13 votes
Accepted

Why aren't ground-based observatories using adaptive optics for visible wavelengths (circa 2016)?

There's a pretty good discussion at this page. There are several factors at work: The smaller isoplanatic angle, as you note. This limits how much of the sky you can observe with AO, since your ...
Peter Erwin's user avatar
  • 16.7k
8 votes
Accepted

Why are Shack-Hartmann sensors so expensive (4k+ USD)?

I have been deeply involved in both Shack-Hartmann and lateral-shear polarization interferometers. Now I want something simple and slow for hobby projects and had the same question. I don’t think such ...
Jon Twichell's user avatar
7 votes

Why not use only one support vane for the secondary mirror, to avoid multiple diffraction spikes?

I can't speak for the JWST, but I do work at a company that designs telescopes for spacecraft and I can tell you that a lot of the structure of the telescope is to protect the instruments during the ...
Timothy Barrett's user avatar
7 votes

Why was the (small) Hubble better able to find KBO targets for New Horizons than large adaptive optics ground telescopes?

I suspect it's a combination of two things: Stable, guaranteed high-resolution imaging across the entire field of view, something not possible with ground-based adaptive optics; Very low background ...
Peter Erwin's user avatar
  • 16.7k
6 votes
Accepted

Instead of using flexible mirrors, could software be used for adaptive optics?

Short answer: no. Why not? because the mirror is placed at a position in the optical train such that it controls the wavefront phase and tilt of each subsection, "subaperture" of the incoming beam. ...
Carl Witthoft's user avatar
6 votes
Accepted

How do they keep lights from guided lasers in adaptive optics from interfering with the image?

The laser guide stars are indeed outside the science field of view. In the setup for the Wide Field Mode of the MUSE instrument, there is a mirror with a large hole in the middle: the light for the ...
Peter Erwin's user avatar
  • 16.7k
5 votes

Are digital adaptive optics possible?

The problem with your approach is that the deformable mirror changes the phase of the light across the mirror, where the light is not focussed. The light at the sensor array is focussed, and what you ...
Carl Witthoft's user avatar
5 votes

Why not use only one support vane for the secondary mirror, to avoid multiple diffraction spikes?

I'm an amateur telescope maker. Single vane designs do exist. Their main problem is lack of stability. You would have to use a very thick vane to hold the secondary in place in a stable way. That ...
Florin Andrei's user avatar
5 votes
Accepted

How large does refraction become in radioastronomy?

The refractive deviations in position are very similar for both radio and optical astronomy, until you consider very low frequency radio waves ($<200$ MHz) when the effect becomes rapidly larger. ...
ProfRob's user avatar
  • 149k
4 votes

Why was the (small) Hubble better able to find KBO targets for New Horizons than large adaptive optics ground telescopes?

Adaptive optics only mitigate the air turbulence that blurries the images - and even that is only a partial recovery. All other issues remain. Air absorbs various wavelengths. Air has a certain ...
Florin Andrei's user avatar
4 votes

Why aren't ground-based observatories using adaptive optics for visible wavelengths (circa 2016)?

The simple answer for the wavelength part is that performance of AO systems degrades the shorter in wavelength you look. The basics of what happens is as you go to shorter the wavelengths of light, ...
veda905's user avatar
  • 225
4 votes

When using adaptive optics, what is the shortest timescale of atmospheric changes in refractive index that astronomers have to deal with?

I apologise, I'm posting an answer because I can't comment. What is the fastest abberation coming from atmospheric turbulence in any measurement setting (sun observation during day, night-time ...
Sudharshan Saranathan's user avatar
4 votes
Accepted

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

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 ...
ProfRob's user avatar
  • 149k
4 votes

What is tilt anisoplanatism for Laser-guide-star usage in adaptive optic systems?

Isoplanatism commonly refers to a region of angles over which a ground-based telescope observes effectively the same atmospheric turbulence (e.g. an "isoplanatic patch"), such that a laser guide star ...
E. Douglas's user avatar
4 votes

Number of lenslets in wavefront sensor array

Generally speaking as many as possible because the number of lenslets determines the lateral wavefront resolution. But in reality there are a few factors to be considered. Wavefront sensor Say your ...
WDC's user avatar
  • 454
4 votes
Accepted

Why is the Nancy Grace Roman Space Telescope's field of view shaped like a PacMan "ghost"?

By offsetting the SCA columns, the Field of View layout follows the natural annular curve of an off-axis TMA optimized field. Short answer: Yes, off axis aberration magic. Longer answer: The ...
Dave Content's user avatar
3 votes
Accepted

Adaptive Optics Shack-Hartmann Sensor and Deformable Mirror

Just adding some math. Say your wavefront is $w(x,y)$, based on the diagram above you have sub-aperture local slope movements $\delta(x,y)$, and $$\nabla w = \delta(x,y) / f$$ Since for Shack-...
WDC's user avatar
  • 454
3 votes

Adaptive Optics Shack-Hartmann Sensor and Deformable Mirror

Speaking as one who has dealt with those equations (and those problems) since 1984. Yes, the simplest approach is to use matrix methods to force the phases at the edges of every subaperture to be the ...
Carl Witthoft's user avatar
3 votes
Accepted

Why weren't the Hubble light-echo images of V838 Monocerotis supplemented by ground telescopes?

Adaptive optics does not work (or at least did not 10+ years ago) on ground based astronomical telescopes at visible wavelengths. No images remotely comparable to HST could be obtained for faint$^*$ ...
ProfRob's user avatar
  • 149k
3 votes
Accepted

V471 Tauri's circumbinary brown dwarf non-observation; Applegate, or over-restrictive assumptions?

For all of these direct imaging results, the critical parameter is the contrast as a function of separation. This lets you know how much fainter an object you can see around the much brighter primary ...
astrosnapper's user avatar
  • 8,347
3 votes

Why not use only one support vane for the secondary mirror, to avoid multiple diffraction spikes?

I cannot answer about the JWST. Regarding amateur telescopes, there is the option of using curved supports, which are intended to eliminate diffraction spikes and reduce overall diffraction. http://...
astromath's user avatar
  • 316
3 votes

How do tip tilt mirrors correct distortion in adaptive optics?

Not an expert, but offer one solution: Question 1 The adaptive optics correction is accomplished by a tip-tilt mirror and a deformable mirror. Usually the atmospherical wavefront distortion (or say, ...
WDC's user avatar
  • 454
3 votes
Accepted

Do point spread functions from large single telescopes using adaptive optics still look like Airy functions for narrow-band filters?

Yes, narrow-band images taken with adaptive optics on ground-based telescopes produce point-spread functions resembling Airy disks. To answer this, I went fishing for some data, and randomly caught a ...
giardia's user avatar
  • 2,078
3 votes

JWST mirrors each can be "positioned in tip, tilt, piston, horizontal & vertical decentering and clocking". What does this mean?

Each of the primary mirrors has seven degrees of freedom, not six. It's just the secondary mirror that only has six degrees of freedom. The six degrees of freedom for the eighteen primary mirrors and ...
David Hammen's user avatar
  • 33.7k
3 votes
Accepted

JWST mirrors each can be "positioned in tip, tilt, piston, horizontal & vertical decentering and clocking". What does this mean?

Can I assume that "piston, horizontal & vertical decentering" correspond to 3D positioning in space and "tip, tilt, and clocking" correspond to attitude? Could someone help ...
BradV's user avatar
  • 737
2 votes

Why was the (small) Hubble better able to find KBO targets for New Horizons than large adaptive optics ground telescopes?

I think you have hit the nail on the head in your question. KBOs are seen in reflected sunlight and they are incredibly faint, since the amount of reflected light reaching the earth goes as the ...
ProfRob's user avatar
  • 149k
2 votes

How difficult is adaptive optics computationally?

If you look around, you should be able to find home-user AO systems which run, albeit at slightly lower bandwidth than you might like, on your PC (or an equivalent chip in the vendor's box). It ...
Carl Witthoft's user avatar
2 votes
Accepted

How difficult is adaptive optics computationally?

If you're only trying to correct a small field, using a single guide star (or a single laser guide star), then the requirements aren't very arduous. The MACAO adaptive-optics module used on the Very ...
Peter Erwin's user avatar
  • 16.7k
2 votes

Why was the (small) Hubble better able to find KBO targets for New Horizons than large adaptive optics ground telescopes?

The magnitude of these Kuiper Belt Objects is incredibly small, to begin with. The atmosphere distorts stars normally and scatters light even on the clearest of nights. In addition to that, these ...
t.sand's user avatar
  • 122
2 votes

How large does refraction become in radioastronomy?

I found some interesting information in this vulgarization paper by Ian Poole. A first point is electron density in ionosphere changes between day and night, so the resulting bend will be different: ...
J. Chomel's user avatar
  • 1,452

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