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Comment by appleflaxen | Hacker News Reader

the article makes it sound like the approach is so effective that it makes space-based telescopes unnecessary.

is that true? does this mean that we should simply use ground-based 'scopes with adaptive optics?

We can achieve a very high resolution from the ground but only in a very small field of view. To cover one typical HST image with MUSE at the VLT, we would need a mosaic of hundreds of exposures. The reason for this are the four artificial guiding stars from the lasers. The closer they are together on the sky, the more atmospheric distortion you can correct.

Here is an image of them: https://www.eso.org/public/unitedkingdom/images/vlt-laser-cc...

Some parts of the electromagnetic spectrum are also not possible to observe from the ground. That's mainly UV and shorter wavelengths (X-ray, gamma-rays). We will always need space telescopes if we want to have these photons.

The Hubble field of view is pretty small too, something like 25 arcseconds the internet tells me.

I was comparing it to HST WFC3 with a field of view of 160 x 160 arcsec^2 (https://www.spacetelescope.org/about/general/instruments/wfc...). Thats about 450 times larger than the MUSE narrow-field mode FOV.

I think you mean the high-resolution mode of the ACS instrument (https://www.spacetelescope.org/about/general/instruments/acs...) but that is broken and it was not repaired during the last HST service mission.

Actually what I was looking at was the field of view of an individual MAMA detector in the Space Telescope Imaging Spectrograph, that instrument has about ~ 100 x 100 arcsec2 of total field of view apparently.

Do the lasers create light pollution problems for the telescopes?

Absolutely! When the lasers are used, MUSE uses a filter at the correct wavelength (Natrium D) to get rid of the laser photons.

Only one telescope is currently equipped with lasers. The other ones can't observe the same region of the sky when the lasers are activated.

Does adding more lasers fix the field of view problem?

Yes. You need one dot for each patch, where the distortion inside each patch is approximately constant inside a single instant. Now, handling these multiple dots in a good way, that's another story. Compare the patch size/discussion in https://publikationen.uni-tuebingen.de/xmlui/handle/10900/49...

This is very much unexplored territory, but ESO thinks so. The ELT (https://www.eso.org/public/teles-instr/elt/) will use more lasers but the exact configuration is still work in progress, as far as I know.

Not if we want to look in the infrared.

Adaptive optics is really only effective in the infrared. And really only in the near-infrared, as past 5 microns, we can't really see through the atmosphere. In the visible, ground-based can't match space observatories (in the visible, the atmospheric turbulence is way harder to correct for).

> In the visible, ground-based can't match space observatories (in the visible, the atmospheric turbulence is way harder to correct for).

The image this article is about is mostly in the optical (MUSE only goes from 465nm to 930nm; and the synthetic filters used in the MUSE image [4] seem to be quite close to the used HST filters).

> And really only in the near-infrared, as past 5 microns, we can't really see through the atmosphere.

Not quite true [1] (at least if only considering absorption), it's just that the background becomes more and more of a problem (both continuum and narrow emission lines), and one has less nicely defined windows of transmission and lots of strongly variable absorption lines (picking dry places for the telescopes and selecting nights with low water vapour column densities helps). At the VLT for example there is VISIR [2], which does mid-IR imaging and spectroscopy.

Of course the sensitivty from the ground is much lower than from space or somewhere in between (for example there is SOFIA [3] which is a 2.5m telescope on an airplance) and some bands of interest are indeed absorbed. But there are indeed projects that involve mid-IR observations that can be done from the ground.

[1] https://www.gemini.edu/sciops/telescopes-and-sites/observing... [2] http://www.eso.org/sci/facilities/paranal/instruments/visir/... [3] https://en.wikipedia.org/wiki/Stratospheric_Observatory_for_... [4] https://www.eso.org/public/unitedkingdom/images/eso1824c/

Ah my bad, I mean that in the visible, you can't reach the diffraction limit with AO like you can in the near infrared. Certainly impressive matching HST from the ground.

I don't think past 5 microns there's been a lot of science done from the ground (not counting SOFIA). Practically, I think everyone is waiting for JWST. A lot of the interesting molecular lines also get absorbed by the Earth's atmosphere.

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