Telescopes Analyzing electromagnetic spectra to search for understanding of celestial objects.

What is a Telescope? A telescope is simply a device that captures as many photons as possible from a given region of sky and concentrates them into a focused beam for analysis. A telescope is simply a device that captures as many photons as possible from a given region of sky and concentrates them into a focused beam for analysis. Optical telescopes are ones designed to collect visible wavelengths Optical telescopes are ones designed to collect visible wavelengths Other “exotic” or high energy telescopes are used to collect “invisible” (high freq.) wavelengths. Other “exotic” or high energy telescopes are used to collect “invisible” (high freq.) wavelengths.

OPTICAL TELESCOPES Light gathering instruments that allow us to see objects Galileo was the first to make practical application of the telescope. Galileo was the first to make practical application of the telescope. Refracting telescope - gathers light with a convex lens Refracting telescope - gathers light with a convex lens Reflecting telescope - gathers light with a concave mirror Reflecting telescope - gathers light with a concave mirror

REFRACTING TELESCOPES Refraction refers to the bending of light as it passes from one medium to another. Refraction refers to the bending of light as it passes from one medium to another. Convex objective lens gathers light Convex objective lens gathers light Upper limit of 40 inches diameter objective lens Upper limit of 40 inches diameter objective lens Commonly small telescopes for amateurs Commonly small telescopes for amateurs Most terrestrial telescopes are refractors Most terrestrial telescopes are refractors Largest (40 inch) is at Yerkes Observatory in Wisconsin Largest (40 inch) is at Yerkes Observatory in Wisconsin

REFRACTING TELESCOPE

REFLECTING TELESCOPES Concave mirror gathers and focuses light Concave mirror gathers and focuses light Mirrors may be much larger than the objective lenses of refracting telescopes Mirrors may be much larger than the objective lenses of refracting telescopes Most really serious observatory telescopes are reflectors Most really serious observatory telescopes are reflectors The largest reflector in the US is the 6.5 m Mt Hopkins telescope The largest reflector in the US is the 6.5 m Mt Hopkins telescope The famous Hale telescope is located at Palomar observatory in San Diego, CA. The famous Hale telescope is located at Palomar observatory in San Diego, CA. A technology called adaptive optics allows computers to control several small mirrors to gather and focus light A technology called adaptive optics allows computers to control several small mirrors to gather and focus light

Reflection One problem that must be surmounted with a reflecting telescope is how to place an observer at the focus. In the example shown above, the focus is inside the telescope. This is called the prime focus, and in some large telescopes observations can be made at the prime focus. More commonly, various mirror arrangements are used to transport the light from the focus to an external observer. Two common ones are a Cassegrain focus and a Newtonian focus.Cassegrain focus Newtonian focus

Types of Reflector Designs Each design uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. Notice that the secondary mirrors shown in (c) and (d) are actually slightly diverging, so that they move the focus outside the telescope.

Reflector or Refractor A number of factors tend to favor reflecting over refractors A number of factors tend to favor reflecting over refractors 1. Passing light through lens creates a deficiency known as chromatic aberration 2. Some light is actually absorbed as by the lens as light passes through it 3. Large lens can be heavy and hard to support 4. Lens have two surfaces (to be polished and machined) where a mirror has only one.

Imaging & Non-imaging Detectors Camera – photographs Camera – photographs Photometer – device to measure the amount of light received (intensity) Photometer – device to measure the amount of light received (intensity) Spectrometer – analyze and study of spectrum Spectrometer – analyze and study of spectrum

Image Processing Use of cameras becoming rare Electronic detectors known as charge- coupled devices (CCDs) are widespread. Their output goes directly to a computer. A CCD consists of a wafer of silicon divided into a two-dimensional array of many tiny picture elements, known as pixels. Pixels detect more photons; and thus light intensity. Much more efficient than photographic plates

Telescope Size Larger telescopes are preferred for two reasons Greater light gathering power Greater detail can be seen (resolving power) Bigger = Better

Light Gathering Power The standard measure of a telescope is its : Expressed as the total collecting area of the telescope. The standard measure of a telescope is its Light Gathering Power: Expressed as the total collecting area of the telescope. The greater the area, the more light collected. The greater the area, the more light collected. We usually We usually express the size of a telescope in terms of the diameter of its primary collecting optic: Light Gathering Power increases as the square of the diameter of the primary collecting optic.

Resolving Power Larger telescopes have finer angular resolution Resolution refers to the ability of any device, such as a camera or telescope, to form distinct, separate images of objects lying close together in the field of view. The finer the resolution, the better we can distinguish the objects and the more detail we can see Diffraction of light as it enter a telescope causes the rays to spread out, making it impossible to focus the beam into a sharp point and introduces a “fuzziness” into the optical system. For light of any given wavelength, large telescopes produce less diffraction than small ones.

Resolution Detail becomes clearer in the Andromeda Galaxy as the angular resolution is improved some 600 times, from (a) 10', to (b) 1', (c) 5", and (d) 1". (AURA)

Atmospheric Blurring The resolution of ground-based optical telescopes is actually limited by seeing – the blurring effect of Earth’s atmosphere, which smears the stars image into what is referred to a seeing disk. Atmospheric turbulence effects long waves less Atmospheric turbulence effects long waves less Higher elevation experience less atmospheric turbulence Higher elevation experience less atmospheric turbulence

Radio Telescopes Use antennas to detect cosmic radio waves: Use antennas to detect cosmic radio waves: Radio-wavelength lines of Hydrogen and Molecules (CO) from cold interstellar gas Radio-wavelength lines of Hydrogen and Molecules (CO) from cold interstellar gas Radio waves are emitted by hot electrons, or electrons accelerated by strong magnetic fields. Radio waves are emitted by hot electrons, or electrons accelerated by strong magnetic fields. Interferometry: Trick of ganging together many smaller telescopes to achieve the higher resolution of a large single dish (aperture synthesis).

Radio vs. Optical ( a) VLA radio “photograph” (or radiograph) of the spiral galaxy M51, observed at radio frequencies with an angular resolution of a few arc seconds; (a) shows nearly as much detail as (b), an actual (light) photograph of that same galaxy made with the 4-m Kitt Peak optical telescope. (NRAO/AURA)

Why Do We Have to Go to Space to See All of the Electromagnetic Spectrum?

Detecting Other Radiation From Space Infrared telescopes are located on tall, dry mountains, or in airplanes! Why??? Infrared telescopes are located on tall, dry mountains, or in airplanes! Why??? Ultraviolet, gamma, and x-ray telescopes must be located in outer space because the atmosphere absorbs much of this radiation. Ultraviolet, gamma, and x-ray telescopes must be located in outer space because the atmosphere absorbs much of this radiation. Used to detect planets, newly formed stars, quasars, pulsars, super novas and black holes. Used to detect planets, newly formed stars, quasars, pulsars, super novas and black holes.