telescopes
DESCRIPTION
TRANSCRIPT
Observing
The Electromagnetic spectrum
The electromagnetic spectrum is a series of transverse waves (composed of the electric field and the magnetic field), all travelling at the same speed through a vacuum e.g. space, with a velocity of 300 000 000 ms-1 (3 x 108
ms-1).
The Electromagnetic spectrum
Earth’s orbital period around the Sun = 365.25 days, so add extra day every 4 years.
Doppler Shift
If an object is moving towards us, its light will be ‘squeezed’ in our direction, i.e. the frequency will increase and light will be bluer (shorter wavelength - blueshift). A receding object’s light waves are ‘stretched’ - the frequency is lower than normal and the light will be shifted to the red (redshift).
Towards us Away from us
Telescopes
Two main types: refractors and reflectors.
Refractors use lenses Reflectors use mirrors.
Refractors
These have an objective lens (object-glass) and an eyepiece. If the objective has a diameter (D) of 3”, the telescope is called a 3” refractor.
Incident light
Eyepiece
O
P
Distance OP = focal length of objective, F.
F/D = focal ratio e.g. 36”/3” = 12 (this telescope has a f/12 ratio)
The objective collects the light, but the eyepiece (of focal length, f) does the magnification:
F/f = magnification, M
Refractors
Usually have several eyepieces for the telescope: (a) One to give a low M and a wide field of view (looking at the stellar
background)(b) One with moderate M (for observing the moon, planets and double
stars)(c) One with a high M (for detailed views on clear nights)
Must compromise between magnification and focal ratio. E.g. Using an objective of
D = 3” and F = 36” and an eyepiece of f = 0.125”, M = 288. However the objective
is too small - so the image although detailed will be too faint!
Could use a 6” refractor, F = 72” and an eyepiece with f = 0.25”, so M = 288.
The longer focal length of the new objective requires a longer tube, could use a 6”
lens with F = 54” so focal ratio is now 9 instead of 12 (smaller tube). However a short
focal ratio delivers low magnifying power.
Refractors - problems
The lenses refract different wavelengths of light by different amounts - this causes chromatic aberration. Could use an achromatic objective made of several component lenses of different types of glass.
Incident lightRed focus
Objective
A refractor produces an inverted image. Could add a correcting lens but this increases light loss further as it travels through the lenses.
Blue focus
Mountings
The higher the magnification, the smaller the field of view so the telescope must be
moved slowly and steadily. Simplest form of mounting is the altazimuth - this can
move in altitude (up and down) and azimuth (left and right). However must
continuously track objects as they move in the sky.
Easier to have an equatorial mount where the telescope is mounted on one end of a
polar axis (parallel to axis of the Earth) and a counterweight keeps it steady on the
other end. The telescope is moved round in azimuth (right ascension) and the
altitude is already corrected for. For bigger telescopes there is a computerised
electrical drive whereby you can type in the RA and declination of the object.
Reflectors
In a Newtonian reflector light reflects off a main mirror and is deflected by the small flat mirror through a side eyepiece.
Incident light
Eyepiece
MF
The mirror is coated with a thin layer of silver or aluminium to increase its reflectivity.
The flat mirror contributes to some light loss but it is not great.
Main mirror
Flat
Reflectors
In a Cassegrain reflector light reflects off a main mirror onto a secondary mirror, passing through a hole in the main to the eyepiece.
Incident light
Eyepiece
The focal ratio and magnification are the same as for refractors.
Reflectors have a small field of view so there is a finder attached to the side of the telescope. This is a small refractor with low magnification but has a wide field of view so it’s easier to find an object and track it. A permanently mounted motor driven equatorial telescope can be set to any RA and dec (above the horizon!).
Main mirror
Convexsecondary
mirror
Refractors vs Reflectors
Refractors are easy to use and need little maintenance. The min useful aperture is
3”. They are usually portable and useful for looking at the Sun. However they suffer
from chromatic aberration.
Reflectors have mirrors that are less effective than a lens - a min aperture of 6” is
needed. Also any small error in the curve of the primary will give distorted images.
However a greater magnification can be acquired (all major telescopes are
reflectors). Also there are no refraction problems.
Binoculars are made up of two small refractors. A 7 x 50 pair have a magnifying
power of 7, with each objective lens having a diameter of 50 mm. Binoculars with
M greater than x12 have a small field of view and need a tripod.
Major Telescopes
Ground-based telescopes are set up at high altitudes where atmospheric turbulence
is less. This is particularly important for infra-red observations as water vapour
absorbs certain wavelengths in the infra-red (IR). Detectors in IR telescopes have to
be kept cold as thermal emission from the telescope itself can affect the observations.
Radio telescopes are large parabolic dishes - the radio waves are collected and
brought to focus above the dish, where they are detected by an electrical sensor.
Observations can be made during the day! Interferometery is a series of telescopes
spaced out over large distances. All of the signals are combined - having a large
array improves angular resolution for these long-wavelength observations.
Space-based telescopes are in orbit around the Earth and are primarily used for
observations in the high energy end of the spectrum and in the sub-mm and
microwave regions - wavelengths to which the atmosphere is opaque.
SWIFT, orbiting at 600 km
Gamma rays, X-rays, UV and optical
Gamma Ray Bursts
NGC 4321 + Supernova
Optical Ultraviolet X-ray
XMM-Newton, orbiting at 7000 - 100 000 km
X-ray
Chandra, orbiting at 139 000 km
X-ray
X-ray Binaries - Galactic Centre (near a black hole)
Gemini N Mauna Kea (4213 m), Hawaii
Gemini S Cerro Pachon (2722 m), Chile
8.2 m, Optical and Infrared
M74
M20
Quasars
NGC 246
Twin Keck telescopes 10 m, Mauna Kea, Hawaii
Optical and Infrared
PPN (IR)
Jupiter (IR)
NGC 891
Saturn (IR)
South African Large Telescope (SALT) 10 m, Sutherland (1759 m), South Africa
Optical and Infrared
Hobby-Eberly telescope 9.2 m, Texas (2026 m), USA
Optical
Large Binocular telescope 2 x 8.4 m, Mt Graham (3200 m), Arizona, USA
Optical and IR
NGC 6946
NGC 891 (blue)
William Herschel telescope 4.2 m, La Palma (2400 m), Canary Islands, Spain
Optical and IR
Anglo-Australian telescope (AAT) 3.9 m, Siding Spring Mountain (1134 m), Australia
Optical and IR
Hubble Space Telescope (HST) 2.4 m, orbiting at 600 km
Ultraviolet, Optical and Near-IR
M100
Orion Nebula
Helix Nebula Red Rectangle (HD 44179) White Dwarf stars
United Kingdom Infrared Telescope (UKIRT) 3.8 m, Mauna Kea, Hawaii
Infrared UKIDSS (UK IR Deep Sky Survey)
NGC 3132
Spitzer 0.85 m, orbiting at 100 000 km
Infrared
Orion Nebula HST
Orion Nebula HST and Spitzer
Cosmic Background Explorer (COBE), orbiting at 900 m
Microwave
Wilkinson Microwave Anisotropy Probe (WMAP), orbiting at 1.5 million km
Microwave
Cosmic Microwave Background
Composition of the Universe
Arecibo radio telescope 305 m, Puerto Rico
RadioFeatured in GoldenEye and Contact
Effelsberg radio telescope 100 m, Effelsberg, Germany
Radio
Jodrell Bank radio telescope 100 m, Manchester, UK
Radio
Atacama Large Millimetre Array (ALMA), 64 x12 m antennae Andes Mountains, Chile 5000 m
Radio
The new HST: James Webb Space Telescope (JWST) 6.5 m, orbiting beyond the moon at 1.5 million km (earth - moon distance = 400 000 km)
Optical, Infrared