<p>There's a paper by <a href="https://arxiv.org/abs/1007.1612" rel="nofollow noreferrer">Gillesen et al. (2010)</a> that discusses the GRAVITY (4-telescope) interferometer at the VLT.</p>
<p>It presents the following image based on a 9 hours observation towards the Galactic centre. This is said the be &quot;The PSF for one night of VLTI observations.&quot; It's not very satisfactory because there is no indication of what the color scale means! But I presume white/yellow is good and blue is poor. The square is 0.1 arcseconds across.</p>
<p><a href="https://i.sstatic.net/ZVqmi.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZVqmi.png" alt="PSF for VLTI" /></a></p>

CC BY-SA 4.0
2022-05-22T11:46:27.267Z
2022-05-22T11:46:27.267Z
ProfRob
<p>In interferometric radio astronomy UV plots are the first step in understanding what a point spread function (PSF) will look like for a given location in the sky observed over a period of time. The larger the number of dishes used, the better the choice of locations for each dish (pseudo-random spiral-like ground patterns) and the longer the observation (making longer arcs in the UV plane) all help to reduce the size of the PSF and reduce any bright secondary spots within it.</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/26413/7982">Math behind a uv plot in interferometry?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/41107/7982">In astronomical interferometry, what values do the points in the uv-plane have?</a></li>
</ul>
<p>Bits of a discussion in comments below the currently unanswered question <a href="https://astronomy.stackexchange.com/q/28619/7982">Are they really sure this isn&#39;t an Airy disk? How was that ruled out?</a> goes something like:</p>
<blockquote>
<p>I think part of the answer is that interferometric point-spread functions generally do not look like the PSF of single-aperture telescopes, with a circular, symmetric Airy disk.</p>
</blockquote>
<p>and</p>
<blockquote>
<p>where the image is built up over several nights from several different sets of three<sup>1</sup> &quot;holes&quot; rather than interfere them all at once, (they move the telescopes around in between) and I don't know know how they synthesize that to get one image.</p>
</blockquote>
<p><strong>Question:</strong> So I'm now wondering, with the VLT's large <em>but sparse</em> interferometric aperture (there are only four primary telescopes though I think there are some secondary (auxiliary) telescopes that feed into the interferometry as well): <em>What does a &quot;point spread function&quot; look like for long exposures?</em></p>
<hr />
<p><sup>1</sup>should be four<sup>2</sup> I think, plus those auxiliary telescopes</p>
<p><sup>2</sup>No, <a href="https://astronomy.stackexchange.com/questions/41213/what-does-a-point-spread-function-look-like-for-long-exposures-from-the-vlts?noredirect=1#comment84372_41213">three was right</a> for that image, hat tip to astrosnapper</p>

CC BY-SA 4.0
2021-02-04T00:38:40.767Z
I. Can't. Resist. Point spread: Chiefs -3 (-105), Buccaneers +3 (+105)
David Hammen
CC BY-SA 4.0
2021-02-04T18:34:22.957Z
The image linked in the previous question was made with [AMBER](https://www.eso.org/sci/facilities/paranal/decommissioned/amber.html) a previous generation beam combiner and instrument which only supported 3 telescopes/beams. The [current instruments](https://www.eso.org/sci/facilities/paranal/telescopes/vlti/configuration/P105.html) `MATISSE, PIONIER, GRAVITY` support combining 4 beams.
astrosnapper
CC BY-SA 4.0
2021-06-24T01:07:11.150Z
different but related: [Do point spread functions from large telescopes using adaptive optics still look like Airy functions for narrow-band filters?](https://astronomy.stackexchange.com/q/44526/7982)
uhoh
CC BY-SA 4.0
2022-05-22T11:16:02.920Z
It totally depends on where the source is on the sky and how long the observations were. So I take it you just want *an* example.
ProfRob
<observational-astronomy><photography><optics><interferometry><very-large-telescope>
What does a narrow-band "point spread function" look like for long exposures from the VLT's large interferometric aperture?
uhoh
# What does a narrow-band "point spread function" look like for long exposures from the VLT's large interferometric aperture?

(Asked by: username_0 on 2021-02-04)

In interferometric radio astronomy UV plots are the first step in understanding what a point spread function (PSF) will look like for a given location in the sky observed over a period of time. The larger the number of dishes used, the better the choice of locations for each dish (pseudo-random spiral-like ground patterns) and the longer the observation (making longer arcs in the UV plane) all help to reduce the size of the PSF and reduce any bright secondary spots within it.

* [Math behind a uv plot in interferometry?](https://astronomy.stackexchange.com/q/26413/7982)
* [In astronomical interferometry, what values do the points in the uv-plane have?](https://astronomy.stackexchange.com/q/41107/7982)

Bits of a discussion in comments below the currently unanswered question [Are they really sure this isn't an Airy disk? How was that ruled out?](https://astronomy.stackexchange.com/q/28619/7982) goes something like:

> I think part of the answer is that interferometric point-spread functions generally do not look like the PSF of single-aperture telescopes, with a circular, symmetric Airy disk.

and

> where the image is built up over several nights from several different sets of three1 "holes" rather than interfere them all at once, (they move the telescopes around in between) and I don't know know how they synthesize that to get one image.

**Question:** So I'm now wondering, with the VLT's large *but sparse* interferometric aperture (there are only four primary telescopes though I think there are some secondary (auxiliary) telescopes that feed into the interferometry as well): *What does a "point spread function" look like for long exposures?*

---

1should be four2 I think, plus those auxiliary telescopes

2No, [three was right](https://astronomy.stackexchange.com/questions/41213/what-does-a-point-spread-function-look-like-for-long-exposures-from-the-vlts?noredirect=1#comment84372_41213) for that image, hat tip to astrosnapper

**Tags:** observational-astronomy, photography, optics, interferometry, very-large-telescope
> **Comments:**
> **username_1**: I. Can't. Resist. Point spread: Chiefs -3 (-105), Buccaneers +3 (+105)  
> **username_2**: The image linked in the previous question was made with [AMBER](https://www.eso.org/sci/facilities/paranal/decommissioned/amber.html) a previous generation beam combiner and instrument which only supported 3 telescopes/beams. The [current instruments](https://www.eso.org/sci/facilities/paranal/telescopes/vlti/configuration/P105.html) `MATISSE, PIONIER, GRAVITY` support combining 4 beams.  
> **username_0**: different but related: [Do point spread functions from large telescopes using adaptive optics still look like Airy functions for narrow-band filters?](https://astronomy.stackexchange.com/q/44526/7982)  
> **username_3**: It totally depends on where the source is on the sky and how long the observations were. So I take it you just want *an* example.  

---

## 1 answers

### Answer by username_3 on 2022-05-22. Score: 1 | Accepted answer

There's a paper by [Gillesen et al. (2010)](https://arxiv.org/abs/1007.1612) that discusses the GRAVITY (4-telescope) interferometer at the VLT.

It presents the following image based on a 9 hours observation towards the Galactic centre. This is said the be "The PSF for one night of VLTI observations." It's not very satisfactory because there is no indication of what the color scale means! But I presume white/yellow is good and blue is poor. The square is 0.1 arcseconds across.

[![PSF for VLTI](https://i.sstatic.net/ZVqmi.png)](https://i.sstatic.net/ZVqmi.png)


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