The X-ray Astronomy Imaging Chain

Pop quiz (1): which of these is the X-ray image?

The dying star (“planetary nebula”) BD +30 3639

Pop quiz (2): which of these is the X-ray image?

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

telescopemirror 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, 1999Supernova 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

Central Orion Nebula region, X-raytime step 1

X-ray image cube example: space vs. time

Central Orion Nebula region, X-raytime step 2

X-ray image cube example: space vs. time