# Tag Info

10

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 target needs to be within the isoplanatic angle of a bright enough references star. (Even with laser guide stars, there is still a need for a reference star for "...

6

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. Once an optical system has formed an image, all phase information is lost. There are some software methods which can correct for simple aberrations such as ...

6

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 in the optical for HST (Hubble), versus a very high background for ground-based AO in the near-infrared. Most adaptive optics systems are only able to correct ...

5

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 would block off a significant amount of light and may impact the performance of the instrument. Active systems, while theoretically possible, would be very ...

5

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 initial ascent to space where the launch vibrations and g-loading will be the worst. Plus mirrors for telescopes that big have really precise alignment ...

4

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 ...

4

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 provides effective correction of atmospheric seeing. an-isoplanatism refers to a lack of isoplanatism, or a way in which the science target and the reference ...

4

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 amount of glow from various sources (light pollution, etc) which masks faint objects. Etc. There is no real substitute for a large telescope operating in vacuum.

4

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 get is the intensity which is, roughly, the Fourier Transform of the phase front at the mirror. By the time you have intensity, the phase information has ...

3

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, the phase screen) looks like this: Note its ramp-like shape. In other word, the 2nd and 3rd Zernike terms are large. The deformable mirror has limited ...

3

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, you need a finer plate scale to detect variations in seeing which requires very expensive (and in some cases non-existant) hardware. You also need a higher AO ...

3

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$^*$ objects from the ground in the BVI wavebands. I checked for papers discussing adaptive optics at other wavelengths and couldn't find any. The field of view is a ...

3

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://www.fpi-protostar.com/crvmnts.htm Athough diffraction spikes are eliminated by curved supports, diffraction occurs symmetrically around bright objects.

3

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-Hartmann the spatial resolution is usually low, so people fit wavefront $w$ to a Zernike polynomial. Denote the Zernike basis be matrix $Z$ and coefficients be $a$, ...

3

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 same. The assumption is that there is no phase tearing , and if there were, adaptive optics pretty much would be unable to handle such disruptions. Once the ...

2

tl;dr: Using one vane may not provide any optical advantage over three vanes since the primary mirror is already so highly segmented and "edgy", and it certainly provides a lot of mechanical disadvantages in terms of stiffness against lateral vibration translation and breaking of symmetry during thermal expansion! note: I'll be updating this answer ...

2

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 depends a lot on the number of actuators on your deformable mirror, as do most matrix operations. You might be interested in the offerings at ThorLabs , ...

2

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 Large Telescope at the European Southern Observatory dates from the early 2000s and is based around two 400 MHz PowerPC 604 chips (one for the "supervisory ...

2

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 inverse fourth power of their distance from us (see my answer to this question on trying to view Oort cloud objects). To see such objects requires deep imaging ...

2

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 closer objects can be found with infrared detection. The atmosphere absorbs infrared wavelengths extremely well, which makes space based observations a necessity. ...

2

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 object whose light has been suppressed by the coronograph (the black circle in the center of the star). From the change in eclipse timings (Figure 1 in their ...

2

From looking at the E-ELT website, it appears that at first light, the AO will only work for near-IR. Specifically, the instrument that can use AO is the MICADO instrument. The description page for this instrument states MICADO, or the Multi-Adaptive Optics Imaging Camera for Deep Observations, is one of the first-light instruments for the European ...

1

Why are Shack-Hartmann sensors so expensive (4k+ USD)? I had the same feeling the first time I saw their prices. In principle they are just an array of microlenses one focal length in front of a CCD or CMOS imaging chip. It can even be a pinhole array in some specific cases, depending on what you need to resolve and the nature of your incoming wavefront. ...

1

@CarlWitthoft's answer is misleading if not wrong. The Fourier transform of a field does not lose phase information. If you let the light drift another focal length and then use an identical mirror, you completely recover the initial electric field distribution incident at the telescopes entrance aperture. Information is not lost here. The problem is not ...

1

Each part of the mirror contributes to every pixel in the image, and the pockets of atmospheric distortion may be only 10-20 cm wide. If you look at a bright planet through a 30+ cm telescope in poor seeing conditions, the image looks like a stack of several sub-images shifting in and out of alignment, each from a different part of the mirror. To ...

1

As far as microlensing is concerned: Not likely This is a bit of a dodge on the question, but it is hard to come up with a scenario where microlensing becomes the dominant PSF term. Microlensing occurs during conjunction with a stellar or planetary object, and results in a momentary (seconds-to-days timescale) increase in the brightness of the observed ...

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