# 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 "...

7

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 a project is as daunting as might be imagined. One would like 12 bits in the camera, the Sony CMOS cameras are so good, the last bit is nearly noise free. The ...

7

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

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. For plane parallel refraction an approximation for the deviation you are talking about is $$\Delta \theta \simeq (n-1) \cot \theta,$$ where $\theta$ is the ...

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 ...

5

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 instrument goes through the hole, while the light from the adjacent part of the sky, where the laser guide stars appear (and also the natural star that's used as ...

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.

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

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

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 lenslet array is of focal length $f$ and subaperture size $D$, given a maximum desired detection angle $\alpha$ (in your case is 2'' \approx 1\times10^{-5}\, \... 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 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 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 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 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: This very interesting site explains notably that there is a cut-off frequency for the ionosphere beyond which it loses its capacity to reflect shortwaves. ... 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 ... 2 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 ... 2 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 observations)? It depends on the astronomical seeing conditions at the location of the telescope, and also varies quite a lot. It could be anywhere from1-100~ms\$. ...

1

Given your reply it seems your definition of diffusion is slightly different from my sense. But the phenomenon is true that at segmentation locations wavefronts are being deflected not in a uniform manner, causing the diffusion issue posted in this question. Before addressing this issue, let me briefly describe how adaptive optics works in practice. In a ...

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