From The Observatory:

Anyone who missed any of the AAS 239 press conferences can see them on the AAS Press Office's YouTube channel! Here's the full program.

From there I found the Monday, January 10, 2022 presentation NASA’S Newest X-Ray Eyes: an Update on the Imaging X-ray Polarimetry Explorer (IXPE) by Martin C. Weisskopf, IXPE Principal Investigator NASA/Marshall Space Flight Center

Below is a screenshot from page 4 which shows a diagram of IXPE and indicates that it has three parallel and identical X-ray telescopes.

Question: How does IXPE measure polarization, and why does it have three identical X-ray telescopes?

cropped from slide #4 of Martin C. Weisskopf, IXPE Principal Investigator's NASA’S Newest X-Ray Eyes: an Update on the Imaging X-ray Polarimetry Explorer (IXPE)

  • $\begingroup$ @PM2Ring ya it seems to have been the result of an edit gone wrong; the current answer's author didn't catch it either, so I've deleted the first "telescope" in all places on the page (except of course your comment. Thanks! $\endgroup$
    – uhoh
    Commented Jan 16, 2022 at 14:31

1 Answer 1


How does IXPE measure polarization?

Short answer: Electrons in the detector are preferential emitted in the direction of the polarization of the incident photon and then detected.

Quoting from NASA's IXPE: Polarization Detection page:

IXPE will have three identical X-ray telescopes, with polarization detectors at each focus. The detectors, called Gas Pixel Detectors (GPD) are based on proportional counters (see Mullard Space Science Laboratory's Introduction to Proportional Counters). Polarized X rays interacting with a gaseous medium create photoelectons that are preferentially emitted in the polarization direction. Photoelectron tracks mark the path of the photoelectron from the position of the initial X-ray interaction to its stopping point. Analysis of the distribution of the initial directions of the tracks gives the degree of polarization and the position angle from the incident X ray. Depiction of polarization selection process in the X-ray detector

Why does it have three identical X-ray telescopes?

Quoting from IXPE - Satellite Missions - eoPortal Directory:

Short answer: Redundancy and the three smaller systems could be accommodated better than one larger equivalent telescope.

These trades were completed and the result is the three telescope system described here which meets science objectives and requirements with margin while placing reasonable and achievable demands on the spacecraft, launch vehicle, and the deployable optical bench. Specifically, three identical systems provide redundancy, a range of detector clocking angles to mitigate against any detector biases, shorter focal length for given mirror graze angles (i.e., given energy response) and thinner/lighter mirrors compared to a single telescope system.


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