astronomical observational techniques and instrumentation

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Astronomical Observational Techniques and Instrumentation

RIT Course Number 1060-771 Professor Don Figer

X-ray Astronomy

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Aims of Lecture• review x-ray photon path• describe x-ray detection• describe specific x-ray telescopes• give examples of x-ray objects

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X-ray Photon Path

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Atmospheric Absorption• X Rays are absorbed by Earth’s atmosphere

– lucky for us!!!

• X-ray photon passing through atmosphere encounters as many atoms as in 5-meter (16 ft) thick wall of concrete!

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X RaysVisible Light

How Can X Rays Be “Imaged”?• X Rays are too energetic to be reflected “back”, as is possible

for lower-energy photons, e.g., visible light

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X Rays (and Gamma Rays “”) Can be “Absorbed”

• By dense material, e.g., lead (Pb)

Sensor

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Imaging System Based on Selective Transmission

Input Object(Radioactive Thyroid)

Lead Sheet with Pinhole “Noisy” Output Image(because of small number

of detected photons)

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How to “Add” More Photons1. Make Pinhole Larger

Input Object(Radioactive Thyroid

w/ “Hot” and “Cold” Spots)

“Fuzzy” Image Through Large Pinhole

(but less noise)

“Noisy” Output Image(because of small number

of detected photons)

“Fuzzy” Image

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How to “Add” More Photons2. Add More Pinholes

• BUT: Images “Overlap”

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How to “Add” More Photons2. Add More Pinholes

• Process to combine “overlapping” images

Before Postprocessing After Postprocessing

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Coded Aperture Mask• A coded aperture mask can be used to make images by relating intensity

distribution to the geometry of the system through post-processing, as described in the thyroid example.

• Six coded aperture mask instruments are operational in space: XTE-ASM, HETE-WXM, SWIFT-BAT and the three instruments on INTEGRAL.

• EXIST is being considered as a Black Hole Finder Probe in NASA's Beyond Einstein program.

Instrument1 Principle Investigator Science Operational Period

Detector/ E-range

(keV) E-resolution

Pattern3 Config4

Mask mat.

FOV5 (degrees)/ Ang. Res. (arcmin)

Det. Area (cm2)

No. of

Modules6

Instruments flying

ASM RXTE

H. Bradt (MIT) USA

XB,AGN,GRB 95- PSPC 2-10 25%@6 keV

ORA R|S|1 Al/Au

6X90 (HM) 3X15

90 3

WXM HETE

E. Fenimore (LANL) & N. Kawai (RIKEN) USA & Japan

GRB, X-ray bursts

00- PSPC 2-30 15%@6 keV

ORA(33%) R|S/C|1 Al/Au

90X90 (ZR) 40

400 2

IBIS INTEGRAL

P. Ubertini (IAS) Italy

XB,AGN 02- CdTe/CsI 20-10000 8% @ 100 keV

MURA R|C W

8X8 (FC) 12

2500 1

SPI INTEGRAL

V. Schoenfelder (MPE) Germany

Galactic diffuse emission

02- Ge 20-8000 0.2% @ 1.33 MeV

HURA H|C W

16 (FC) 120

500 1

JEM-X INTEGRAL

N. Lund (DSRI) Denmark

XB,AGN 02- PSPC 3-35 13%@10 keV

ORA(25%) H|C W

4.8 (FC) 3

500 2

BAT SWIFT

N. Gehrels (GSFC) USA

GRB, hard X-ray survey

04- CdZnTe 15-150 6 keV throughout

URA H|C

120 17

5200 1

Instruments being studied

EXIST Balloon

J. Grindlay (Harvard) USA

Survey CdZnTe 5-600 1% @ 100 keV

20X6 (HM) 5

2500 4

1Instruments between parentheses experienced difficulties in flight 2 XB - galactic X-ray binaries, GRB - gamma ray burst phenomenon, ASM - all-sky monitoring, SSM - selected-sky monitoring, AGN - active galactic nuclei, Technology - instrument technology, GC - the galactic center 3 Pattern: FZP = Fresnel Zone Plate, ORA = Optimized RAndom pattern, URA = Uniformly Redundant Array, HURA = Hexagonal URA; p = pseudo-noise subset, h = hadamard subset, tp = twin prime subset; triadic = near-URA with 0.33 open fraction 4 Configuration: R = rectangular, H = hexagonal, C = cyclic, S = simple, 1 = 1-dimensional 5 HM=full width at half maximum, ZR=full width at zero response, FC=full width of fully coded field of view 6 Number of modules

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Would Be Better to “Focus” X Rays• Could “Bring X Rays Together” from Different Points in

Aperture– Collect More “Light” Increase Signal– Increases “Signal-to-Noise” Ratio

• Produces Better Images

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X Rays and Grazing Incidence

X Ray at “Grazing Incidenceis “Deviated” by Angle

(which is SMALL!)

X-Ray “Mirror”

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Why Grazing Incidence?• X-Ray photons at “normal” or “near-normal” incidence

(photon path perpendicular to mirror, as already shown) would be transmitted (or possibly absorbed) rather than reflected.

• At near-parallel incidence, X Rays “skip” off mirror surface (like stones skipping across water surface)

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X-ray Collecting Mirrorsn.b., Distance from Front End to Sensor is LONG due to Grazing Incidence

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X-Ray Mirrors• Each grazing-incidence mirror shell has only a very small

collecting area exposed to sky– Looks like “Ring” Mirror (“annulus”) to X Rays!

• Add more shells to increase collecting area: create a nest of shells

“End” View of X-Ray Mirror

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X-Ray Mirrors• Add more shells to increase collecting area

– Chandra has 4 rings (instead of 6 as proposed)

• Collecting area of rings is MUCH smaller than for a Full-Aperture “Lens”!

Nest of “Rings” Full Aperture

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X-ray Detection

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The Perfect X-ray Detector• What would be the ideal detector for satellite-borne X-ray astronomy? It would

possess:– high spatial resolution – a large useful area, – excellent temporal resolution – ability to handle large count rates– good energy resolution – unit quantum efficiency – large bandwidth– output would be stable on timescales of years – internal background of spurious signals would be negligibly low– immunity to damage by the in-orbit radiation environment – no consumables– simple design– longevity– low cost– low mass – low power – no moving parts – low output data rate– (and a partridge in a pear tree)

• Such a detector does not exist!

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X-ray Detectors• Principle measurements

– flux– position– energy– time of arrival

• Specific types of detectors– gas proportional counter: x-ray photoionizes gas and induces voltage

spike in nearby wires– CCD: x-ray generates charge through absorption in silicon– microchannel plates: x-ray generates charge cascade through

photoelectric effect– active pixel sensor (i.e. CMOS hybrid detector array): x-ray generates

charge through absorption in silicon– single photon calorimeters: x-ray heats a resistive element an amount

equal to the energy of the x-ray

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X-ray Detector Materials• Light sensitive materials must be sensitive to x-ray energies

(~1-100 keV)– must allow some penetration– must have enough absorption or photoionization

• Good materials– gas– CZT (CdZnTe)– CdTe– silicon

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X-ray Image of Fe55 Source• For precise measurements of conversion gain (e-/ADU) or

Charge Transfer Efficiency (CTE), one often uses an Fe-55 X-ray source.

• Fe55 emits K-alpha photons with energy of 5.9KeV (80%) and K-beta photons with energy of 6.2KeV (20%). Impacting silicon, these will free 1616 and 1778 electrons, respectively.

Image of Fe55 x-rays obtained in the RIT Rochester Imaging Detector Lab (RIDL) with a silicon detector.

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Fe55 Experiment with Silicon Detector

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CCDs as X-Ray Detectors

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CCDs “Count” X-Ray photons• X-Ray events happen much less often

– fewer available X rays – smaller collecting area of telescope

• Each absorbed x-ray has much more energy– deposits more energy in CCD– generates many electrons

• Each x-ray can be counted– attributes of individual photons are measured independently

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Measured Attributes of Each X Ray1. Position of Absorption [x,y]2. Time when Absorption Occurred [t]3. Amount of Energy Absorbed [E]

• Four Pieces of Data per Absorption are Transmitted to Earth:

, , ,x y t E

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Why Transmit [x,y,t,E] Instead of Images?• Images have too much data!

– up to 2 CCD images per second– 16 bits of data per pixel (216=65,536 gray levels)– image Size is 1024 1024 pixels 16 10242 2 = 33.6 Mbps– too much to transmit to ground

• “Event Lists” of [x,y,t,E] are compiled by on-board software and transmitted– reduces required data transmission rate

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Image Creation• From event list of [x,y,t,E]

– Count photons in each pixel during observation• 30,000-Second Observation (1/3 day), 10,000 CCD frames are obtained

(one per 3 seconds)• hope each pixel contains ONLY 1 photon per image

• Pairs of data for each event are plotted as coordinates– Number of Events with Different [x,y] “Image”– Number of Events with Different E “Spectrum”– Number of Events with Different E for each [x,y] “Color Cube”

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X-ray Telescopes

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Chandra

Chandra in Earth orbit (artist’s conception)http://chandra.nasa.gov/

Originally AXAFAdvanced X-ray Astrophysics Facility

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Chandra Orbit• Deployed from Columbia, 23 July 1999• Elliptical orbit

– Apogee = 86,487 miles (139,188 km) – Perigee = 5,999 miles (9,655 km)

• High above LEO Can’t be Serviced• Period is 63 h, 28 m, 43 s

– Out of Earth’s Shadow for Long Periods– Longer Observations

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Chandra Mirrors Assembled and Aligned by Kodak in Rochester

“Rings”

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Mirrors Integrated into spacecraft at

TRW (NGST), Redondo Beach, CA

(Note scale of telescope compared to workers)

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Chandra ACIS CCD Sensor

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X-ray Objects

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First Image from Chandra: August, 1999Supernova remnant Cassiopeia A

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Example of X-Ray Spectrum

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Example of X-Ray Spectrum

Gamma-Ray“Burster”GRB991216

http://chandra.harvard.edu/photo/cycle1/0596/index.html

Counts

E

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Chandra/ACIS image and spectrum of Cas A

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Light Curve of “X-Ray Binary”

http://heasarc.gsfc.nasa.gov/docs/objects/binaries/gx301s2_lc.html