Light and Telescopes Chapter 5

Traditional Telescopes The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

Advances in Modern Telescope Design (I) Modern computer technology has made possible significant advances in telescope design: 1. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

Advances in Modern Telescope Design (II) 2. Lighter mirrors with lighter support structures, to be controlled dynamically by computers. Floppy mirror Segmented mirror

The Keck Telescopes The two Keck Telescopes on Mauna Kea, Hawaii. Each telescope has a mirror diameter of 10 meters.

Examples of Modern Telescope Design The Large Binocular Telescope (LBT) The Very Lage Telescope (VLT) in Chile

The Future of Optical Telescopes The European Extremely Large Telescope (E-ELT): 906 segments in a 42-m mirror! The Giant Magellan Telescope (2016)

Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence. Distortions by the atmospheric turbulence are measured using a laser beam.

Interferometry Recall: Resolving power of a telescope depends on diameter D:  min = 1.22 /D. This holds true even if the entire surface is not filled out. → Combine the signals from several smaller telescopes to simulate one big mirror → Interferometry

What a telescope does Typical resolution of eye is about 0.5 minutes of arc or 30 arc seconds. HST has a resolution of 11.6/240 =.048 arc seconds. With HST we can resolve objects that are almost 1000 times smaller. NOTE the equation used above was given on page 77 of the textbook.

CCD Imaging CCD = Charge-coupled device More sensitive than photographic plates Data can be read directly into computer memory, allowing easy electronic manipulations False-color image to visualize brightness contours

Negative Images The galaxy NGC 891 as it would look to our eyes (i.e., in real colors and brightness) Negative images (sky = white; stars = black) are used to enhance contrasts.

The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object.

Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna). Amplified signals are stored in computers and converted into images, spectra, etc.

Radio Maps In radio maps, the intensity of the radiation is color-coded: Red = high intensity; Violet = low intensity Just like optical telescopes, radio telescopes should be built in regions with low average rainfall and cloud cover, and low radio noise.

Radio Interferometry The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Just as for optical telescopes, the resolving power of a radio telescope is  min = 1.22 /D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light. → Use interferometry to improve resolution!

The Largest Radio Telescopes The 100-m Green Bank Telescope in Green Bank, WVa. The 300-m telescope in Arecibo, Puerto Rico.