Land Based Telescopes Telescopes light buckets Primary functions

Land Based Telescopes

Telescopes: "light buckets" Primary functions: Gather light from a 1. ______ given region of sky. Focus light. 2. ______ Secondary functions: Resolve detail in image 1. ______ Magnify angular size 2. _______ of objects.

Optical Telescopes wavelengths of light • Designed to collect ______ visible that are ____ to the human eye. • Data observed by human eyes or recorded on photographs or in computers.

The Human Eye: Shortcomings size • Eye has limited _____. – limited light gathering power. frequency response • Eye has limited __________. – only detects E-M in visible wavelengths. multiple • Eye distinguishes new image _____ times/second _______. – cannot be used to accumulate light over long period to intensify faint image. cannot store • Eye _______ image for future reference. – unlike photographic plate or CCD.

Optical Telescope Design • Basic telescope has two parts: Objective 1. _____ gather light Function: to ______ Lens/mirror of longer focal Materials: _____ length & larger diameter than the eyepiece Eyepiece 2. _______ magnify image made by Function: to ______ objective lens with a shorter focal length Material: ____ than the objective

Optical Telescopes • ______ Refractors refraction –Focus light with _____: bend light path in transparent medium –Use _______ lenses –First kind made, used by Galileo • Reflectors _____ reflection –Focus light by _____: bounce light off a solid medium –Use ____ mirror –First designed and created by Sir Isaac Newton • ______ Catadioptric lenses and _______ mirrors -Uses both ______

First Optical Telescopes: Refractors Image of source is formed on focal plane and magnified by eyepiece.

Refracting Telescopes

The Yerkes 40” Refracting Telescope

Refractors: Disadvantages high tolerance • Quality optics require _______ perfect – surfaces must be _______ absorb light, especially IR and UV. – glass will _______ – changes in orientation, ___________ temperature may flex lenses Large size very heavy, hard to support – _____ aberration • Chromatic __________ –light passes through glass –refraction a function of wavelength –all wavelengths focus different distances from lens –correctable with compound lenses, expensive

Chromatic Aberration • Dispersion of light through optical material causes blue component of light passing through lens to be focused slightly closer to lens than red component. • Known as chromatic aberration.

Reflecting Telescopes: Designs

Why build reflectors instead of refractors (advantages)? chromatic 1. Mirrors don’t have _____ aberration _____. absorb light 2. Mirrors don’t ______ (especially infrared and UV). 3. Mirrors can be supported by their edge ____ and back; lenses by ONLY their edge. one surface to be 4. Mirrors have only ______ machined correctly; lenses have two.

Why build reflectors instead of refractors? 5. Telescopes made with mirrors compact in design; can be _____ reflectors cannot. 6. Telescopes using mirrors can have _______ larger objective ends (because they have bigger mirrors), which means ____________. more light-gathering power

Powers of the Telescope 1. Magnifying Power The ability to enlarge an image. 2. Light Gathering Power The ability to see faint objects. 3. Resolving Power The ability to see fine details.

Magnification and Focal Length focal length of objective focal length of eyepiece focal length of objective Magnification = focal length of eyepiece

Light-Gathering Power • The objective’s area collects light. • The larger the area, the greater the light-gathering power of telescope. Light-gathering power proportional to (objective diameter)2 __________. diameter

Resolving Power diameter of • Varies directly with the ______ objective _____. • Also depends on wavelength of light being observed and – ______ atmospheric seeing conditions. – _________

Resolving Power: Diameter and Wavelength

Site Selection • Where are the best places for ground-based observatories? • Important factors dark/light pollution – _____ – good _____ weather – dry ___ air turbulence – _______

Closer to Sea Level, More air to pass through

Higher Altitude, telescopes in the high mountains

Earth At Night

U. S. A. At Night (circa 1994 -95)

Detection • Collected light detected in many ways. – image observed and recorded • eye, photographic plate, ____ CCD – measurements • intensity and time variability of source –______ photometer • spectrum of source –______ spectrometer

CCD Imaging • A charge-coupled device(CCD) – Wafer of silicon divided into a twodimensional array of many tiny elements, known as pixels. – When light strikes a pixel, electric charge builds up on device. – Charge buildup monitored electronically.

Radio Telescopes larger than • Much _______ reflecting optical telescopes • Resemble satellite TV dishes • Used to collect radio waves from space • AM, FM, and TV signals interfere, so must be in a radio “protected” area

Radio Astronomy: Wavelength Advantages • • dependent on time of day/night NOT _____ NOT as _____ dependent on weather Use of interferometry visible light Information other than _______ – Quasars, pulsars • Generally _______ not absorbed traveling space – pass through clouds of interstellar dust in our galactic plane Accuracy of dish shape not as hard to • _____ create or maintain – not need to be highly polish, often light weight

Arecibo Observatory: Largest Radio and Radar Dish • 1000 -ft radio dish • Used to – create maps of Moon, Venus, and Mars – discover pulsars and galaxies – measure the rotation rate of Mercury – discover planetary systems outside of our solar system

Very Large Array(VLA) in New Mexico 27 antennas, each 25 m in diameter Effective diameter = 36 km Yields radio-image details comparable to optical resolution

Interferometry or more telescopes used • Two ______ –to observe same object –at same wavelength and –at the same time. • Uses wave interference to yield high resolution. Cheaper than one • ____ (impossibly) large telescope. • Farthest 2 _ telescopes act like the end of one telescope. –Baseline: • distance between 2 farthest ‘scopes. • equals the relative ‘scope size.