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Review for Test #1 Feb 13 Telescopes

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Review for Test #1 Feb 13 Telescopes Molecules Molecule vibration and rotation Two or more atoms joined together. They occur in atmospheres of cooler stars, cold clouds of gas, planets. Examples H2 = H + H CO = C + O CO2 = C + O + O NH3 = N + H + H + H (ammonia) CH4 = C + H + H + H + H (methane) They have - electron energy levels (like atoms) - rotational energy levels - vibrational energy levels Review for Test #1 Feb 13 Telescopes Topics: • Fundamentals of Astronomy • The Copernican Revolution • Radiation and the Electromagnetic Spectrum • Atoms and Spectroscopy • Telescopes Methods • Conceptual Review and Practice Problems Intro-Chap 3 • Review lectures (on-line) and know answers to clicker questions • Try practice quizzes on-line Bring: • Two Number 2 pencils • Simple calculator (no electronic notes) Reminder: There are NO make-up tests for this class Light rays from a distant source, parallel to the Light Hitting a Telescope Mirror "mirror axis" all meet at one point, the focus. small mirror far from 2 stars CCD small mirror far from 2 stars Light rays from any single point of light are essentially parallel. But the parallel rays from the second star come in Parallel light rays at another angle meet at another point in at a different angle. same vertical plane, the “focal plane”. 1 Optical Telescopes - Refracting vs. Reflecting Chromatic Aberration Refracting telescope Lens - different colors focus at different places. Focuses light with a lens (like a camera). white light <-- object (point of light) image at focus Mirror - reflection angle doesn't depend on color. Problems: - Lens can only be supported around edge. - "Chromatic aberration". - Some light absorbed in glass (especially UV, infrared). - Air bubbles and imperfections affect image quality. Refracting Telescope Reflecting Telescope Reflecting telescope Yerkes 40-inch (about 1 m). Cerro-Tololo 4 -m reflector. Largest refractor. Focuses light with a curved mirror. <-- object image - Can make bigger mirrors since they are supported from behind. - No chromatic aberration. - Reflects all radiation with little loss by absorption. Reflector Types Reflector Types Prime focus Cassegrain focus Prime focus Cassegrain focus (GMRT) (AT) Offset Cassegrain Naysmith Offset Cassegrain Naysmith (VLA) (OVRO) Beam Waveguide Dual Offset Beam Waveguide Dual Offset (NRO) (ATA) 2 Clicker Question: Clicker Question: Compared to radio waves, gamma-rays An advantage of refracting telescopes over travel: reflecting telescopes is: A: faster A: Big lenses are lighter than big mirrors. B: slower B: The focus is easy to get to. C: at the same speed C: They don’t suffer from chromatic aberration D: They don’t suffer from altitude sickness E: All of the above Clicker Question: Types of Antenna Mount The blackbody curve of a star moving towards Earth would have its peak shifted: A: To lower intensity B: Toward higher energies C: Toward longer wavelengths + Beam does not rotate + Lower cost D: To higher intensity + Better tracking accuracy + Better gravity performance E: To lower energies. − Higher cost − Beam rotates on the sky − Poorer gravity performance Mirror size Mirror with larger area captures more light from a cosmic Image of Andromeda galaxy with Image with telescope of twice the object. Can look at fainter objects with it. optical telescope. diameter, same exposure time. Keck 10-m optical telescope. 30-100 m optical telescopes now being considered! 3 The Two Main Types of Observation Resolving Power of a Mirror (how much detail can you see?) Imaging (recording pictures) Spectroscopy (making a spectrum) usually using a diffraction grating fuzziness you would In both cases, image or spectrum usually recorded on a CCD see with ("charge-coupled device") your eye. detail you can see with a telescope. Seeing "Angular resolution" is the smallest angle by which two objects * can be separated and still be distinguished. Air density varies => bends light. No longer parallel Parallel rays enter For the eye, this is 1' (1/60th of a degree). Looking at the Moon, atmosphere you can distinguish features separated by > 100 km. wavelength dome angular resolution α mirror diameter No blurring case. * Sharp image For a 2.5-m telescope observing light at 5000 Angstroms (greenish), Rays brought to CCD on CCD. resolution = 0.05". same focus. But, blurring by atmosphere limits resolution to about 1" for light. Blurring. Rays This is called seeing (radio waves, for example, don't get blurred). not parallel. Can't Blurred be brought into image. focus. North America at night Example: the Moon observed with a 2.5 m telescope 1" => 2 km 0.05" => 100 m So where would you put a telescope? 4 Astronomy at Yet Other Wavelengths Telescopes also observe infrared, UV, X-rays and gamma rays. Kitt Peak National Mostly done from space because of Earth's atmosphere. Observatory, near Tucson Spitzer Space Telescope - infrared Longer infrared wavelengths allow you to see Mauna Kea Observatory, radiation from Hawaii warm dust in interstellar gas. Shorter infrared wavelengths allows you to see stars through dust. Dust is X-ray Astronomy good at blocking visible light but infrared gets through better. Chandra X-ray Observatory Trifid nebula in visible light Trifid nebula with Spitzer Crab pulsar and nebula in X-rays Gamma-ray Astronomy Hubble Space Telescope and its successor-to-be: the James Webb Space Telescope Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images. GLAST - Gamma-ray Large Area Space Telescope Artists conception of a jet from an active galaxy Center of M51: HST (left; 0.05” resolution) vs. ground-based (right; 1” resolution) 5 The JWST Radio Telescopes Large metal dish acts as a mirror for radio waves. Radio receiver at focus. Surface accuracy not so important, so easy to make large one. But angular resolution is poor. Remember: Jodrell Bank 76-m (England) “It’s only a model” angular resolution α wavelength Will have diameter 6.5 meters (vs. HST 2.5 meters) – much higher mirror diameter resolution and sensitivity. Will also observe infrared, whereas Hubble is best at visible light. Expected launch 2013. D larger than optical case, but wavelength much larger (cm's to m's), e.g. for wavelength = 1 cm, diameter = 100 m, resolution = 20". Andromeda galaxy – Andromeda radio map with optical 100m Effelsberg telescope Parkes 64-m (Australia) Effelsberg 100-m (Germany) • Our Galactic center (GC) is 25,000 ly away (8000 pc) • GC lies behind 30 visual magnitudes of dust and gas Green Bank 105-m telescope (WV) Arecibo 300-m telescope (Puerto Rico) 6 Aperture Synthesis – Basic Concept If the source emission is unchanging, there is no need to collect all of the VLA image at incoming rays at one time. λ=90 cm ~45” resolution One could imagine inner few degrees sequentially combining of the Galaxy pairs of signals. If we break the aperture into N sub- apertures, there will be N(N−1)/2 pairs to combine. This approach is the basis of aperture synthesis. Example: wavelength = 5 cm, separation = 2 km, resolution = 5" Interferometry A technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry. D Very Large Array (NM). Maximum separation of dishes: 30 km Consider two dishes with separation D vs. one dish of diameter D. VLA and optical By combining the radio waves from the two dishes, the achieved images of M51 angular resolution is the same as the large dish. General Antenna Types Large Blade Antenna Wavelength > 1 m (approx) Wire Antennas Dipole Yagi Helix or arrays of these Feed Wavelength < 1 m (approx) Reflector antennas 7 The Long Wavelength Array (LWA) Very Long Baseline Array. Maximum separation 1000's of km An LWA Station State of New resolution: few arcsec resolution: 0.05 arcsec Mexico, USA 20-80 MHz tuning range (at least) Baselines up to 400 km for resolution [8,2]’’ @ [20,80] MHz 53 “stations” - mJy-class sensitivity Important astrophysical & ionospheric science resolution: 0.001 arcsec! Clicker Question: Clicker Question: The biggest telescopes on Earth are: When multiple radio telescopes are used for A: Gamma-ray telescopes. interferometry, resolving power is most B: X-ray telescopes. improved by increasing: C: Optical telescopes A: the distance between telescopes; D: Radio telescopes B: the number of telescopes in a given area; E: Infra-red telescopes C: the diameter of each telescope; D: the power supplied to each telescope Radio Frequency Interference Grote Reber’s telescope and Radio Frequency Interference in 1938 8.
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