Why Use a Telescope?

Transcription

1 1 Why Use a Telescope? All astronomical objects are distant so a telescope is needed to Gather light -- telescopes sometimes referred to as light buckets Resolve detail Magnify an image (least important of the three) Telescopes accomplish these by using a combination of lenses and/or mirrors

2 2 Refracting Telescope Refraction = as light passes from one medium to another (e.g. air to glass) it is bent Light is gathered and focused by a curved lens.

3 3 Refracting Telescope First telescopes were of this type No longer used for astronomical research Very difficult to make large, defect-free lenses Weight of large lenses make them deform over time. Lenses exhibit chromatic aberration and spherical aberration

4 4 Reflecting Telescope A curved mirror is used to collect and focus the light.

5 5 Reflecting Telescope Used in modern telescopes Mirror can be supported from the back, allowing the mirror to be much less massive than a comparable lens Do not need large, defect free glass because surface is coated with reflective material. Mirrors can be constructed in a parabolic shape, minimizing spherical aberration Mirrors do no exhibit chromatic aberration

6 6 Telescope Designs Distance from lens or mirror to focus = focal length. Objective produces an image at the focus An eyepiece (usually a lens) is used to magnify the image.

7 7 Reflecting Telescope Designs For a reflecting telescope, a secondary mirror is used to reflect the image to a detector outside of the telescope. Prime Focus Newtonian Cassegrain

8 8 Eyepiece and Magnification Magnification focal length of objective focal length of eyepiece

9 9 Telescope Properties: Gathering Light The larger the area of the primary mirror, the more light can be collected and the fainter the object we can detect. Light Gathering P ower Diameter 2

10 10 Gathering Light Can combine many smaller mirrors to make one large area Keck Observatory Keck telescope primary mirror made of 36 hexagonal mirrors Keck Observatory

11 11 Comparing Telescopes

12 12 Telescope Properties: Angular Resolution The smallest separation in angle which can be observed by the telescope

13 13 Angular Resolution Absolute limit of angular resolution: D θ = best angular resolution in arcseconds = wavelength (meters) D = diameter of mirror (meters) Small (the minimum resolvable angle) means good resolution Smaller wavelengths = better resolution Larger mirror = better resolution

14 14 Angular Resolution Resolution usually limited by motions in the atmosphere ( twinkling ) Need site with calm, dry weather, little atmosphere above the telescope to reduce effect. Adaptive optics: sensors monitor distortions due to atmosphere and correct shape of mirror times per second Very Large Telescope (8.2 meters), many others

15 15 Adaptive Optics Object viewed through typical telescope Object viewed with adaptive optics

16 16 Angular Resolution

17 17 Observing the Image Astronomers rarely look through a research telescope Photographic film is now replaced by CCD detectors Detector is put at the focus to record a digital image. Data is processed by a computer 900 megapixel CCD detector for the Subaru 8.2-m telescope CCD Chip

18 18 LBT The Large Binocular Telescope (LBT) is an optical telescope for astronomy located in southeastern Arizona, and is a part of the Mount Graham International Observatory. The LBT is currently one of the world's most advanced optical telescopes; using two 8.4-m (27-ft) wide mirrors, with a 14.4 m center-center separation, it has the same light gathering ability as an 11.8-m (39-ft) wide single circular telescope and resolution of a 22.8-m (75-ft) wide one.

19 19 Hubble Space Telescope

20 20 Spectroscopy Light coming though telescope is often separated by a prism or a diffraction grating to produce a spectrum of intensity vs. wavelength (color).

21 21 Observing at Other Wavelengths All telescopes discussed so far have been optical (visible wavelengths) telescopes. Other wavelengths are very useful because they often yield information that visible light cannot Earth s atmosphere blocks some wavelengths of light

22 22 Atmospheric Opacity Opacity = percentage of light blocked by atmosphere Low opacity: Optical and Radio Medium opacity: Infrared and UV High opacity: Gamma Rays, X-rays & some UV

23 23 Infrared Telescopes IR light can pass through dust Used to observe star formation, center of galaxies, low T objects (i.e. planets) Telescope design much like optical telescope, but with different detector. Best results if telescope is placed above much of the atmosphere (Spitzer Space Telescope, Herschel Space Observatory)

24 24 Spitzer Space Telescope

25 25 Radio Telescopes Can observe day or night Not affected by Earth s atmosphere Radio light can pass though dust in space Because wavelength is long, we need large telescope to get good resolution 21-cm wavelength allows astronomers to map the hydrogen concentration

26 64 m telescope at Parkes Observatory in Australia 305 m telescope at Arecibo Observatory in Puerto Rico 26

27 27 Interferometers More than one radio telescope is used to increase resolution. Large effective diameter Image made only after much computer processing Very Large Array in New Mexico

28 28 Ultraviolet, X-rays and Gamma Rays All blocked by atmosphere Telescopes must be above atmosphere Chandra X-ray Telescope

29 29 Sky at Many Wavelengths Figure 5.36 Chaisson/McMillan Astronomy Today, 7 th ed., (Pearson Education, 2011)