1. Astronomical Observational Techniques and Instrumentation
RIT Course Number
Professor Don Figer
X-ray Astronomy

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

3. X-ray Photon Path

4. 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!

5. 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
X Rays
Visible Light

6. X Rays (and Gamma Rays “”) Can be “Absorbed”
By dense material, e.g., lead (Pb)
Sensor

7. Imaging System Based on Selective Transmission
Input Object
(Radioactive Thyroid)
Lead Sheet with Pinhole
“Noisy” Output Image
(because of small number
of detected photons)

8. How to “Add” More Photons 1. Make Pinhole Larger
 “Fuzzy” Image
Input Object
(Radioactive Thyroid
w/ “Hot” and “Cold” Spots)
“Noisy” Output Image
(because of small number
of detected photons)
“Fuzzy” Image
Through Large Pinhole
(but less noise)

9. How to “Add” More Photons 2. Add More Pinholes
BUT: Images “Overlap”

10. How to “Add” More Photons 2. Add More Pinholes
Process to combine “overlapping” images
Before Postprocessing
After Postprocessing

11. 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.

12. 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

13. X Rays and Grazing Incidence
X-Ray “Mirror” 
X Ray at “Grazing Incidence
is “Deviated” by Angle 
(which is SMALL!)

14. 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)

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

16. 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

17. 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

18. X-ray Detection

19. 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!

20. X-ray Detectors Principle measurements Specific types of detectors
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

21. 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

22. 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.

23. Fe55 Experiment with Silicon Detector

24. CCDs as X-Ray Detectors

25. 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

26. Measured Attributes of Each X Ray
Position of Absorption [x,y]
Time when Absorption Occurred [t]
Amount of Energy Absorbed [E]
Four Pieces of Data per Absorption are Transmitted to Earth:

27. 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   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

28. 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”

29. X-ray Telescopes

30. Chandra in Earth orbit (artist’s conception)
Originally AXAF
Advanced X-ray
Astrophysics Facility
Chandra in Earth orbit (artist’s conception)

31. 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

32. Chandra Mirrors Assembled and Aligned by Kodak in Rochester
“Rings”

33. Mirrors Integrated into spacecraft at TRW (NGST), Redondo Beach, CA (Note scale of telescope compared to workers)

34. Chandra ACIS CCD Sensor

35. X-ray Objects

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

37. Example of X-Ray Spectrum

38. Example of X-Ray Spectrum
Gamma-Ray
“Burster”
GRB991216
Counts E

39. Chandra/ACIS image and spectrum of Cas A

40. Light Curve of “X-Ray Binary”