<p>I asked myself the same question and back then, I found the <a href="https://de.wikipedia.org/wiki/Large_Binocular_Telescope#Konzept" rel="nofollow noreferrer">reasoning on the German Wikipedia page</a> pretty comprehensive. I am translating, summarizing and expanding the corresponding section in the following:</p>
<p><strong>By using two mirrors instead of a single one has a couple of advantages:</strong></p>
<ul>
<li>The double telescope collects the same amount of light as an 11.8-metre mirror, but is from an engineering point of view easier to built than a single large mirror, because the center of mass is outside the telescope(s). An example structural problems of large moving telescope was e.g. the failure of the <a href="https://en.wikipedia.org/wiki/Green_Bank_Telescope" rel="nofollow noreferrer">Green Bank Telescope</a> in November 1988. In radio astronomy, the approach of having multi-antennas on a single moveable structure is used for long, e.g. in the <a href="https://en.wikipedia.org/wiki/Pluton_(complex)" rel="nofollow noreferrer">Pluton Complex</a></li>
<li>The LBT's optical resolution can be increased by interferometry to that of a 22.8-metre mirror.</li>
<li>Interferometry will also be used to virtually block out the light from central stars in order to make their possible planets directly visible, sometimes called <a href="https://en.wikipedia.org/wiki/Nuller" rel="nofollow noreferrer">nulling interferometer</a>.</li>
</ul>
<p>Last but not least, I remember hearing that LBT's mirror size is the largest feasible non-segmented mirror. This hypothesis is somewhat supported if you look at the <a href="https://en.wikipedia.org/wiki/List_of_largest_optical_reflecting_telescopes" rel="nofollow noreferrer">list of largest optical telescopes</a>.</p>

CC BY-SA 4.0
2021-03-11T14:24:59.993Z
Thank you for reviving this old question and writing up a thorough answer!
uhoh
CC BY-SA 4.0
2021-03-11T14:18:53.797Z
2021-03-11T14:27:38.997Z
2021-03-11T14:27:38.997Z
B--rian
<p><strong>edit:</strong> The short form of the question is <em>Why are there TWO telescopes??</em> A longer version follows. </p>

<p>The <a href="https://en.wikipedia.org/wiki/Large_Binocular_Telescope" rel="noreferrer">Large Binocular Telescope</a> (and <a href="http://www.lbto.org/leech-hr8799.html" rel="noreferrer">LBT blog as source of images below</a>) has two 8.4 meter diameter mirrors, allowing for the possibility of interference in one dimension.</p>

<p>I'm aware that a single baseline, two-element <a href="https://en.wikipedia.org/wiki/Michelson_stellar_interferometer" rel="noreferrer">stellar interferometer</a> can be used to measure the separation between a pair of stars by looking at the Fourier transform of the intensity vs time of the interference. A single frequency peak is produced by one star, but if there are two close together, there will be "beating" in the time series that shows up as two peaks in the FT. </p>

<p>But I'm wondering if the LBT was designed and built with a plan to do more than that. <em>Just for example</em>, did plans include an ability to use interferometry to help reconstruct 2D images, or enhance the resolution beyond that which is already provided by the <a href="https://en.wikipedia.org/wiki/Large_Binocular_Telescope#Adaptive_optics" rel="noreferrer">adaptive optics</a> used in each half separately? Were there other plans or justifications?</p>

<p>Possibly helpful; <a href="https://arxiv.org/abs/1608.00037" rel="noreferrer">Current Status of the Facility Instrumentation Suite at The Large Binocular Telescope Observatory</a></p>

<p>Open image in new window for full size:</p>

<p><a href="https://i.sstatic.net/rxwF9.jpg" rel="noreferrer"><img src="https://i.sstatic.net/rxwF9.jpg" alt="enter image description here"></a></p>

CC BY-SA 3.0
2017-12-25T13:37:53.717Z
Great question. This is something that I, too, would like to know. I expect that it involves multiple exposures, and a good deal of computer processing to extract the maximum amount of information from a rotating field of view. I suspect, also, that the dark arcs that the planets are embedded in are artefacts of the image processing algorithms.
Mick
CC BY-SA 3.0
2017-12-25T23:04:53.190Z
Due to technical problems, the LBT until very recently (beginning 2016), couldn't use their two mirrors in interferometric mode. Thus all images you see and quote from before that period have been taken with single mirror exposures.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-12-26T01:45:25.053Z
@AtmosphericPrisonEscape thanks for that! I've revised the question accordingly.
uhoh
CC BY-SA 3.0
2017-12-26T23:31:25.167Z
@uhoh: Then I think the answer is 'not at all, until recently...'
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-12-27T00:02:58.477Z
@AtmosphericPrisonEscape I'm assuming you're helping me fine tune the *tense* of the question in order to set the best possible stage for an excellent answer. At this point I think anybody reading in good faith will understand what I'm asking.
uhoh
CC BY-SA 3.0
2018-01-14T09:40:34.033Z
@AtmosphericPrisonEscape I can probably post an answer next week, but if you'd like to, I'll hold off.
uhoh
CC BY-SA 3.0
2018-01-14T11:45:08.337Z
@uhoh: Well I know my answer, but I'm too lazy to look up all the references you'd be interested in. So go ahead.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-04-14T09:02:00.313Z
Not to nitpick, but *raison d'être* is closer in meaning to "purpose" or "reason for being", not to "explanation for how it works" or "how come that is possible". ;)
Florin Andrei
CC BY-SA 4.0
2019-04-14T09:33:40.950Z
@FlorinAndrei that's exactly the meaning intended. I've asked why this telescope exists, or what is the justification for its existence? Maybe I'm being a little poetic for extending *raisons d'être* to scientific instrumentation, but it's how I felt at the time.
uhoh
<interferometry><post-processing><large-binocular-telescope>
What are the raisons d'être for the Large Binocular Telescope "binocularity"?
uhoh
# What are the raisons d'être for the Large Binocular Telescope "binocularity"?

(Asked by: username_0 on 2017-12-25)

**edit:** The short form of the question is *Why are there TWO telescopes??* A longer version follows.

The [Large Binocular Telescope](https://en.wikipedia.org/wiki/Large_Binocular_Telescope) (and [LBT blog as source of images below](http://www.lbto.org/leech-hr8799.html)) has two 8.4 meter diameter mirrors, allowing for the possibility of interference in one dimension.

I'm aware that a single baseline, two-element [stellar interferometer](https://en.wikipedia.org/wiki/Michelson_stellar_interferometer) can be used to measure the separation between a pair of stars by looking at the Fourier transform of the intensity vs time of the interference. A single frequency peak is produced by one star, but if there are two close together, there will be "beating" in the time series that shows up as two peaks in the FT.

But I'm wondering if the LBT was designed and built with a plan to do more than that. *Just for example*, did plans include an ability to use interferometry to help reconstruct 2D images, or enhance the resolution beyond that which is already provided by the [adaptive optics](https://en.wikipedia.org/wiki/Large_Binocular_Telescope#Adaptive_optics) used in each half separately? Were there other plans or justifications?

Possibly helpful; [Current Status of the Facility Instrumentation Suite at The Large Binocular Telescope Observatory](https://arxiv.org/abs/1608.00037)

Open image in new window for full size:

[![enter image description here](https://i.sstatic.net/rxwF9.jpg)](https://i.sstatic.net/rxwF9.jpg)

**Tags:** interferometry, post-processing, large-binocular-telescope
> **Comments:**
> **username_1**: Great question. This is something that I, too, would like to know. I expect that it involves multiple exposures, and a good deal of computer processing to extract the maximum amount of information from a rotating field of view. I suspect, also, that the dark arcs that the planets are embedded in are artefacts of the image processing algorithms.  
> **username_2**: Due to technical problems, the LBT until very recently (beginning 2016), couldn't use their two mirrors in interferometric mode. Thus all images you see and quote from before that period have been taken with single mirror exposures.  
> **username_0**: @AtmosphericPrisonEscape thanks for that! I've revised the question accordingly.  
> **username_2**: @uhoh: Then I think the answer is 'not at all, until recently...'  
> **username_0**: @AtmosphericPrisonEscape I'm assuming you're helping me fine tune the *tense* of the question in order to set the best possible stage for an excellent answer. At this point I think anybody reading in good faith will understand what I'm asking.  
> **username_0**: @AtmosphericPrisonEscape I can probably post an answer next week, but if you'd like to, I'll hold off.  
> **username_2**: @uhoh: Well I know my answer, but I'm too lazy to look up all the references you'd be interested in. So go ahead.  
> **username_3**: Not to nitpick, but *raison d'être* is closer in meaning to "purpose" or "reason for being", not to "explanation for how it works" or "how come that is possible". ;)  
> **username_0**: @FlorinAndrei that's exactly the meaning intended. I've asked why this telescope exists, or what is the justification for its existence? Maybe I'm being a little poetic for extending *raisons d'être* to scientific instrumentation, but it's how I felt at the time.  

---

## 1 answers

### Answer by username_4 on 2021-03-11. Score: 4 | Accepted answer

I asked myself the same question and back then, I found the [reasoning on the German Wikipedia page](https://de.wikipedia.org/wiki/Large_Binocular_Telescope#Konzept) pretty comprehensive. I am translating, summarizing and expanding the corresponding section in the following:

**By using two mirrors instead of a single one has a couple of advantages:**

* The double telescope collects the same amount of light as an 11.8-metre mirror, but is from an engineering point of view easier to built than a single large mirror, because the center of mass is outside the telescope(s). An example structural problems of large moving telescope was e.g. the failure of the [Green Bank Telescope](https://en.wikipedia.org/wiki/Green_Bank_Telescope) in November 1988. In radio astronomy, the approach of having multi-antennas on a single moveable structure is used for long, e.g. in the [Pluton Complex](https://en.wikipedia.org/wiki/Pluton_(complex))
* The LBT's optical resolution can be increased by interferometry to that of a 22.8-metre mirror.
* Interferometry will also be used to virtually block out the light from central stars in order to make their possible planets directly visible, sometimes called [nulling interferometer](https://en.wikipedia.org/wiki/Nuller).

Last but not least, I remember hearing that LBT's mirror size is the largest feasible non-segmented mirror. This hypothesis is somewhat supported if you look at the [list of largest optical telescopes](https://en.wikipedia.org/wiki/List_of_largest_optical_reflecting_telescopes).

> **Comments:**
> **username_0**: Thank you for reviving this old question and writing up a thorough answer!  

---

