@KeithMcClary's comment under lousy mirror corrected by software links to Bad Astronomy's First Light for the Exoplanet Hunter Mission CHEOPS Goes Tetrahedral which shows the image below.

I understand that in order to do precision photometry with a pixelated sensor one wants to spread the light over several pixels in a controlled way, but is this particular PSF the result of some painstaking optimization, some problem with the figure of one of the telescope's elements (oh no I hope it's not 1990 all over again!) or just because it's preliminary and they'll tweak something to make it more symmetrical?

If it stays this way, will it degrade CHEOPS' performance?

CHEOPS launched on 18 December, 2019, and on 07 February, 2020, it took its very first image of a star! The target chosen was HD 70843, a star about 155 light years away that’s a little bit hotter and brighter than the Sun.

The image, traditionally called First Light by astronomers, is itself … surprising.

CHEOPS first light in Bad Astronomy

  • $\begingroup$ This really IS the Cheops PSF, as mentioned in an answer below. $\endgroup$ Feb 22, 2020 at 14:02
  • $\begingroup$ @AtmosphericPrisonEscape and the question above doesn't say it isn't. But why is it so weird looking that Phil Plait wrote a whole blogpost about how weird it looks? Who intentionally builds a telescope with a PSF that looks like a squashed tetrahedron? $\endgroup$
    – uhoh
    Feb 22, 2020 at 14:04
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    $\begingroup$ @uhoh Note that the "hot spots" are only a factor of 2 or so brighter than the faint regions (figure of the PSF in the file I linked to). That's very different from the several-orders-of-magnitude variation you get in an unblurred PSF (between, say, the core and the first Airy ring). So the actual variation in integrated intensity will be pretty small. $\endgroup$ Feb 24, 2020 at 12:38
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    $\begingroup$ Also, note this comment from Magrin et al. (2014; "Shaping the PSF to nearly top-hat profile: CHEOPS laboratory results"), which was about experimenting with methods of making smoother blurred PSFs [continued in next comment] $\endgroup$ Feb 24, 2020 at 12:40
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    $\begingroup$ "At the end, for CHEOPS mission, it has been decided to avoid any kind of PSF shaper device, because it has been demonstrated through simulations that the shape of the PSF has much smaller impact with respect to other noise sources. On the contrary, the PSF stability and the straylight background are dominant noise factors and the introduction of an additional optical element in the intermediate pupil could be a source of deterioration for them." $\endgroup$ Feb 24, 2020 at 12:40

1 Answer 1


As the article you reference makes clear, the defocusing is deliberate. It spreads the light of bright stars (the main targets for CHEOPS) over more pixels and hence mitigates saturation and non-linearity problems in the detectors.

The first light images look very similar to simulated pre-flight images (e.g. Hoyer et al. 2020; Futyan et al. 2020). The first light images are in line with expectations. There is therefore no unexpected problem with the telescope or optics, as there was with HST, and therefore no unexpected compromise of the mission sensitivity or goals.

From Hoyer et al.

The focal plane is defocussed to deliver a large PSF with a 12 pixels radius encircling 90% of the flux. As a result of the combination of the Ritchey-Chrétien design and other specific features of the building of the telescope, the PSF exhibits sharp and peaky features at sub-pixel level.

The reasons for the weird defocused PSF shape are discussed in section 11.1 of Benz et al. (2020). The triangular pattern is caused by strain on the primary mirror from a three-point fixation mechanism. This fixing mechanism was added to ensure that it wasn't shaken out of alignment during launch. In fact, Benz et al. say that the measured PSF (in zero gravity) is a little bit more symmetric, wider and smoother than found in the laboratory calibration tests. As a result there are some positives for observing bright stars - reducing the possibility of image saturation, but the situation is slightly worse for fainter stars because the wider PSF introduces more background signal into the photometry aperture.

  • $\begingroup$ I've mentioned in the question that I understand that Phil Plait explains that "one wants to spread the light over several pixels in a controlled way", but do slightly defocused spots from good telescopes really look this irregular? The paper mentions "the extended irregular point spread function" so I think this is more than a deliberate defocus which would tend to look a little more regular and perhaps donut-like. $\endgroup$
    – uhoh
    Feb 22, 2020 at 9:52
  • $\begingroup$ I'll read through it again tomorrow to see if I can figure out what those "other specific features of the building of the telescope" might be. I would think that "sharp and peaky features at sub-pixel level" are the absolute last thing one would want to have on one's pixels if one's raison d'etre were precision photometry. In other words, to me it looks like a significant problem. $\endgroup$
    – uhoh
    Feb 22, 2020 at 14:09
  • $\begingroup$ Section 4.3 of Expected performances of the Characterising Exoplanet Satellite (CHEOPS) I. Photometric performances from ground-based calibration: "In addition, the number of photoelectrons in the photometric aperture is limited by the fact that the flux is adjusted such as the value of strongest peak of the PSF in a pixel never reaches the saturation of the detector." While the "sharp and peaky features" may be an "existential" limitation, it might not impact the required performance. If it did, that wouldn't say so $\endgroup$
    – uhoh
    Nov 18, 2020 at 6:05

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