Land Based Telescopes

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

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

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

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

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

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

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)? 1.Mirrors don’t have _________ __________. 2.Mirrors don’t ____________ (especially infrared and UV). 3.Mirrors can be supported by their ____ and back; lenses by ONLY their edge. 4.Mirrors have only ____________ to be machined correctly; lenses have two. chromatic aberration absorb light edge one surface

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

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.

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

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 ___________________. diameter (objective diameter) 2

Resolving Power Varies directly with the ____________ _________. Also depends on –___________ of light being observed and –__________________ conditions. diameter of objective wavelength atmospheric seeing

Resolving Power: Diameter and Wavelength

Site Selection Where are the best places for ground-based observatories? Important factors – __________ pollution – _____ weather – ___ air – _____________ dark/light good 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 )

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

CCD Imaging A charge-coupled device(CCD) – Wafer of silicon divided into a two- dimensional 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 Much _______ than 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 larger

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

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