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

The Large Binocular Telescope (and LBT blog as source of images below) 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 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 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

Open image in new window for full size:

enter image description here

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    $\begingroup$ 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. $\endgroup$
    – Mick
    Commented Dec 25, 2017 at 13:37
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    $\begingroup$ 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. $\endgroup$ Commented Dec 25, 2017 at 23:04
  • $\begingroup$ @AtmosphericPrisonEscape thanks for that! I've revised the question accordingly. $\endgroup$
    – uhoh
    Commented Dec 26, 2017 at 1:45
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    $\begingroup$ @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. $\endgroup$ Commented Jan 14, 2018 at 11:45
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    $\begingroup$ @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. $\endgroup$
    – uhoh
    Commented Apr 14, 2019 at 9:33

1 Answer 1


I asked myself the same question and back then, I found the reasoning on the German Wikipedia page 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 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
  • 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.

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.

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    $\begingroup$ Thank you for reviving this old question and writing up a thorough answer! $\endgroup$
    – uhoh
    Commented Mar 11, 2021 at 14:24

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