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The X-ray Astronomy
Imaging Chain
Pop quiz (1): which of these is the
X-ray image?
Pop quiz (2): which of these is the
X-ray image?
The dying star (“planetary nebula”) BD +30 3639
Medical vs. astronomical
X-ray imaging
Medical: X-rays from source form shadowgram of object on film
Astronomical: X-rays from high-energy astronomical source are collected by telescope
The Need to Go to Space
• Because X-rays are
absorbed by Earth’s
atmosphere,
telescope must go
above atmosphere to
detect celestial
objects
• Like its predecessor
X-ray observatories,
Chandra was
designed as a space
telescope
Chandra in Earth orbit
(artist’s conception)
How are X-rays focused?
• X-ray
telescopes use
grazing
incidence
optics
• Mirrors are
arranged in
concentric
shells
Chandra
X-ray
telescope
mirror
design
X-rays strike the nested mirrors...
…are “gently” redirected toward
the detectors...
…and the detectors capture the
resulting image of the source.
X-ray Mirrors
• Why grazing incidence?
– X-ray photons at near-normal incidence would be transmitted or
absorbed rather than reflected
– At near-parallel incidence, X-rays “skip” off mirror surface (like
stones slipped across surface of a pond)
• Why nested mirror shells?
– Each grazing-incidence mirror shell has a small area exposed to
sky
– Therefore, beef up collecting area by nesting shells
• What limits resolution?
– No atmosphere (X-rays not susceptible to scintillation anyway)
– Also, diffraction limit is very small because lambda is small
– However, exceedingly difficult to produce mirror surfaces that are
smooth at X-ray wavelength scales
The Chandra mirrors were assembled by
Kodak, right here in Rochester
The mirrors
were
integrated
into the
Chandra
spacecraft at
TRW in
Redondo
Beach, CA
On the Road Again...
Travels of the Chandra mirrors
The main Chandra X-ray detector system uses CCDs
Advanced CCD Imaging Spectrometer (ACIS)
CCDs as X-ray detectors
CCDs as X-ray detectors
• Operate in “photon counting” mode
• Detected photons (“events”) are identified
and extracted from the CCD image by
software on board the spacecraft
• Each event is “tagged” with essential
attributes:
– position
– energy
– time
X-ray CCDs (continued)
• Why “photon counting” mode?
– In contrast w/ optical, in which large ensembles of photons are
detected in each pixel in each CCD frame, X-rays can be counted
one at a time
– In principle, this allows attributes of each photon to be measured
independently
• Why identify X-rays (in CCD images) on board
spacecraft?
– Chandra takes up to 6 CCD images, each 1024x1024, once every 3
seconds: data rate prohibitively high for transmission to ground
– “Event lists” (with photon x, y, E, and t) compiled by on-board
software represent an enormous reduction in required data
transmission rate
Chandra launch: July 23, 1999
The first image: August, 1999
Supernova remnant Cassiopeia A
Processing: from X-ray events to
images, spectra, etc.
• How was the preceding image produced?
– Recall that events (photons), not CCD images, were
sent to ground
• Answer: bin and produce 2-D histogram
– Establish image cell grid (pixel size)
• Can (but doesn’t have to) correspond to CCD pixel size
– Count up the number of photons that landed in each
grid cell (pixel) during the observation
• For a 30,000 (1/3 day) second observation, 10,000 CCD
frames are obtained
• Hopefully, each grid cell will contain only 1 photon per frame
Processing X-ray data (cont.)
• Spectra and “light curves” produced the
same way
– Both are 1-D histograms
• Spectrum: no. of photons vs. energy of photon
• Light curve: no. of photons vs. time
• Can combine either energy or time data
with image data, to produce image cube
– 3-D histogram
Chandra/ACIS image and spectrum of Cas A
X-ray image cube example:
space vs. time
Central Orion Nebula region, X-ray
time step 1
X-ray image cube example:
space vs. time
Central Orion Nebula region, X-ray
time step 2