<p><strong>How does IXPE measure polarization?</strong></p>
<p>Short answer: Electrons in the detector are preferential emitted in the direction of the polarization of the incident photon and then detected.</p>
<p>Quoting from NASA's <a href="https://ixpe.msfc.nasa.gov/detectors.html" rel="nofollow noreferrer">IXPE: Polarization Detection</a> page:</p>
<blockquote>
<p>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 <a href="http://www.mssl.ucl.ac.uk/%7Emwt/propcounters/intro.htm" rel="nofollow noreferrer">Mullard Space Science
Laboratory's Introduction to Proportional Counters</a>). 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.
<a href="https://i.sstatic.net/IcaV1.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/IcaV1.png" alt="Depiction of polarization selection process in the X-ray detector" /></a></p>
</blockquote>
<p><strong>Why does it have three identical X-ray telescopes?</strong></p>
<p>Quoting from <a href="https://directory.eoportal.org/web/eoportal/satellite-missions/i/ixpe" rel="nofollow noreferrer">IXPE - Satellite Missions - eoPortal Directory</a>:</p>
<p>Short answer: Redundancy and the three smaller systems could be accommodated better than one larger equivalent telescope.</p>
<blockquote>
<p>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.</p>
</blockquote>

CC BY-SA 4.0
2022-01-16T12:27:05.323Z
2022-01-16T14:30:00.330Z
2022-01-16T14:30:00.330Z
GrapefruitIsAwesome
<p>From <a href="https://chat.stackexchange.com/transcript/message/60155601#60155601">The Observatory</a>:</p>
<blockquote>
<p>Anyone who missed any of the AAS 239 press conferences can see them on the AAS Press Office's YouTube channel! Here's <a href="https://aas.org/meetings/aas239/2nd-media-advisory#program" rel="nofollow noreferrer">the full program</a>.</p>
</blockquote>
<p>From there I found the Monday, January 10, 2022 presentation <a href="https://aas.org/sites/default/files/2022-01/AAS239_Mon2_MartinWeisskopf.pdf" rel="nofollow noreferrer">NASA’S Newest X-Ray Eyes: an Update on the Imaging X-ray Polarimetry Explorer (IXPE)</a> by Martin C. Weisskopf, IXPE Principal Investigator NASA/Marshall Space Flight Center</p>
<p>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.</p>
<p><strong>Question:</strong> How does IXPE measure polarization, and why does it have <strong>three</strong> identical X-ray telescopes?</p>
<p><a href="https://i.sstatic.net/bzDE5.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/bzDE5.jpg" alt="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)" /></a></p>

CC BY-SA 4.0
2022-01-16T14:31:09.707Z
@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!*
uhoh
<observational-astronomy><space-telescope><instruments><x-ray>
How does IXPE measure polarization, and why does it have three identical X-ray telescopes?
uhoh
# How does IXPE measure polarization, and why does it have three identical X-ray telescopes?

(Asked by: username_0 on 2022-01-16)

From [The Observatory](https://chat.stackexchange.com/transcript/message/60155601#60155601):

> 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](https://aas.org/meetings/aas239/2nd-media-advisory#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)](https://aas.org/sites/default/files/2022-01/AAS239_Mon2_MartinWeisskopf.pdf) 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)](https://i.sstatic.net/bzDE5.jpg)](https://i.sstatic.net/bzDE5.jpg)

**Tags:** observational-astronomy, space-telescope, instruments, x-ray

> **Comments:**
> **username_0**: @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!*  

---

## 1 answers

### Answer by username_1 on 2022-01-16. Score: 3 | Accepted answer

**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](https://ixpe.msfc.nasa.gov/detectors.html) 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](http://www.mssl.ucl.ac.uk/%7Emwt/propcounters/intro.htm)). 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](https://i.sstatic.net/IcaV1.png)](https://i.sstatic.net/IcaV1.png)

**Why does it have three identical X-ray telescopes?**

Quoting from [IXPE - Satellite Missions - eoPortal Directory](https://directory.eoportal.org/web/eoportal/satellite-missions/i/ixpe):

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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