1/17/2020
Topics in Observational astrophysics
Ian Parry, Lent 2020 Lecture 1
• Electromagnetic radiation from the sky. • What is a telescope? What is an instrument? • Effects of the Earth’s atmosphere: transparency, seeing, refraction, dispersion, background light. • Basic definition of magnitudes. • Imperfections in imaging systems.
Lecture notes
• www.ast.cam.ac.uk/~irp/teaching • Username: topics • Password: dotzenblobs
• Email me ([email protected]) so that I can put you on my course email list.
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Introduction • Observational astronomy is mostly about measuring electromagnetic radiation HERE (on Earth and nearby) and NOW. Astronomy is an evidence based science. • We measure the intensity, arrival direction, wavelength, arrival time and polarisation state. • Astronomical sources are so far away that the parts of the spherical wavefronts that are ultimately collected by the telescope aperture are essentially flat just before they enter the Earth’s atmosphere. • A telescope is a device that collects pieces of incoming wavefronts and focuses them, i.e. turns them into converging spherical wavefronts. • An instrument is a device that comes after the telescope. It receives the wavefronts and further processes them by either optically manipulating them or converting the energy into measurable signals, or both.
Telescope Instrument
Primary Optional Optional Detector optics and telescope instrument initial pupil optics optics
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Examples • The human eye can be thought of as a telescope (pupil + lens) and an instrument (retina). • Similarly the camera in a phone or a laptop can be thought of as a telescope (pupil + lens) and an instrument (cmos detector). • The lens of an SLR camera is the telescope and the camera body is the instrument. • When a person looks through a telescope their eye is the instrument. • Stonehenge was an instrument (the first working telescopes appeared in 1608). • Precisely where the telescope ends and the instrument begins is subjective.
The human eye
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Detrimental effects of the Earth’s atmosphere
1) Blocks some or all of the light. Wavelength dependent. Clouds are a major problem. 2) Distorts the plane wavefronts. Blurs images and causes twinkling. 3) Refracts the light causing apparent change in observed wavefront direction. 4) Atmosphere scatters sunlight and it also glows of its own accord adding background light which makes faint objects more difficult to see.
The transparency of the Earth’s atmosphere
Much of the EM radiation from space does not get to the Earth’s surface – it’s absorbed by atoms and molecules.
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Atmospheric absorption of the solar spectrum
Air mass = sec (zenith distance) 1.5=sec(48deg)
Atmospheric transmission (model)
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Turbulence in the Earth’s atmosphere • The plane wavefronts from space are distorted by turbulence in the atmosphere. • Can think of the atmosphere as being made up of lots of cells each with slightly different refractive indices. • The structure of these cells varies with time and there is a range of cell sizes. • Characteristic timescale (cell lifetime), ~0.01 sec • Characteristic length scale, 10‐20cm • Atmospheric “seeing” is a measure of the blurred size of an image for a point source due to atmospheric turbulence. At a good observing site this will usually be in the range 0.4 – 1.5 arcsec with a median seeing of ~0.7 arcsec FWHM. • Adaptive optics is a technique that flattens the wavefronts, i.e. removes the wavefront errors due to atmospheric turbulence and therefore sharpens the images (see later lecture). • Turbulence also causes intensity variations (spatial and Light intensity variations due temporal) across a wavefront (like the light pattern at the bottom of a swimming pool) and so the integrated intensity of a to scintillation are greater for wavefront sampled by the telescope pupil will vary with time. smaller apertures. This is called scintillation and the variations are stronger for smaller apertures. The human eye has a small pupil and scintillation is what causes stars to twinkle.
Refraction by the Earth’s atmosphere • The refractive index of the atmosphere is slightly greater than unity so in general, light rays are bent and objects in the sky appear closer to the zenith than they ought to (atmospheric refraction). • Zenith distance is the angle between an object and the zenith. • For 75 a good approximation to the amount of refraction is given by