Telescopes Flashcards
Law of Reflection
Angle at which the light beam hits the mirror of a telescope, angle of incidence, is exactly equal to the angle at which it is reflected, angle of reflection, applies to all flat mirrors
Focal Point
Where light beams are reflected to, all the beams crossing makes the image brighter
Snell’s Law
Light bends as it crosses into glass, if light enters at an angle it bends towards the normal, as the light exits it speeds up and bends away from the normal
Objective Lens
Primary lens, bends the light to the focal point, the bigger the lens the more light collected so the better the image
Refracting Telescope
Light is collected through a glass objective lens and light from the focal point is reflected to the eye piece for magnification
Chromatic Aberration
Since light of different frequencies travel at different speeds within the medium they have different focal points, causes the image to be blurry and have a colored haze surrounding it, only a problem for refracting telescopes
Achromatic lenses
Uses multiple types of glass to match color focal points
3 Powers of a Telescope
Light gathering power, resolving power, magnification power
Light-Gathering Power
larger the objective lens the more light gathered, allows you to see fainter objects proportional to the area of the objective lens
Resolving Power
Ability to distinguish fine details and separate two closely spaced objects, higher power gives better image
Magnification Power
How much larger an object appears through the telescope compared to naked eye, least important power
Magnifying Scale
Angular size (how much space it takes up in your field of view) in telescope / angular size to naked eye
Cons of high magnification
reduces field of view, darker image, can make things blurry if resolving power isn’t high enough, heightens imperfections in atmosphere, harder to keep objects in sight
How to get difference in light-gathering power
6m lens vs. 9m lens
(9^2 pie) / (6^2 pie) = x stronger
Magnification Power Equation
focal length of objective / focal length of eye piece
How to get better resolving power
larger objective lens, resolving power is reported by the smallest angular separation between objects that can be distinguished
Newtonian Telescope
Uses a concave primary mirror, then a flat 45 degree secondary mirror redirecting light to the side of the telescope at an eye piece, solved the problem of chromatic aberration
Cassegrain Telescope (Reflecting Telescope)
Uses a concave primary mirror then a convex secondary mirror, the reflected back down through the middle of the objective to an eye piece
Why reflecting telescopes are better than refracting telescopes
Can collect more light using mirrors, no chromatic aberration, mirrors are cheaper and easier to support, more compact and easier to maintain (Use reflecting in all major telescopes)
Why do stars twinkle?
There is turbulence in our atmosphere, moving pockets of warm and cold air, causing images to blur on and off
Turbulence
Mixing of air of different densities in the atmosphere causing light to hit the focal point off-center or at different times creating a blurry image
Adaptive Optics
Technology that helps with turbulence, uses flexible mirrors that can adjust in real-time, self-correcting loop
Top 5 Ground-Based Optical Telescopes
Gran Telescopio Canarias (GTC), Keck I and II, Very Large Telescope (VLT), Subaru Telescope, Southern African Large Telescope (SALT)
Gran Telescopio Canarias (GTC)
Canary Islands, Spain, 10.4 m, high-altitude, Largest single-aperture optical telescope in the world
Keck I & II
Mauna Kea, Hawaii, 10 m each, can work independently and together, high-altitude
Very Large Telescope (VLT)
Chile, 8.2m each, 4 units, detailed study of black holes, can work independently or together
Subaru Telescope
Mauna Kea, Hawaii, 8.2m, National Astronomical Observatory of Japan, large field of view
Southern African Large Telescope (SALT)
South Africa, 10.4m, largest optical telescope in southern hemisphere, for spectroscopy
Charged-coupled devices (CCD)
Captures light at pixel level and processes that into an image, high resolution, very light sensitive, low noise
Top 5 Space-Based Telescopes
Hubble Space Telescope (HST), James Webb Space Telescope (JWST), Chandra x-ray observatory, Spitzer space telescope, Kepler space telescope
Hubble Space Telescope (HST)
1990, 570km above earth, 2.4 primary mirror, most detailed images in optical, UV, and mear-infrared wavelengths
James Webb Space Telescope (JWST)
2021, designed for the infrared spectrum, 6.5m, orbits at L2 lagrangian point, helps us understand early universe
Chandra X-ray Observatory
1999, 139,000 km from earth, provides images that invisible to optical telescopes, designed to detect x-rays from very hot objects
Spitzer Space Telescope
2003, retired 2020, designed for infrared light and seeing very cold objects that are too cold to emit light, trails earth to stay cool (heliocentric orbit)
Kepler Space Telescope
2009, retired 2018, designed to find exoplanets, trails earth (heliocentric orbit), 0.95m, not for photos but measuring changes in starlight
Radio Telescopes
Uses parabolic dish to focus waves onto receiver, turning waves into electrical signal, dish doesn’t need to be smooth of continuous since waves are so long
Radio Telescope Advantages
Operate 24/7 regardless of weather or time of day, can look through dust and gas clouds, can pick up emissions from cold and far away objects, large field of view
Radio Telescope Disadvantages
Lower resolution, huge and expensive to make, sensitive to interference from human made objects, complex data to process and understand
Colors of radio telescopes
Images are false color since we can’t see the actual waves, just assigning frequencies to colors, important to read photo captions, cm listed is what element is being looked at
Interferometry
Uses multiple radio telescopes spread apart to get better resolution, baseline (how far apart the dishes are, this distance is then the total dish size), must line up signals since they are hit by waves at different times and altitudes
5 Major Radio Telescopes
Very Large Array (VLA), Event Horizon Telescope (EHT), Arecibo Observatory, Atacama Large Millimeter/submillimeter Array (ALMA), Five-hundred meter Aperature Spherical Telescope (FAST), Parkes Radio Telescope
Very Large Array (VLA)
27 dishes, 25m each, New Mexico,
Event Horizon Telescope (EHT)
Global network, earth’s diameter is total dish size based on baseline, used for blackholes, first photo of black hole
Arecibo Observatory
Puerto Rico, 305m fixed dish, collapsed, on the equator, large field of view, largest single aperture at the time
Atacama Large Millimeter/submillimeter Array (ALMA)
Chile, 66 Atenas, largest is 12m, observes cold universe, detailed images
Five-hundred meter Aperature Spherical Telescope (FAST)
Extremely sensitive, China, Fixed dish, largest single aperature
Parkes Radio Telescope
Australia, 64m dish, 1961, helped with Apollo 11 landing
