Procrastinating, I clicked on a suggested video (How Large Can a Telescope Be?) and after about 01:20 I saw a clip of a telescope shown below.

I was confused as I saw the reflections in the mirror until I decided that there are actually two small objects in front of the primary mirror, one far from the primary where you'd expect to see a secondary, which I've labeled "A", and one very close to the primary, which I have labeled "B".

Both are supported by four vanes, and there does not seem to be a hole in the primary.

I've labeled the reflected image of the back side of B as B' (B prime), and I've indicated the reflections of an "A vane" and a "B vane" with arrows.

But "A" looks very small compared to what I'd expect, only perhaps a few percent of the primary's diameter, and it looks like the telescope is not in a conventional dome, but perhaps has only a sliding or otherwise openable roof (currently off screen) to protect it when not in use.

Question: How does this telescope work, and what is it for? Is it for imaging, or performing some other function?

screen shot of a telescope


  • 1
    $\begingroup$ I am not quite sure, but this could be an SLR (Satellite Laser Ranging) telescope. $\endgroup$
    – SergiusPro
    Aug 25, 2018 at 13:24
  • $\begingroup$ Looks Cassegrainish en.wikipedia.org/wiki/Cassegrain_reflector except B appears to be a 45° diagonal. Perhaps drilling a hole through the primary would've been tos expensive, or mess with adaptive optics? However, SergiusPro may well be right. $\endgroup$ Aug 25, 2018 at 16:26
  • $\begingroup$ @WayfaringStranger the relative diameter of the secondary (A) to the primary is so small that the curvature of the mirror would have to be enormous, while the tolerance on the surface would still have to be quite small if it were an imaging telescope, so it would be a fairly difficult and expensive secondary to make. I've never seen such a tiny hyperbolic (or similarly curved) secondary! I think this is quite a peculiar optical system. I also think that SergiusPro has a good theory. A collimator for a laser system of some kind, or some kind of lower re light collector seems more likely. $\endgroup$
    – uhoh
    Aug 25, 2018 at 16:34
  • $\begingroup$ @uhoh From tube length, it looks pretty short focus, but SP seems more likely correct. $\endgroup$ Aug 25, 2018 at 16:50
  • 1
    $\begingroup$ Does not quite look quite like Apache point lunar ranging telescope google.com/…: but might serve a similar purpose. $\endgroup$ Aug 25, 2018 at 19:38

1 Answer 1


I'm pretty sure this is one of the four auxiliary (or "outrigger") telescopes for the European Southern Observatory's Very Large Telescope Interferometer, part of the VLT complex at Paranal Observatory in Chile. It's used in conjunction with the four main 8 m ("unit") telescopes in an optical/infrared interferometric system, so its purpose is to send images of the sky in the optical or near-IR to be combined with images from the unit telescopes and the other auxiliary telescopes (via underground tunnels) in the interferometric system. (The octagonal gray frame is kind of a giveaway, and the fact that this video is ESO-based helps confirm this.)


(Note that, per the link above, the primary mirror in each auxiliary telescope has a diameter of 1.8 m, while the secondary mirror is only 0.14 m -- very small, as you guessed.)

The object you identify as "B" is the tertiary mirror, which is flat, elliptical, and tilted at a 45-degree angle so that light from the secondary mirror is redirected off to the side. It sits in a mount (supported by the "B vanes") located just in front of the primary mirror, so there is no need for a hole in the primary mirror. You're probably used to versions of this (Nasmyth) design where the tertiary mirror is located behind the primary mirror, requiring a hole in the primary. Since the tertiary mirror in a Nasmyth design needs to sit along the elevation rotation axis of the telescope, whether it goes in front of or behind the primary depends on the overall telescope balance.

(Sometimes there is a hole in the primary, and the tertiary sits on top of a small "tower" that protrudes up through the hole from behind the primary; this would help disguise the existence of the hole. That's how the tertiary mirrors in the 8 m unit telescopes of the VLT work, though I don't think they have support vanes the way this one does. The existence of the support vanes and the nature of the B' reflection suggests there really isn't a hole in the primary here.)

The "dome" is an example of a "clamshell" design, in which two halves fold down out of the way; you can see one of the two hinges in the front, near the bottom right of your images. If you google for images of these telescopes, you can see them in the daytime, folded-up configuration.

Here's an image of one of the telescopes when it was still in the factory (before the mirrors were installed), which may help you understand the design better:

VLTI auxiliary telescope in the AMOS factory (credit:ESO)

  • $\begingroup$ Aha!! The folded beam path allows them to be compact, close to the ground for mechanical stability, and yet "portable" so they can be easily reconfigured (a little bit like VLA or ALMA are). This is great, thank you very much! $\endgroup$
    – uhoh
    Dec 4, 2018 at 12:20
  • $\begingroup$ Per Wikipedia and this and this while the interferometer can use three or four 8 meter telescopes, it can use three or four of these instead, freeing the large apertures up for independent work while still keeping one of the interferometers busy. $\endgroup$
    – uhoh
    Dec 4, 2018 at 12:21
  • $\begingroup$ Note, in the question Are they really sure this isn't an Airy disk? How was that ruled out? the image was produced using the 1.8 meter auxiliary telescopes! $\endgroup$
    – uhoh
    Dec 4, 2018 at 12:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .