low to medium resolution spectrometer in range 55-210 µm. Four detector arrays, two bolometer arrays and two Ge:Ga photoconductor arrays. The bolometer arrays are dedicated for wideband photometry, photoconductor arrays are exclusively for spectroscopy. PACS can be operated either as an imaging photometer, or as an integral field line spectrometer. • SPIRE is a camera and low to medium resolution spectrometer Wavelengths in the range 194-672 µm. An imaging photometer and a Fourier Transform Spectrometer (FTS), use bolometer detector arrays. • HIFI is a very high resolution heterodyne spectrometer covering the 490-1250 GHz and 1410-1910 GHz bands. It utilises low noise detection using superconductor-insulator-superconductor (SIS) and hot electron bolometer (HEB) mixers, together with acousto-optical and autocorrelation spectrometers. HIFI is not an imaging instrument, it observes a single pixel on the sky at a time. CERN Summer School May 2009 Biggest space telescope ever !

  16. PACS • Ge:Ga photoconductor - application of stress shifts the cut-off wavelength to higher values by breaking the degenerate valence band forcing the heavy hole band closer to the acceptor state thus allowing less energetic photons to excite free charge carriers • Responsivity changes to irradiation! TopScanning electron microscope (SEM) micrograph of part of one PACS 16 × 16 bolometer matrix. BottomThe detail, marked by the red rectangle, shows the structure of one pixel: a (coarser) grid in the front acts as a resonant absorber while the (finer) grid in the back acts as a λ/4 back-reflector. CERN Summer School May 2009

  17. SPIRE • SPIRE Photometer Detector Assembly • The SPIRE photometer has three arrays of bolometer detectors each dedicated to one of the instrument's three wavelength bands centred on approximately 250, 350 and 500 μm. This image shows the whole detector assembly for the 500 μm band. The 43 holes in this front view of the assembly are the entrances to feedhorns that direct the light onto the actual detectors: 43 bolometers lying behind the feedhorns in the assembly. CERN Summer School May 2009

  18. SPIRE • Close-up of a single bolometer, hexagonal in shape, with parts of surrounding bolometers also visible. • The tiny crystal (towards the bottom of the bolometer), reacts to changes in temperature. When a source on the sky is observed, the green circular area of the bolometer absorbs the energy from the light that falls on it and heats up by a small amount. • Measurement of brightness is carried out by measuring crystal the change in resistance CERN Summer School May 2009

  19. Herschel Cryostat • A large cryostat surrounds the instruments maintaining an operational temperature of 1.7 K for a nominal mission lifetime of 4 years. Helium-II conditions arranged by the use of vacuum pumping for several days to below the Lambda point at about 2.17 Kelvin then lowered even further by continued vacuum pumping to about 1.7 Kelvin. • Continually refilled with additional He & pumping until four days before the launch • Helium gas keeps the cryostat cold • After the launcher fairing placed over the Herschel and Planck spacecraft access points used to ensure the cryostat tank itself is kept as cold as possible for the following days by flushing helium gas • The helium is used to cool the three science instruments' focal plane units (FPU) and the shields.  • A porous plug allows the separation of the gas from the boiling liquid helium such that only gas leaves the tank. It slowly flows from the tank into pipes around the payload (highlightedin blue) to cool it to between 1.7 K, 4 K and about 10 K. CERN Summer School May 2009

  20. Planck Planck has the ability to: • Detect much smaller temperature variations in the CMB than previous missions • Perform CMB measurements with a higher angular resolution than ever before • Measure over a wider band of frequencies to enhance the separation of the CMB from interfering foreground signals • Possibly see first anisotropies in Universe and undermine Copernican Principle? CERN Summer School May 2009

  21. Planck Instruments • Planck’s Low Frequency Instrument (LFI), and the High Frequency Instrument (HFI), are equipped with a total of 74 detectors covering nine frequency channels. These detectors cooled to temperatures < 20 K. 3-stage refrigeration chain which takes over after the passive cooling system ~ 50 K. Dilution refrigerator 0.1K • The bolometric detectors, located behind the horns, absorb the light and heat up slightly. A thermometer reads the temperature rise and converts it to an electrical signal which travels down wires connecting the low- and high-temperature ends of the instruments. CERN Summer School May 2009

  22. Sensors and Cooling • Cryogenic photon detectors offer advantages in terms of • Higher sensitivity in terms of NEP • Better energy resolution E / ΔE = λ / Δλ CERN Summer School May 2009

  23. System Design Issues • Decision to adopt cryogenic design will be based on trade-off analysis considering among others the mass savings, power consumption and reliability • Total energy balance depends critically on the efficiency of the system: • Passively cooled subsystem has positive energy balance, while actively cooled (eg Stirling cooler) has a critical energy balance • Choice between passively and actively cooled design depends on operating temperature and even the type of orbit of the satellite CERN Summer School May 2009

  24. System Design Issues • Cooler must have heat lift compatible with satellite size and power resources • Low temperature equipment must be properly supported, insulated from the room temperature satellite bus & protected from solar/earth radiation • Cold parts have to be accessed for wiring and for the optical input • Ancillary equipment has also to be cooled (heat links, heat switches, filters thermometers) • Testability of the system (instrument performance, environment qualification tests) has to be ensured CERN Summer School May 2009

  25. System Design Issues • Cooling system must survive vibrations of launch (compromise between cross-section for support and for thermal isolation) • Cooler must operate in zero gravity for period time ~years • Payload needs to be built from materials compatible for both space and cryogenic environment • No vibrations or pick-up sensitive elements • Lifetime (MTBF) of equipment should be compatible with and exceed nominal mission duration and operational cycles CERN Summer School May 2009

  26. Types of coolers • Passive radiator to deep space 2.73K (~σεAreaT4 ) efficient to 100K until parasitic inulation loads • Open cycle stored cryogens (work done by liquefier on-ground) No energy to radiate but gas to be released – limited life • Closed cycle mechanical cooler, work done continuously . For lowest temperatures require multiple stages CERN Summer School May 2009

  27. Bolometers for High Energies • Not immediately apparent this technology would be applicable: • The total energy deposited by X-rays from the sky via focussing telescopes over 30-40 years of observatories would heat a teaspoon of water ~1 microKelvin CERN Summer School May 2009

  28. X-Ray Optics CERN Summer School May 2009

  29. X-ray Calorimeter V Single pixel calorimeter Thermometer Absorber Thermallink ColdBath CERN Summer School May 2009

  30. Microcalorimeter Arrays • Development issues: • Each pixel the same thermal link to the cold bath (independent of the amount of pixels) • Pixels closely packed together (efficiency) => (limited space for wiring and thermal link) • Small electrical and thermal cross talk (do not degrade the energy resolution) • Fabrication feasibility, ruggedness (thermal cycling) • Response to charged particles (need for anti coincidence, dead times) CERN Summer School May 2009

  31. X-ray CCD • Essentially the same operation as for visible band, except read out fast enough to guarantee single photon counting • Energy absorbed and photoelectron cascade liberates 1 e/h pair per 3.65eV • ΔE (fwhm) = 2.35 * √(r² +FE/3.65) ~100eV CERN Summer School May 2009

  32. XMM-Newton CCDs • XMM EPIC imager used two different sorts of CCD- conventional CMOS and a pn structure with heritage from drift detectors • Also CCDs used to readout dispersed spectrum from grating instrument CERN Summer School May 2009

  33. Simultaneous Imaging and Spectroscopy • Tycho Supernova remnant CERN Summer School May 2009

  34. Radiation Damage • Very low individual photon count rate • Most frames empty and trap sites from damage remove charge during transfer CERN Summer School May 2009

  35. Data selection and artefacts • Data rate limited to ~16kbit/sec • Need to select valid X-ray events, reject CR events and ensure correct background local zero estimation CERN Summer School May 2009

  36. Data selection and artefacts CERN Summer School May 2009

  37. Light Filter • For X-ray CCDs and bolometers need to ensure that X-rays detected without the effect of light and IR loading that increases noise • Visible:X-ray ratio for stars typically 107. • Aluminised plastic window (<100nm A) must not prevent soft X-ray absorption yet robust to launch! CERN Summer School May 2009

  38. Gamma Ray detectors • Historically needed large , thick proportional counters. • Compton effect and pair production energy regime? • Compound semiconductors and scintillators – but as yet immature technology cf. silicon CERN Summer School May 2009

  39. Integral SPI CERN Summer School May 2009

  40. Integral Imager (IBIS) CERN Summer School May 2009

  41. Alternate Imaging Method • IBIS can reconstruct images of powerful events like gamma-ray bursts (GRBs) using the radiation that passes through the side of INTEGRAL's imaging telescope. • IBIS uses two detector layers, the higher energy gamma rays can Compton scatter in first detector layer (called ISGRI), losing energy, but not being completely absorbed. • The deflected and now less energetic rays then pass through to the layer below where they can be captured and absorbed The blue-shaded part of the image describes the fully coded field of view of the instrument. CERN Summer School May 2009

  42. Gamma Ray focusing • Coded mask telescopes have only 50% open factor, and detectors must have frontal area equivalent to collecting area • Large background could be decoupled by focussing advantage – but gamma ray focussing is not easy. Bragg-relation 2 d sin θB = n λ CERN Summer School May 2009

  43. Laue Lens Energy bandpasses 460 – 522 keV 825 – 910 keV Effective area in ~ 1 m2 bandpasses Field of view > 30 arc seconds Angular resolution ~ 10 arc seconds Focal Length 400m • Rings of Cu and Si crystals for energy bandpass around nuclear emission lines ~ tonnes payload • Separated telescope and detector spacecraft • But detector size only <10cm – much lower background CERN Summer School May 2009

  44. Testing • Space-bound scientific instruments are subjected to extensive ground testing before launch to ensure successful launch and on-orbit operation. • This is a radical departure from testing terrestrial laboratory instruments since these instruments typically operate in a more benign environment, and are generally accessible for repair during their useful life. Among the various tests conducted for space environmental testing are thermal, structural (including vibration and acoustic) and electromagnetic interference (EMI). • The test series is intended to validate design, assembly, performance and reliability of flight qualified units. • The qualification tests will demonstrate performance margins over and above requirements under operating environmental conditions. • Random vibration tests and mechanical shock tests will be performed at 1.5 times the load level specified for acceptance requirements. • The random vibration tests simulate launch conditions and will induce stresses to uncover any potential structural deficiencies that might exist. The mechanical shock tests will simulate potential impacts incurred as a result of handling or transport. CERN Summer School May 2009

  45. Major Design Issues for Space-Borne Astronomy • Mass (= M€) • Power (= large solar arrays = mass) • Volume (= support structure and mass) • Radiation (require shielding = mass) • Cosmic ray removal (processing = mass) • Calibration (on-ground time and in-orbit efficiency) • Vacuum (materials, testing time, design effort) • Thermal (testing, design, power, ) • Filters (single point failure) • Emc (test on-ground with spacecraft ?) • Stray light (Baffles, operations = mass / time) • Pointing (Mechanisms, stability, lifetime) • Compression (processing=mass, robustness) • Downlink (Bandwidth = power= mass) CERN Summer School May 2009

  46. Future Missions – ESA Cosmic Visions Programme 2015-2025 • Following proposals from the science community, 6 missions are being studied for a down-selection at the end 2009 • 2 medium-class missions (€300-400M) and 1 large mission (€600M) will be defined further • Covers range of solar-system, planetary and astronomy missions CERN Summer School May 2009

  47. CV M-Class Astronomy • PLATO – planet finder (transits and asteroseismology) • Stable conditions in space for accurate photometry • Long duration (2yr) high duty cycle (95%) at one field • Low background • Euclid Dark Energy/Matter • Weak lensing signal not subject to systematics of atmosphere stable PSF • Wide waveband for Vis/near-IR to cover redshift range • Simultaneous field spectroscopy to calibrate redshift sample • Baryon Acoustic Oscillations in correct redshift range to measure standard ruler (spectroscopy with low background) CERN Summer School May 2009

  48. The instrumental concept of PLATO (model payload) wide Field-of-view + large collecting area - ensemble of 28 telescopes with 10cm pupil - each equipped with 4 CCD 3584 x 3584 x 18 - all telescopes observe the same field - field of view: 557 deg2 - 26 identical: cadence 30 sec, white light dynamical range mV = 8 - 14 - 2 specific: 1 sec, frame transfer, 2 broadband filters dynamical range mV = 4 - 8 injection into large Lissajous L2 orbit continuous observation, field rotation every 3 months CERN Summer School May 2009

  49. Euclid • WL survey cover 20,000 deg2 and provide 40 galaxies per amin2 usable for WL with a median redshift z>0.8. • shear systematics variance σsys2 <10-7 • 100M galaxies spectroscopic redshift measurement shall be σz < 0.001 CERN Summer School May 2009

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