Telescopes Lecture 7 2/7/2018
Tools to measure electromagnetic radiation Three essentials for making a measurement: A device to collect the radiation A method of sorting the radiation A device to detect the radiation
Light Hitting a Telescope Mirror small mirror far from star 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 at a different angle.
Why are the light rays parallel? earth
Light rays from a distant source, parallel to the mirror axis, all meet at one point the focus. Parallel light rays at another angle meet at another point in the same vertical plane, the focal plane.
Light rays from a distant source, parallel to the lens axis, are refracted based on wavelength to a single focus
Refraction Waves bend when they pass through material of different densities. air swimming pool water The deflection depends on the wavelength too!
Chromatic Aberration Lens different colors focus at different places white light Mirror: - reflection angle doesn t depend on color
Reflecting telescope Can make bigger mirrors since they are supported from behind No chromatic aberration Reflects all radiation with little loss by absorption One surface to machine, unlike a refracting lenses
Refracting Telescope Yerkes 40-inch (about 1m) Largest refractor Reflecting telescope Cerro-Tololo 4-m reflector
Resolution Angular size of the smallest detail that can be distinguish. Measured in arc seconds (1/3600 of a degree) Larger telescopes have better resolution. Longer wavelengths are harder to resolve. If your resolution is 1 arc second, you can resolve a quarter from 5 km away. The Hubble Space Telescope has a 0.1 arc second resolution.
Exposure Time Matters The longer the exposure time. The more light collects. This scales linearly with exposure time. For example an exposure time of 2 hours will collect twice as much light as a 1 hour exposure. Because there are so many astronomers compared to telescopes, exposure time is the limiting factor for most observations.
Mirror Size Matters The quality of an image depends on the collecting area of the telescope. The larger the area, the more light that can be collected. The area of a circle is radius 2 x pi So a 2-m telescope collects 4 times more light than a 1-m telescope
Image of Andromeda galaxy with optical telescope Image with telescope of twice the diameter, same exposure time, 4x light collected
Charge Coupled Device (CCD)
Raw CCD image pre processing Analog Readout of CCD Image
Why use a CCD over photographic film? Film typically collects about 5% or less of all light, while a CCD typically collects 75%.
The two main types of observation Imaging: recording pictures Spectroscopy: making a spectrum, usually using a diffraction grating In both cases, image or spectrum usually recorded on a CCD (charged coupled device)
Filters: Color Images red green blue combined
A spectrum of a star
Where would you put a telescope? North America at Night
Criteria for best observations At the best sites, the weather is clear as much as 75% of the time. Even on a clear night, the atrophic moister filters out starlight. Dry sites are preferred, generally found at high altitudes. Need dark skies! Observatories are best located at least 100 miles from the nearest large city. Air is often unsteady; light passing through this turbulent air is disturbed, resulting in blurred star images. Astronomers call these effects bad seeing. Can be corrected with adaptive optics.
Ground based inferred astronomy Infrared is heat radiation. Objects that are ~300 Kelvin emit IR radiation IR telescopes must be very cold, 1-3 Kelvin, in order to not be blinded by the IR radiation of the telescope. IR telescopes can see through objects that visible light cannot
Radio Telescopes Large metal dish acts as a mirror, radio receiver at focus. Detecting wavelengths between 1 mm (0.039 in) and 100 km (62 mi) Surface accuracy not so important, so easy to make large one. The longer the waves, the harder it is to resolve fine detail in the images or maps we make
Andromeda galaxy optical Andromeda radio map with 100m Effelsberg telescope
GC lies behind 30 visual magnitudes of dust and gas
Interferometry A technique to get improved angular resolution using an array of telescopes. Most common in radio but also limited optical interferometry. Consider two dishes with separation D vs. one dish of diameter D. By combining the radio waves from the two dishes, the achieved angular resolution is the same as the large dish.
Very Large Array (NM) Very Long Baseline Array.
Space based telescopes
Optical Space Telescopes: Hubble and James Web
Astronomy at Yet Other Wavelengths Spitzer Space Telescope Infrared Longer infrared wavelengths allow you to see radiation from warm dust in interstellar gas.
Shorter infrared wavelengths allows you to see stars through dust. Dust is good at blocking visible light but infrared gets through better. Trifid nebula in visible light Trifid nebula with Spitzer
X-ray astronomy Chandra X-ray Observatory Crab pulsar and nebula in X-rays
Gamma-ray astronomy GLAST- Gamma-ray Large Area Space Telescope Artists conception of a blazar jet
Hubble-Space Telescope Advantage of space for optical astronomy: get above blurring atmosphere much Sharper imager Center of M51: HST (left, 0.05 resolution), vs. ground based (right, 1: resolution)
Friday: 11-1 1-3 Monday 11-1 Test Review Possible times.