# Tag Info

67

Yes and no. Yes, it's true that the apparent size of the Moon is 30 arcmin. It's true that the visual acuity of most people is 1 arcmin. So it's true that if you take the angular size of the smallest detail you can see on the Moon, and you put a bunch of those lined up straight in a row, you could span a Moon diameter with only a few dozen of them. In that ...

52

It doesn't seem so far-fetched to me. Sure, you might be off by a few pixels, due to differences between the human eye and a computer monitor, but the order of magnitude seems about right — the detail in your images, viewed closely, more or less matches what I see when I look at the full moon. Of course, you could fairly easily test it yourself: go ...

34

Forget about magnification. People who know telescopes don't think in terms of magnification. What matters is the angular resolution, or the resolving power: the angular size of the smallest details that you could see in an instrument. Rule of thumb: the resolving power of a telescope with a diameter of 10 cm is 1 arcsecond when using visible light. The ...

22

When you gaze at the moon "live", you are not seeing a still image. You're seeing a "video": your retina is gathering multiple images over time. Those pixels have to be taken into account; they amount to extra pixels. Suppose that 60x60 pixel images are taken of a scene using a tripod-mounted camera which slightly jitters. From the multiple images, a higher-...

10

This has to do with the angular resolution of the Hubble telescope and the ratio between the distance of an object in space and its size in space. The galaxies that the Hubble telescope can see are bigger in size than they are far in light years away compared to pluto from earth. Take the galaxy NGC 5584 for example: It spans 50,000 light-years and it's 72 ...

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 "...

10

The Hubble space telescope has a 2.4m mirror and is pretty much diffraction limited, so at near-UV wavelengths of say 240nm it has an angular resolution of about $10^{-7}$ radians. Mars' closest distance to Earth is about 54.6 million km, so the theoretical minimal resolution is between 5 ad 6 km. So large cities might be visible if they have lots of ...

8

After a bit of searching, I found this blog page, which has several charts about various observatories, including this one: Image courtesy of Olaf Frohn under the Creative Commons Attribution-Share Alike 4.0 License. The majority are space-based, although the radio telescopes are largely land-based. They cover existing and future telescopes, at energies ...

8

Drizzling can't actually do any better than the theoretical resolving power of the combination of your telescope and the atmosphere. What it can do is at least partly compensate for having pixels that are too large to properly sample the resolution of your telescope + the atmosphere. The Nyquist Sampling Theorem basically says that you should have at least ...

7

In fact, the techniques of adaptive optics are already being used in radio astronomy. They are implicit in the basic imaging algorithms (e.g., CLEAN) used to produce maps from radio interferometers. In those cases, they are usually being used to correct for the artificial structure introduced by the way the interferometer samples the sky, rather than for ...

7

I'm not familiar with the design of the ProjectBlue telescope, but I think you have answered your own question. The habitable zones for Alpha Cen A and B, are approximately centred at 1.25au and 0.7au. Both are at a distance of 4.37 light years. 1au at 4.37 light years, subtends an angle of 0.74 arcseconds. If working at blue wavelengths (the aim appears ...

7

No, the telescope doesn't measure the parallax. A sextant or any other angle measuring device fit on the telescope does. And, we don't(can't) directly measure the parallax angle. Instead, we just track the position of the star/object throughout the year. A little bit of spherical astronomy math shows us that the path of a star in the celestial sphere ...

7

The diffraction pattern at the focal plane created by a circular aperture is called an Airy Disk or Airy Pattern. Both the outer opening and the inner hole plus secondary contribute to the exact function. This is usually not easily observed with ground based telescopes because the seeing fluctuations due to air turbulence smears it out. These images use ...

7

The actual math is a bit complicated, but there's a simple intuitive explanation. Longitudinal chromatic aberration happens because, when you cut a convergent lens in two, and you look at the cross-section, the edge of the lens looks a bit like a prism, doesn't it? (look at the diagram that you've posted above, the top of the lens) And it does exactly what ...

6

Drizzling is just one technique in the field of super resolution imaging. Note that claims that you can't "pass the resolution limit" of a camera are wrong, in principle. In practice, it's really hard. What limits your ability to construct an image of what the camera is looking at are: the amount of random noise in the pixels, and your understanding of the ...

6

As far as I know, "seeing" (or rather the effects influencing optical wave propagation) is caused by turbulence in the atmosphere. Using the Reynolds number Number $Re = \dfrac{\rho L v}{\mu}$ as a measure for turbulence: density $\rho$ drops due to the reduced pressure (about 1/100 earth pressure), additionally the gravity is smaller than on earth ...

6

A 130m baseline operating at 2 microns gives a theoretical resolution of $2\times 10^{-6}/130$ radians. At a distance of 400,000 km this translates to 6m. My guess is that Genzel is referring to the accuracy with which the position of a point source of light can be measured. This not the same thing as the smallest thing you can image. The centroid of a ...

5

The relation you cited holds for a single telescope. But, as also noted in the lecture you linked "One thing that is possible in radio astronomy is to use interferometry, which combines the signals from an array of antennas as if they were all part of the same aperture. That means that the resolving power of a radio telescope is not just what it would be ...

5

The probably most advanced system for determination of parallaxes is AGIS as used for Gaia. It's able to go far beyond the angular resolution of the telescopes. Angular resolution is just one parameter. Actually it's just necessary to determine the luminosity centroids of the stars, almost independent of the resolution of the telescopes. That's mainly a ...

5

Parallax measurement in practice is not as is explained above using the popular diagram you have used. The parallax causes the star to prescribe an ellipse in the sky, the semi-major axis of whose is equal to the parallactic angle. The telescopes generally measure the shift in co-ordinates of star(RA and Dec) and then translate the information to that of ...

5

Adaptive optics, as it says on the GMT website: One of the most sophisticated engineering aspects of the telescope is what is known as “adaptive optics.” The telescope’s secondary mirrors are actually flexible. Under each secondary mirror surface, there are hundreds of actuators that will constantly adjust the mirrors to counteract atmospheric ...

4

After all these astronomic answers, I will add a computer one. Pixels are not the same on all monitors. Take a 1990's monitor and take the latest smartphone screen, the 60 pixels won't be the same. How did you calculate the pixel size according to the vision accuracy ?

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

This is a two part windscreen designed to minimize the effects of windshake on the telescope and to avoid the deterioration in image quality that wind would cause. The AAT is in a tall 6 story dome on a pretty exposed part of Siding Spring Mountain and so is likely more affected by wind gusts. Initially there were issues with the mount being too flexible and ...

3

1. Resolving power and diffraction Diffraction happens anywhere there's an edge. "It is defined as the bending of light around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle." (Wikipedia) So all you need for diffraction is an obstacle of any shape - at the edge of the obstacle, the light will bend a little. In ...

3

I'll attempt to answer as many of these questions, to the best of my abilities, as I can at this late hour. 1) Lenses have an index of refraction that is different than vacuum, and air, and they are curved. ie: corners when light is concerned. 2) The entrance and exit pupils are the pupil size required to pass an "extreme" ray through an optic. Meaning ...

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

The answer that you are looking for rests on two main pillars. Are you limited in your ability to resolve objects because of the atmosphere like most ground-based telescopes, or because of the fundamentals of the wave nature of light (as per space telescopes)? I am assuming the latter in your question. One of the biggest rule-of-thumb equations when it ...

3

The resolution of a telescope is the resolution of the image created by the primary mirror at the focal plane. It provides the minimum separation between two equal brightness stars that appear separate in that image. Often, astronomers put a photographic plate or a CCD at the focal plane, create long exposure photos and these can then be examined at ...

3

The claims that you have seen refer to the possibility of attaining near diffraction-limited images using adaptive optics. Both telescopes will feature wavefront sensors that can apply corrections to the telescope optics using either natural guide stars or laser guide stars. However, the comparison is not quite what it seems. The AO systems on both ...

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