1. The X-ray Astronomy Imaging Chain

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

3. The dying star (“planetary nebula”) BD +30 3639 Pop quiz (2): which of these is the X-ray image?

4. 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

5. 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)

6. How are X-rays focused? X-ray telescopes use grazing incidence optics Mirrors are arranged in concentric shells Chandra X-ray telescope mirror design

7. X-rays strike the nested mirrors...

8. …are “gently” redirected toward the detectors...

9. …and the detectors capture the resulting image of the source.

10. 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

11. The Chandra mirrors were assembled by Kodak, right here in Rochester

12. The mirrors were integrated into the Chandra spacecraft at TRW in Redondo Beach, CA

13. On the Road Again... Travels of the Chandra mirrors

14. The main Chandra X-ray detector system uses CCDs Advanced CCD Imaging Spectrometer (ACIS)

15. CCDs as X-ray detectors

16. 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

17. 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

18. Chandra launch: July 23, 1999

19. The first image: August, 1999 Supernova remnant Cassiopeia A

20. 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

21. 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

22. Chandra/ACIS image and spectrum of Cas A

23. Central Orion Nebula region, X-ray time step 1 X-ray image cube example: space vs. time

24. Central Orion Nebula region, X-ray time step 2 X-ray image cube example: space vs. time