Astronomy-Ch.6 – Flashcards

-Your eye focuses light by bending light. Light rays (from a distant star for example) are parallels that are then focused onto one point (called the focus) at the back of the eye. A glass lens is similar b/c it bends light just like our retina -The place where the image appears in focus is called "focal plane" of the lens- located on your retina (for a person with perfect vision).

A CCD, or charge-couple device, is a chip of silicon engineered to be very very sensitive to photons, or light pieces. Basically, light excites pixels making an electric charge. Numbers will represent the light from each pixel as an image. The main advantage of a CCD camera over the human eye is the ability to have a recorded copy, or image, of what you're looking at. Also, some human eyes are not perfect (for example because of myopia, hyperopia, and presbyopia) while a camera lens is perfect.. MAINLY: you can expose CCD for a minute and be able to see much fainter stuff than your eye can see

-The light-collecting area= how much total light the telescope can collect at one time. A telescope's size is the diameter of its light-collecting area. A 10-meter telescope has a light-collecting area more than 1,000,000x the human eye. Area is proportional to the square of a telescope's diameter -Its angular resolution= the smallest angle over which we can see two stars are distinct (which depends on their actual separation and their distance from us) -These are both important because you couldn't collect the light from stars without a light-collecting area, and without a good angular resolution, all stars would simply clump together and appear as one big bright glob of star. Basically, without these you'd have absolutely no detail.

Diffraction limit is the angular resolution that a telescope could achieve if it were limited only by the interference of light waves. It depends on the diameter of the primary mirror and the wavelength of the light being observed such that: a larger telescope has a smaller diffraction limit, which means it can achieve a smaller angular resolution. The diffraction limit is also larger (worse) for longer wavelengths of light.

The light-collecting area would be 4x greater on a 6-meter (which has 36) telescope than a 3-meter telescope (which has 9). The resolution of the 6-meter telescope would be half that of the 3-meter telescope because of the resolution formula.

Refracting telescopes work like the eye does, using curved transparent glass lenses to collect and focus light while a reflecting telescope uses a precisely curved primary mirror to gather light and then to a secondary mirror that lies in front of it. This secondary mirror focuses the light to a focus at a place where we can observe it (for the lab telescopes its on the side where you put the eyepiece).

-In refractors, it's one big long tube that has a transparent glass lens at the top of the tube that collects the light and focuses it on to the focus. There are no mirrors in this type. -In reflectors, it depends on which variation you're using because there are "cassegrain," "Newtonian," and "Nasmyth/Coude" Focuses. For all three types though, the primary mirror is located at the back or bottom of the tube and the secondary mirror is located nearer the top in the center (either flat or tilted [Newtonian]). In reflectors, the focus is also at varying locations depending on which focus telescope it is. -More specifically, in the Cassegrain design, the secondary mirror reflects the light through a hole in the primary mirror so that the light can be observed beneath the telescope. In the Newtonian design, the secondary mirror reflects light out to the side of the telescope. And in the Nasmyth and Coude designs, a third mirror is used to reflect light out the side but lower down than in the Newtonian.

Astronomers use reflector much more commonly for two reasons: -Light passes through the lens of a refracting telescope, lenses must be made from clear, high-quality glass with precisely shaped surfaces on both sides. Basically, that's difficult....ONLY the reflecting surface of a mirror must be precisely shaped, and the quality of the underlying glass isn't even a factor. Which is much easier. -Large glass lenses are very heavy and can only be held in place by their edges..since it's at the top of the telescope, it makes stabilizing difficult. Also, it's hard to prevent them from deforming. Since a reflecting scope has the mirror at the bottom, its way easier to deal with.

Imaging: yields images of astronomical objects.. w/ a camera Spectroscopy: astronomers obtain and study spectra. Uses diffraction grating to separate various colors of light into spectra Timing: tracks how an object changes with time. Uses light curves, which are graphs that show how objects varies with time

Images made from invisible light means that its an image made from light from any part of the spectrum besides visible light. For example, X-rays and infrared images work this way. For an xray, the X rays are that pass through are recorded on a piece of X-ray sensitive film. Therefore, we're not seeing X-rays, but seeing the image left behind on the film. Colors in these images don't mean anything, but what we defined them as because we can only see visible light and these are invisible light waves. So, we color-code an image by energy (or intensity of the light or by physical properties of objects in the image). (high=blue, low=red, medium=green)

Spectral resolution is the amount of detail we can see. The higher the spectral resolution, the more detail. BUT, b/c the spectral resolution depends on how widely the spectrograph spreads out the light, if the light is spread out more, you need more total light in order for it to be recorded successfully. So making a spectrum of an object requires a longer exposure time than making an image, and higher resolution spectra require longer exposures than low-resolution spectra.

Earth's atmosphere can hinder astronomical observations by: -the scattering of human-made light (light-pollution) =there is no human-made light in space -the blurring of images by atmospheric motion (turbulence) which limits the angular resolution of ground-based telescopes =air is not moving in space -this can also be eliminated with adaptive optics (reverse dancing) -most forms of light cannot reach the ground at all (Only Radio and Infrared) =not a problem if in space b/c things other than visible light can be observed Disadvantages to space telescopes: (ex. Hubble is only a 2.4-meter primary mirror)

No, mainly we're not high enough elevation. Plus, there would be light-pollution from downtown and the constant Sanford Stadium lights. Of course there'd be lots of atmospheric turbulence. =

Adaptive optics can eliminate a lot of the blurring caused by the atmosphere. They make the telescope's mirrors do an opposite dance, canceling out the atmospheric distortions. The shape of the mirror (the secondary, or 3rd/4th mirror) is changed slightly many times each second to compensate for rapidly changing atmospheric distortions.

The Earth's atmosphere is practically opaque to Ultraviolet Light (absorbed by ozone layer), X-Ray, and Gamma Rays, and Infrared (absorbed by water). What we can do about it is put telescopes in space where those wavelengths can reach it. The earth's atmosphere absorbs these wavelengths and doesn't allow them to reach the ground.

-Radio Telescopes: satellite dish, collects radio waves from a satellite in Earth orbit -Infrared Telescopes: Most similar to visible light telescopes. Ex SOFIA=large hole cut in body of a Boeing 747 -Ultraviolet Telescopes: Also close enough to visible light, but not in earth's atmosphere; GALEX and Hubble -X-Ray Telescopes: grazing incidence mirrors (Xrays graze surface) towards focal plane.. Ex. Chandra -Gamma Ray Telescopes: Large Area Telescope on the Fermi Obervatory weighs 3 tons OR Swift. Only 2 b/c they are VERY strong waves

Apply the resolution equation, radio waves have large wavelengths and the proportional relationship to telescopes diameter would make it small.

Interferometry is a way of improving the angular resolution of radio telescopes. Works by linking two or more individual telescopes to achieve the angular resolution of a much larger telescope. It works by taking advantage of the wavelike properties of light that cause interference. Relies heavily on timing when radio waves reach each dish and on using computers to analyze the interference patterns result. -This can improve astronomical observations because you can use it past the radio portion of the spectrum and be able to see more details in many areas of the electromagnetic spectrum. (almost all the way to X rays)