# Telescopes

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Published on November 9, 2008

Source: slideshare.net

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 10 8 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: One to give a low M and a wide field of view (looking at the stellar background) One with moderate M (for observing the moon, planets and double stars) 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.

Usually have several eyepieces for the telescope:

One to give a low M and a wide field of view (looking at the stellar background)

One with moderate M (for observing the moon, planets and double stars)

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 c h r o m a t i c aberration. Could use an achromatic objective made of several component lenses of different types of glass. Incident light Red 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 M F 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 Convex secondary 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 Radio Featured in GoldenEye and Contact