How did VLT's adaptive optics obtain this resolution for Neptune? Is it really working in visible wavelengths?

Question

This image of Neptune taken with the VLT is really impressive. The resolution is achieved by recent improvements in the adaptive optics.

But I don't really understand, is this coming from just one 8 meter mirror, or is this using more than one of the four that make up the VLT? Also, is this really using adaptive optics that work in the visible, including blue wavelengths? Or is this a false-color image? See the excellent answer(s) to Why (actually) aren't ground-based observatories using adaptive optics for visible wavelengths?

ProfRob · Accepted Answer · 2018-07-18T19:41:51.663

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It is an image taken with the new narrow field mode of the MUSE instrument using the GALACSI Adaptive optics module on a single (UT4) VLT telescope using laser guide stars.

I am having a great deal of difficulty (e.g. from this press release) in working out at what wavelength(s) this image was taken. I do not believe that the AO system is working at blue wavelengths and therefore the comparison with HST (which certainly does) is somewhat misleading. I think (from ESO's own instrumentation pages) that the MUSE narrow field mode has zero efficiency at blue wavelengths and the description of GALACSI only talks about AO correction approaching diffraction limited images at 650 nm (i.e. red wavelengths).

The captioning to the press release images found here suggest that the shortest wavelengths used in this false-colour image were 550nm and that most of the detail you see is coming from redder wavelengths (600-920nm).

edited Jul 18 '18 at 19:41

answered Jul 18 '18 at 19:34

ProfRob

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Thanks for looking into this and writing a well-sourced answer! There is a lot of really good material to read here. As far as the collor, the situation reminds me a bit of what happened with Is the “Mars blue dune” actually blue? And what makes it so? – uhoh Jul 19 '18 at 00:35
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Indeed, MUSE has zero efficiency (in any of its modes) blueward of about 480nm; it's one of the few deficiencies of an otherwise amazing instrument. – Peter Erwin Oct 27 '18 at 15:13
fyi I've just quoted some of this in What capabilities of Hubble are unique and irreplaceable? What can it do that simply can't be done by any other ground or space-based telescope?https://astronomy.stackexchange.com/q/44503/7982) – uhoh Jun 23 '21 at 01:25
Generally "true color" is hard. Computer monitors are very limited in terms of producing pure colors and are different from each-other. Eyes are also varied (see: https://www.exploratorium.edu/video/paul-doherty-disagreeing-about-color). Not to mention the small percentage who are tetra-chromatic.
For accurate true color, one may need to measure 12 channels in the visible and reproduce those wavelengths in a special laser projector.
– Kevin Kostlan May 17 '23 at 03:50

How did VLT's adaptive optics obtain this resolution for Neptune? Is it really working in visible wavelengths?

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