# What telescope is this, and how does it work?

In the excellent chemistry series of Periodic Videos' episode Amazing piece of metal (speculum) there is an interesting showing and discussion of Isaac Newton's first telescope.

At 02:36 there is a short cut to a view of a very large, modern telescope mirror. I'm confused by the image because it seems to show what looks like a small 45 degree elliptical, diagonal secondary mirror that you might see near the top end of a long focal length Newtonian telescope, except that it seems to be sitting in front of the primary, facing towards the stars.

Can anyone identify this telescope, and help me understand the basics of how this particular optical system is configured?

edit: I was so focused on the small mirror in the center that I neglected to read the caption. This "very large telescope" may in fact be THE Very Large Telescope, one of it's components at least. But because of all the shiny metal an reflections in mirrors, I still don't really understand what I'm looking at as far as the optical system is configured.

https://www.youtube.com/watch?v=4ESW_NTIhBM

## 1 Answer

This is indeed the VLT, or rather one of the four 8.2m Unit Telescopes (UT in ESO parlance). Looking at the optical layout (found here along with a description of the telescope), the small mirror in the centre of your screenshot is M3, the tertiary mirror. M1 is the primary (in the background), and M2 is out of the frame towards the top- the white supports you see in the foreground hold it above M1.

One advantage of this design is that you can have two large instrument platforms on the vertical tilt axis of the telescope - this way, you don't have to dangle several tons of instrument from the support structure (Nasmyth focus in the image).

• Hmm, the ESO link seemed to refer to the whole optical layout. I guess I'll take the sentence out for now.
– Alex
Jun 3 '17 at 15:37
• Thanks!! OK that makes sense, I appreciate your explanation.
– uhoh
Jun 3 '17 at 15:45