<p>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.</p>
<p>The first light images look very similar to simulated pre-flight images (e.g. <a href="https://www.aanda.org/articles/aa/pdf/forth/aa36325-19.pdf" rel="nofollow noreferrer">Hoyer et al. 2020</a>; <a href="https://arxiv.org/abs/2001.05587" rel="nofollow noreferrer">Futyan et al. 2020</a>). The first light images are in line with expectations. <strong>There is therefore no unexpected problem with the telescope or optics, as there was with HST,</strong> and therefore no unexpected compromise of the mission sensitivity or goals.</p>
<p>From Hoyer et al.</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>The reasons for the weird defocused PSF shape are discussed in section 11.1 of <a href="https://link.springer.com/article/10.1007/s10686-020-09679-4" rel="nofollow noreferrer">Benz et al. (2020)</a>. 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.</p>

CC BY-SA 4.0
2020-02-22T09:52:48.327Z
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](https://astronomy.stackexchange.com/a/34901/7982).
uhoh
CC BY-SA 4.0
2020-02-22T14:09:07.733Z
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.
uhoh
CC BY-SA 4.0
2020-11-18T06:05:27.467Z
Section 4.3 of [Expected performances of the Characterising Exoplanet Satellite (CHEOPS) I. Photometric performances from ground-based calibration](https://www.aanda.org/articles/aa/abs/2020/03/aa35977-19/aa35977-19.html): "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
uhoh
CC BY-SA 4.0
2020-02-22T08:04:10.900Z
2021-02-13T15:15:59.877Z
2021-02-13T15:15:59.877Z
ProfRob
<p><a href="https://astronomy.stackexchange.com/questions/35166/lousy-mirror-corrected-by-software#comment67911_35166">@KeithMcClary's comment</a> under <a href="https://astronomy.stackexchange.com/q/35166/7982">lousy mirror corrected by software</a> links to Bad Astronomy's <a href="https://www.syfy.com/syfywire/first-light-for-the-exoplanet-hunter-mission-cheops-goes-tetrahedral" rel="nofollow noreferrer">First Light for the Exoplanet Hunter Mission CHEOPS Goes Tetrahedral</a> which shows the image below.</p>
<p>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?</p>
<p>If it stays this way, will it degrade CHEOPS' performance?</p>
<blockquote>
<p>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.</p>
<p>The image, traditionally called First Light by astronomers, is itself … surprising.</p>
</blockquote>
<p><a href="https://i.sstatic.net/ECBsw.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ECBsw.png" alt="CHEOPS first light in Bad Astronomy" /></a></p>

CC BY-SA 4.0
2020-02-22T14:02:32.223Z
This really IS the Cheops PSF, as mentioned in an answer below.
AtmosphericPrisonEscape
CC BY-SA 4.0
2020-02-22T14:04:12.797Z
@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?
uhoh
CC BY-SA 4.0
2020-02-22T16:42:46.637Z
The precise shape of the PSF is *irrelevant* to the science they want to do. The actual measurements will be the integrated counts within a large aperture containing the defocussed PSF image. This is not an imaging telescope!
Peter Erwin
CC BY-SA 4.0
2020-02-22T16:55:04.177Z
[This](https://www.cosmos.esa.int/documents/10651/1257879/CHEOPS_FactSheet-Sept2018.pdf/b7a4c58a-2f1e-1c41-40b0-649dc3e6b609?t=1552841370067) is a 2-page PDF describing CHEOPS, produced in 2018. On the second page, you can see an image of the laboratory-measured PSF, which shows a similar curved, three-sided structure. So it looks like what they expected it to look.
Peter Erwin
CC BY-SA 4.0
2020-02-22T23:19:09.767Z
@PeterErwin I may have to ask a followup question then in order to illustrate why [what I've discussed here](https://astronomy.stackexchange.com/questions/35182/does-this-cheops-first-light-image-imply-bad-astronomy?noredirect=1#comment67950_35185) concerns me so much. If the PSF has hot spots that are smaller than a pixel, then one would see a different integrated intensity when said hot spot falls on a pixel than when it fell in the dead space between to pixels. Am I naive to think one would want a smoothly varying PSF for precision photometry instead of one with such intense fine structure?
uhoh
CC BY-SA 4.0
2020-02-24T12:38:26.407Z
@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.
Peter Erwin
CC BY-SA 4.0
2020-02-24T12:40:42.890Z
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]
Peter Erwin
CC BY-SA 4.0
2020-02-24T12:40:57.540Z
"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."
Peter Erwin
CC BY-SA 4.0
2020-02-24T13:00:53.613Z
@PeterErwin thank you for taking the time to discuss this further. I have an actual "Stack Exchange ToDo list and reading up on how CHEOPS works and processes/filters/integrates to get good photometry will be fun. Now that you've written so much, perhaps it could be just copy/pasted into a supplementary answer where future readers are more likely to see it?
uhoh
<observational-astronomy><optics><space-telescope><cheops>
Does this CHEOPS first light image imply bad astronomy?
uhoh
# Does this CHEOPS first light image imply bad astronomy?

(Asked by: username_0 on 2020-02-22)

[@KeithMcClary's comment](https://astronomy.stackexchange.com/questions/35166/lousy-mirror-corrected-by-software#comment67911_35166) under [lousy mirror corrected by software](https://astronomy.stackexchange.com/q/35166/7982) links to Bad Astronomy's [First Light for the Exoplanet Hunter Mission CHEOPS Goes Tetrahedral](https://www.syfy.com/syfywire/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](https://i.sstatic.net/ECBsw.png)](https://i.sstatic.net/ECBsw.png)

**Tags:** observational-astronomy, optics, space-telescope, cheops

> **Comments:**
> **username_1**: This really IS the Cheops PSF, as mentioned in an answer below.  
> **username_0**: @username_1 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?  
> **username_2**: The precise shape of the PSF is *irrelevant* to the science they want to do. The actual measurements will be the integrated counts within a large aperture containing the defocussed PSF image. This is not an imaging telescope!  
> **username_2**: [This](https://www.cosmos.esa.int/documents/10651/1257879/CHEOPS_FactSheet-Sept2018.pdf/b7a4c58a-2f1e-1c41-40b0-649dc3e6b609?t=1552841370067) is a 2-page PDF describing CHEOPS, produced in 2018. On the second page, you can see an image of the laboratory-measured PSF, which shows a similar curved, three-sided structure. So it looks like what they expected it to look.  
> **username_0**: @username_2 I may have to ask a followup question then in order to illustrate why [what I've discussed here](https://astronomy.stackexchange.com/questions/35182/does-this-cheops-first-light-image-imply-bad-astronomy?noredirect=1#comment67950_35185) concerns me so much. If the PSF has hot spots that are smaller than a pixel, then one would see a different integrated intensity when said hot spot falls on a pixel than when it fell in the dead space between to pixels. Am I naive to think one would want a smoothly varying PSF for precision photometry instead of one with such intense fine structure?  
> **username_2**: @username_0 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.  
> **username_2**: 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]  
> **username_2**: "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."  
> **username_0**: @username_2 thank you for taking the time to discuss this further. I have an actual "Stack Exchange ToDo list and reading up on how CHEOPS works and processes/filters/integrates to get good photometry will be fun. Now that you've written so much, perhaps it could be just copy/pasted into a supplementary answer where future readers are more likely to see it?  

---

## 1 answers

### Answer by username_3 on 2020-02-22. Score: 8

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](https://www.aanda.org/articles/aa/pdf/forth/aa36325-19.pdf); [Futyan et al. 2020](https://arxiv.org/abs/2001.05587)). 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)](https://link.springer.com/article/10.1007/s10686-020-09679-4). 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.

> **Comments:**
> **username_0**: 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](https://astronomy.stackexchange.com/a/34901/7982).  
> **username_0**: 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.  
> **username_0**: Section 4.3 of [Expected performances of the Characterising Exoplanet Satellite (CHEOPS) I. Photometric performances from ground-based calibration](https://www.aanda.org/articles/aa/abs/2020/03/aa35977-19/aa35977-19.html): "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  

---

