Telescopes

Types of Telescopes There are telescopes in all emag. wavelengths. Optical telescopes use visible light. Examples: Refracting Telescope, Reflecting Telescope,Cassegrain Telescope

The Earth’s atmosphere is very transparent for visible light and radio waves. For that reason, there are two major types of telescopes on Earth: Common Optical Telescopes Radio Telescopes

The larger the telescope, the more light it gathers. Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.

Why do we use telescopes? 1. to brighten an image 2. to magnify 3. to get better resolution Common misconception: magnification is the most important thing….not necessarily

Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting telescopes.

Refraction

Focal Point Focal Length the place where light rays converge to a point Focal Length the distance from a curved mirror or lens to its focus

Typical Small Refractor

Reflection

The 100” Telescope (old school)

Multiple Mirror Telescope

Why use a telescope? Brighten Magnify Resolve

Why are large telescopes reflectors? Large glass lenses sag and warp over time. Cost—it’s cheaper (only one side’s polished and it’s easier to smooth it out)

Low High Light Gather Power Low High Magnification Low High Resolution

The Powers of a Telescope: 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D A=  (r)2

Telescopes Brighten Light-Gathering Power - cross sectional area of the telescope tube (the aperture) Light gathering power α (diameter)2 Ex. If telescope A is 3 inches in diameter and it is compared to telescope B that is 6 inches in diameter….. B is 2x as large as A….so the light gathering power is (2)2 = 4 x light gathering power (If it was 3x larger…then (3)2 = 9 x power…etc.)

Light Gathering Power 10.7 cm camera 15.2 cm camera

Telescopes Magnify Magnification - the number of times larger an object appears through a telescope than as seen by the naked eye

Magnification Telescopes are usually labeled as Aperture -- f / # Example: A telescope is labeled as 500 -- f / 7 Q: What is the aperture? A: 500 mm Q: What is the focal length ? A: f = (500 mm) (7) so f = 3500 mm Q: What would the magnification be if we used a 50mm eyepiece? A: mag. = 3500 mm / 50 mm = 70 x

Example Problem How would you change the magnification for this telescope? Change magnification by changing out eyepieces.

Telescopes Resolve Angular Resolution - measure of the clarity of images Telescope with larger diameters are able to resolve smaller objects.

Resolution

Things that Detect Light for Astronomers Human Eye and Photographic Film Photometers - an electronic device that measures the brightness of stars CCD’s (charge-couple device) - an electronic imaging device that records the intensity of light falling on it

CCD Camera and Color Filters

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

Observing Problems Bad Weather Light Pollution Dispersion Scintillation “twinkling”

Disadvantages of Refracting Telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be corrected, but not eliminated by second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

Observing Problems Atmospheric dispersion is the spreading out of light into a spectrum by Earth’s atmosphere. Venus

Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing

The Best Location for a Telescope Far away from civilization – to avoid light pollution

Why do stars twinkle? Scintillation is the twinkling of stars caused by turbulence in the Earth’s atmosphere. Turbulence - hot air rising and cool air falling Note: Planets seem to not twinkle because so bright.…but can be “fuzzier”.

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 Radio maps are often color coded: Radio maps are often color coded: Like different colors in a seating chart of a baseball stadium may indicate different seat prices, … colors in a radio map can indicate different intensities of the radio emission from different locations on the sky.

Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light  Use interferometry to improve resolution!

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

Radio Interferometry (2) The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

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

The Hubble Space Telescope... …is the largest telescope in space. ...is 30 times more sensitive than ground based telescope (resolves 0.05 arcseconds). ...orbits the Earth every 95 minutes. …gives high resolution images because it does not suffer from the effects of atmospheric turbulence.

Hubble’s best images                                                              http://heritage.stsci.edu/gallery/gallery.html

Advances in Modern Telescope Design (1) Modern computer technology has made possible significant advances in telescope design: Segmented mirror 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror

Examples of Modern Telescope Design (2) The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes

Science of Radio Astronomy Radio astronomy reveals several features, not visible at other wavelengths: Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe. Molecules (often located in dense clouds, where visible light is completely absorbed). Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.

Infrared Astronomy Most infrared radiation is absorbed in the lower atmosphere. NASA infrared telescope on Mauna Kea, Hawaii However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed. Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.

NASA’s Space Infrared Telescope Facility (SIRTF) Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space.

Ultraviolet Astronomy Ultraviolet radiation with l < 290 nm is completely absorbed in the ozone layer of the atmosphere. Ultraviolet astronomy has to be done from satellites. Several successful ultraviolet astronomy satellites: IUE, EUVE, FUSE Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.

The Hubble Space Telescope Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s Avoids turbulence in the Earth’s atmosphere Extends imaging and spectroscopy to (invisible) infrared and ultraviolet

Gamma-Ray Astronomy Gamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory

X-Ray Astronomy X-rays are completely absorbed in the atmosphere. X-rays are completely absorbed in the atmosphere. X-ray astronomy has to be done from satellites. X-rays trace hot (million degrees), highly ionized gas in the Universe. NASA’s Chandra X-ray Observatory