Collecting Light. In a dark-adapted eye, the iris is fully open and the pupil has a diameter of about 7 mm. pupil

Transcription

1 Telescopes

2 Collecting Light The simplest means of observing the Universe is the eye. The human eye is sensitive to light with a wavelength of about 400 and 700 nanometers. In a dark-adapted eye, the iris is fully open and the pupil has a diameter of about 7 mm. pupil

3 Collecting Light Limitations of the human eye: No ability to leave shutter open (exposure = about 1/15 th second) Fixed field of view / magnification Small collecting area Small wavelength coverage

4 Collecting Light Around 1600, it was discovered that glass lenses could be used to correct human vision. During the next decade, several lens makers realized that combinations of lenses could be used to magnify the view of a distant object. The first telescopes used lenses, but later telescopes were developed that used mirrors. In 1609, Galileo Galilei built a much improved telescope and used it to survey the night sky.

5 Collecting Light A reflecting telescope A refracting telescope

6 Collecting Light Modern, large telescopes are all reflectors: 1. Light traveling through glass is bent differently depending on wavelength. 2. Glass is not perfectly transparent. Some light traveling through glass is absorbed. 3. Large lenses can be very heavy, and can be supported only at their edges. Large mirrors can be supported across their back surfaces. 4. Lenses need both front and back polished, mirrors need only the front polished.

7 A small reflecting telescope Newton s telescope (3.5 inch diameter)

8 A big reflecting telescope Mayall 4 meter telescope (Kitt Peak, AZ) Prime focus Cassegrain focus

9 An even bigger reflecting telescope Large Binocular Telescope (Mt. Graham, AZ) (Two 8.4 meter mirrors in tandem or solo operation)

10 A truly huge reflecting telescope The Thirty Meter Telescope (Planned for 2016) (492 segments form a 30 meter collecting area)

11 Collecting Light The light-collecting power of a telescope equals the area of its primary mirror or lens. Area of a circle π = (diameter) 4 2 Collecting area increases as ( size) 2

12 Collecting Light 1-meter diameter mirror 10-meter diameter mirror How many small mirrors equal the area of the larger mirror? A. 10 small mirrors B. 100 small mirrors C. 1,000 small mirrors

13 Recording Light The recording device may form an image from the light (a camera) or break it into a spectrum (a spectrometer):

14 Recording Light The modern standard for collecting visible light is the charge-couple device (CCD). The CCD chip is made of a silicon wafer that absorbs photons of light and records the electric charge in microscopic wells. CCDs can only measure brightness. They do not measure color directly. e -

15 False color or Color composites The assignment of color values in an image may not reflect the wavelengths of the spectral lines shown in the image. Image of Eagle Nebula Red: red sulfur line, green: red hydrogen line, blue: green oxygen line This representation gives more information about the nebula than a true color image. Image credit: T. Rector and B. Wolpa (NOAO)

16 False color or Color composites An image showing another part of the spectrum is always false color since there is no natural color for gamma-rays, x-rays, UV, IR, or radio waves. X-ray Image of the Tycho Supernova Remnant Red, green, and blue represent different temperatures and chemistries. The human eye cannot see x-rays so there is no real color to the image. Image credit: Chandra X-ray Observatory

17 Image Quality Angular resolution is the ability of a telescope to distinguish neighboring objects in the sky as separate. Here is 4 images of a cluster of stars: The image on the left has poor resolution while the image on the right as high resolution.

18 Image Quality Angular resolution is limited by the diameter of the telescope and the wavelength of the light. Telescopes with higher resolution: Larger diameter Observing shorter wavelengths

19 Astronomical Seeing Earth s atmosphere causes additional blurring known as seeing. Seeing is due to 2 factors: Patches of air at slightly different temperatures Patches of air rise & sink and carried by wind

20 Seeing or Twinkling Light bent as it passes between patches of air. Light path from star to observer changes. More or less light reaches the observer, so we see rapid changes in brightness called twinkling.

21 Going to Mountaintops: Most observatories are placed at high altitude to minimize astronomical seeing due to the atmosphere. Large Binocular Telescope atop Mt. Graham, AZ, ft. altitude Kitt Peak National Observatory, AZ, 6900 ft. altitude Cerro Tololo International Observatory, Chile, 7200 ft. altitude Mauna Kea Observatories, Big Island of Hawaii, ft. altitude

22 Going to Space: The primary reason for placing the Hubble Space Telescope in orbit was to get above the blurring effect of the atmosphere. Hubble s mirror is 2.4 meters in diameter.

23 Information across the EM Spectrum Until the 20 th century, only telescopes observing visible light were available. This severely limited what kinds of objects could be observed. Other wavelengths provide information about different processes and different temperatures. Long wavelength light can pass through clouds of gas and dust that block visible or UV light.

24 Radio Telescopes Radio telescopes observe the range of frequencies that pass through Earth s atmosphere (the radio window ). The longer wavelength of radio waves means that radio telescopes must be much larger than other telescopes to achieve good resolution. But there are advantages in radio astronomy: Radio telescopes observe 24 hours a day Clouds and rain don t interfere with observing Different frequencies, different spectral lines

25 Radio Telescopes Radio telescopes are like optical reflecting telescopes: wavelengths are longer, so surface is not polished relaxed engineering means larger telescopes larger diameter partially offsets long wavelength Design of 100-m Green Bank Telescope, WV

26 Largest radio telescope: 300-m dish at Arecibo, PR

27 Interferometry Combine signal collected from widely-spread telescopes as if they came from a single antenna Resolution will be that of antenna whose diameter equals the separation between dishes Very Large Array (VLA) in western New Mexico

28 Interferometry For example, the Very Long Baseline Array uses 10 radio telescopes spread out across the U.S. Images routinely achieve resolution of a few microarcseconds or arcsecond Allows parallaxes of Milky Way and other galaxies!

29 Interferometry The newest radio interferometer is located in the Atacama desert of Chile Atacama Large Millimeter Array Fills gap between radio and infrared. Altitude greater than 10,000 feet

30 Atmospheric Absorption Most of the electromagnetic spectrum is blocked by Earth s atmosphere, only visible light and radio penetrate all the way to Earth s surface.

31 Optical and infrared images of nebulas in Orion. What are you seeing in each image? Optical image from NOAO Infrared image from 2MASS

32 Optical and x-ray images of the Crab supernova remnant. What are the temperatures of the gas emitting visible light and x-rays? Optical image from ESO VLT X-ray image from Chandra Observatory

33 Optical and ultraviolet images of spiral galaxy M81. What kinds of objects are producing most of the visible light? The ultraviolet light?

34 You want to observe the emission lines from cold clouds of interstellar gas (at about 10 Kelvin). What part of the spectrum would you observe? A. gamma rays B. ultraviolet C. visible light D. radio Where could your telescope be built?

35 You want to observe the thermal spectrum of a newly-born white dwarf (about 100,000 Kelvin). In which part of the spectrum would the white dwarf be brightest? A. gamma rays B. ultraviolet C. visible light D. radio Where would your telescope need to be?

36 You want to observe the population of cool G-type and K-type stars across our Milky Way galaxy. In which part of the spectrum would these types of stars be brightest? A. x-rays B. ultraviolet C. visible light D. infrared Where would your telescope need to be?

37 Suppose you wanted to observe the superheated gas (1 million Kelvin) falling into black holes at the centers of galaxies. What part of the spectrum would you observe? A. x-rays B. ultraviolet C. visible light D. infrared Where would your telescope need to be?