Astronomy Notes 4 - Light and Telescopes

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Unformatted text preview: 12/24/11 Astronom Notes 4 - Light and Telescopes Light What's co e re d he re : H W H H W W W W ? ? , ? ? ? ? ? ? I - wh ? L .A ' , ( , ' ' , , ). A , , , .A E ! ? I $25 , oscillat ing elect ro-magnet ic wav e. T , ?A - , , .I, ' a e; .T , . S , .Y , .Y .T " ( ), ( "- .S , ) . W ?I c, P A a e le ngth. O .I 300,000 , .T / = 186,000 / .T ' . - c is a cons tant. , , .A , !D ' , ; !N , , ' ' .I . T F 1. T www.uni.edu/morgans/astro/course/Notes/section1/new4.html , .T 1/13 12/24/11 Astronom Notes 4 - Light and Telescopes travel at the same speed. Let's say we are going to do another experiment with the light. We are going to watch the two types of light pass by and we are going to count the number of waves that pass by each second. We like doing these sorts of experiments, since we are strange scientists. Will we count the same number of peaks passing by each second? Here is a little animation showing the experiment - remember, both waves of light have to move at the same speed, regardless of their wavelength. This is another way to distinguish the different types of light. The one with more peaks squeezed together will give us a higher count, or you could say its peaks pass by more frequently, since both types travel at the same speed. In other words, you will count more peaks passing by each second for the shorter wavelength light. You could also say that it has a higher frequency compared to the longer wavelength light. Fi g re 1. Two dif f erent t pes of light are shown - one wit h a short wav elengt h and anot her wit h a long wav elengt h. Or conv ersel a high f requenc and a low f requenc . The other way to distinguish light is by the fre que ncy, which is denoted by the letter f. Frequency is often given in units of "per second" such as "peaks that pass by each second." Generally it is harder to get a mental picture of light using frequency, so I'll just stick to wavelength to describe the different types of light. Now there is a really neat relation between the speed, frequency and wavelength of light. Here it is c =f This is pretty simple and straight forward (unlike some formulas). Since c must always have the same value, when you change the wavelength, the frequency must also change, but in the opposite sense. When the wavelength increases, the frequency decreases. Long wavelength light has a low frequency and short wavelength light has a high frequency. As I mentioned, we'll distinguish the different types of light based upon their wavelength. It is important to be able to measure the wavelength to distinguish the various types of light since there are many different types of light, each with their own wavelength value. However, wavelength is often so small that it is difficult to describe in units as "large" as cm or mm. Often astronomers use the units known as ngs troms , and in case you were wondering 1 = 10-10 m. These are pretty small units and are good at covering the range of wavelengths very well. The following table gives the various types of light that astronomers study. Since frequency is a bit harder to picture, its value will not be given, though its size is denoted in general terms. The types of light listed in this table are all studied by astronomers, though for the most part a lot of astronomy is done in the visible part of the spectrum - what you can see with you eyes. The table is ordered in terms of wavelength from longest to shortest wavelengths. Don't worry about the last column; we'll get to that later. Light Wave le ngth in me te rs Wave le ngth in www.uni.edu/morgans/astro/course/Notes/section1/new4.html Fre que ncy Blackbody Te mpe rature (K) 2/13 12/24/11 Astronom Notes 4 - Light and Telescopes Radio 10-2 and greater 108 M ic o a e 10-3 to 10-2 107 to 108 0.3 to 2.9 Inf a e d (IR) 7 x 10-7 to 10-3 7000 to 107 2.9 to 4100 Vi ible 4 x 10-7 to 7 x 10-7 4000 to 7000 4100 to 7250 Ul a iole (UV) 10-8 to 4 x 10-7 100 to 4000 7250 to 300,000 X- a 10-11 to 10-8 0.1 - 100 300,000 to 3 x 108 Gamma- a 10-11 and less 0.1 and greater low and less high 0.3 or less 3 x 108 and greater You have heard of all of these types of light in one way or another. Radio is just what you think it is - the type of light that your car radio picks up. The range of radio light is pretty wide, and the stuff that goes into your radio or tv is a pretty small part of the entire range. A lot of radio wavelengths are taken up by various communication systems, including military, CB radio, ham radio, and so on. Radio light is pretty much all over the place, even though you can't see it with your eyes. Another type of light that is vital to the existence of a typical college student is mic o a e light. Some people don't distinguish microwaves from radio, but we'll consider them as different types of light. Yes, they do cook your dinner in the microwave oven. However, most microwave light is harmless; a lot of cell phones use it for communication. The next type of light is actually the type given off by pretty much everything around you - inf a e d or IR. This is the type of light that is also associated with heat. You and all other objects around you are probably giving off IR light - things at room temperature do just that. You may have seen military or police videos that were shot using IR cameras - these are just picking up the light given off by humans and other warm things. The type of light that you experience most of the time is i ible light. You might just think of it as white light from a typical light bulb or the yellow light from the Sun, but if you were to take that light and pass it through a prism you'd get a rainbow. Visible light is made up of actually all sorts of different types of light (different wavelengths). It is often surprising to people to learn that white light is actually made up of all these colors combined together. If you have ever seen a rainbow then you have seen the full range of visible light. It is easy to remember the order of visible light from long to short wavelength since it is the same as the rainbow - Re d,O ange , Ye llo , G e e n, Bl e , Indigo, Viole . Of course, if you don't have a rainbow handy, all you need to remember is Ro G. Bi , the world famous astronomer! Most pictures of objects that you'll see in this course were taken with visible light cameras, since we can easily produce those images and our minds are geared to interpret them. Red has the longest wavelength of visible light (it comes right after infrared, after all), while violet is the shortest wavelength visible light. Continuing on down the light spectrum to shorter wavelengths, after visible light, or in this case after violet light, comes l a iole . Don't add an "n" to the name and make it ultraviolent, even though this type of light is rather nasty stuff. This is the type of light that gives you a sunburn in the summer and can lead to unpleasant things like skin cancer. Right on the heels of ultraviolet light comes an even nastier form, - a . You know this stuff is nasty when the dentist leaves the room when they x- ray your teeth. Finally, we have the shortest wavelength type of light that is studied by astronomers, gamma- a . These are very short wavelength and are also rather rare. There is the whole range of light, from longest to shortest wavelength. www.uni.edu/morgans/astro/course/Notes/section1/new4.html 3/13 12/24/11 Astronom Notes 4 - Light and Telescopes M , all t pe s of light trave l at the s ame s pe e d, name l c. I' ( ) , , , , .W , , - .S , , , , "W , !" T .L ' "H , .Y , ' .W " , . W photon. T ?L , , .L ?T ' , - .T , ( ). T ( ), - E = hf E = hc/ D' ; ' .B Ene rg ( .Y E ) ) ( - , .Y .I .T , ,, , .W , .I , , ' - Energ UP, Frequenc UP, Wav elengt h DOWN. T .I .T .T .T , ,' ;' ! , .W .A .A , As tronomical Spectra L , - , .I ) F ( pris m, 2 www.uni.edu/morgans/astro/course/Notes/section1/new4.html ; ' , , S, s pe ctrum ( . : s pe ctra). 4/13 12/24/11 Astronom Notes 4 - Light and Telescopes Fi g re 2. The album cov er f rom Pink Flo d's " Dark Side of t he Moon" showing a prism spreading out whit e light int o t he colors of t he spect rum. When e look a he pec a f om a io objec o in pace, e don' ee he ame pec a - he e a e ome i h big gap , ome i h colo mi ing, and ome ha a e a eal me . Ho i hi po ible? Wha ca e he diffe en pe of pec a? To nde and hi , e ha e o nde and ho atoms in e ac i h ligh and ho pec a a e p od ced. Pe hap hi o ld be a good ime o ge a nice ong c p of coffee, o pe hap ome highl ga loaded caffeina ed be e age - don' o , I'll ai . Fi of all, o ha e o emembe ome ba ic ph ic abo a om . A pical a om ha a pecific c e ba ed pon he pe of a om i i . Thi c e incl de man po ible o bi fo he e le ctron o elec on if he e i mo e han one. The e o bi a e loca ed a a io le el , hich co e pond o diffe en ene g le el . Yo migh an o hink of hi a a b nch of mo n ain climbe . The climbe i h he mo ene g ill go o he highe pa of he mo n ain, p o a ce ain ledge, hile ho e i h lo e amo n of ene g ill no ge ha high p, b ill a on he lo e edge . If he ge id of hei ene g , he ill go back do n he mo n ain o he bo om le el (kno n a he ground s tate in an a om). Elec on like o be in he g o nd a e ( he lo e le el). Yo migh an o hink of hem a mello pa icle ; ma be he j like o la a o nd he ho e all da in hei nde ea . Wha e e he ca e, he don' like o be e ci ed and in ho e highe o bi (highe ene g le el ). If he do ge in o a highe o bi , he eall an o ge back do n o he g o nd (lo e ene g ) a e. Ho e e , a om a e no e ac l like mo n ain , ince he elec on can onl go f om one le el o ano he if i ab o b o emi he e ac amo n of ene g (ligh ). I can' go o a place in be een - he e a e onl ce ain ene g le el a ailable o he elec on - o of like he e a e onl ce ain place on he mo n ain he e o can op and and afel . Elec on a e e pick abo he pe of ligh (amo n of ene g ) he ill e. Onl ce ain a eleng h of ligh ill in e ac i h he a om, ince ligh i di ing i hed b a eleng h. Le ' a o ha e an a om i h he elec on in he lo e ene g le el (a h d ogen a om i he imple o pic e). Mo a eleng h of ligh a e igno ed b he a om. If he e ac l igh a eleng h come along, he elec on ill ab o b he ene g (ab o b he ligh ) and he elec on ill bo nce p o a highe o bi . The highe he ene g of he ligh i ab o b , he highe he o bi i end p in. Remembe , i ha o be he e ac igh pe of ligh fo hi o happen. I ill igno e all of he pe of ligh ha a e ong. When he elec on mo e o a highe o bi (highe ene g a e), e efe o he a om a being e cite d. Elec on don' an o be in highe o bi ; he like o be in a lo ene g a e (mainl he g o nd a e) emembe , he a e la and like o ha e lo ene gie . The an o ge id of an ene g he pick p. To do hi he gi e off he ligh needed o go do n o lo e ene g le el . Thi ligh ha he gi e off co ld be e ac l like he ligh he ab o bed o a combina ion of diffe en pe of ligh ha add p o he ame ene g a he ligh ha a o iginall ab o bed. The end e l i he a om ge id of he ene g i cked p (i gi e off he ligh in a andom di ec ion) and he elec on i back do n in he g o nd a e. Fo a hile he a om a o ing p e a ene g - hi i hen o o ld call i an e ci ed a om - no beca e i on he lo e , b beca e i ha e a ene g in i . Thi cena io i ho n in Fig e 3. www.uni.edu/morgans/astro/course/Notes/section1/new4.html 5/13 12/24/11 Astronom Notes 4 - Light and Telescopes Fi g re 3. An a om i ho n i h he elec on in he lo e (g o nd) ene g le el. Ligh of j he igh a eleng h i ab o bed b he elec on, ca ing i o go in o a highe ene g le el. A long a he a om ha hi e a ene g i i ef e ed o a being " e ci ed" . To ge back do n o he lo e ene g le el, he elec on ha o ge id of he ene g and doe o b emi ing ligh i h he app op ia e ene g o mak e he an i ion. No i i j a plain ne ci ed a om again. Wha happen if he ene g he elec on ab o b i oo m ch? I he e ch a hing a oo m ch? Yo be he e i . The a om ha a c e ha allo fo o bi p o onl a ce ain ene g le el - o of like he op of he mo n ain. If he elec on ab o b oo m ch ene g and hen goe p, he e i no le el ha i can land in, ince i ha oo m ch ene g . Thi o ld be o like ha ing a ocke pack on o back a o climb a mo n ain, like ha of en happen o he Co o e ing o ca ch he Road- nne - he o e hoo hi a ge . The elec on goe comple el o of he a om. The a om i hen aid o be ioni e d; ee Fig e 4 fo an ill a ion. No he a om i o of o of balance, ince he e a an elec ical balance be een he elec on and p o on . Wi h he lo of an elec on, he e i one le nega i e pa icle. Fo an a om like h d ogen, he e i onl one elec on ha can be lo . O he a om ha e mo e elec on and can lo e mo e and mo e, o long a he ene g a ailable i high eno gh o keep kicking he elec on o . Roman n mbe a e p behind an a om' name o di ing i h ioni ed and nioni ed a om . Fo e ample, eg la , e e da , no mal i on a om i efe ed o a Fe I. If i lo e one elec on, i become Fe II; lo e ano he elec on and i become Fe III, e c. If i on lo all of i elec on o o ld ha e Fe XXVII, ince i ha 26 of hem. If he elec on an o ge back oge he i h he a om, o an o he a om, i ha o ge id of he ene g i cked p. Unde he igh ci c m ance i can gi e off ligh and hen can go back o an a om. Fi g re 4. An a om ge ioni ed, hen ligh i h high eno gh ene g can k nock he elec on o of he a om comple el . The ligh i ab o bed b he elec on o ha i i able o e cape f om he a om. Thi i all e ci ing ( eah, igh ) b ha abo he pec a? J a min e, I'm ge ing o ha . Le ' look a ano he i a ion. Wha if o ha e a b nch of a om ha a e no ge ing bomba ded b ligh b a e j mo ing a o nd a lo - he a e pe hap in a con aine ha i ge ing hea ed p. When o hea p ga e , he a om mo e a o nd fa e . Thi i no good, ince he e ill be a b nch of colli ion . Doe hi mean hei in ance a e go p? No, hi mean ha he e i a lo of e ne rg of motion in ol ed. Yo kno ha a om like o do i h ene g , igh ? The elec on can ake he ene g of he colli ion and change hei o bi . To ge back do n o he lo ene g le el he ha e o gi e off ligh , o he do j ha and go back do n o hei g o nd a e. Thi i a i a ion he e o can ge ligh f om a b nch of a om i ho p ing ligh in o i in he fi place; all o ha e o do i hea i p ( echnicall , hea i ene g , hich i ha ligh i , b i ' no ha o hink of a ligh , igh ?). If he a om a e ho eno gh, he 'll p od ce hei o n ligh . The a om co ld be ioni ed o e ci ed in hi ca e, depending pon ho fa he a e mo ing and ho m ch ene g i in ol ed in he colli ion . See Fig e 5 fo he cena io. www.uni.edu/morgans/astro/course/Notes/section1/new4.html 6/13 12/24/11 Astronom Notes 4 - Light and Telescopes Fi g re 5. T o a om head f o a colli ion. The ene g of he a om ' mo ion i an la ed in o ge ing he elec on b mped p o highe ene g le el . In one ca e, he elec on j mo e p o a highe ene g le el and he a om i he ef o e e ci ed. In he o he ca e, he elec on i comple el popped o of he a om, e l ing in an ioni ed a om. The elec on ill ha e o e en all ge id of he ene g he ha e - b po ibl gi ing of f ligh - ince he an o ge back do n o he g o nd a e. Wha abo olid obj ec ? W , , , .T - ' ' ' .A ( , , ) , .A .F black bod . Y , ' , .T , .T " ' " .I , , , ( ) . A ?I , , .I ' .T K , Kirchhoff's Laws . 1. A , ( ) continuous s pe ctrum - , . 2. A ( 3. W , ) , Fi g re 6. The dif f e en F ( e mis s ion s pe ctrum ' . ) ( ) , .I , , , .T abs orption s pe ctrum . pe of pec a a e ho n - con in o , emi ion and ab o p ion. 6 .O , .A , , - , , - .F www.uni.edu/morgans/astro/course/Notes/section1/new4.html 7/13 12/24/11 Astronom Notes 4 - Light and Telescopes no all of he ligh ha i ab o bed b he cool ga clo d emain in he ga clo d. When he a om in he ga clo d ab o b he ligh , he elec on ill be momen a il e ci ed, b a o kno , he don' like o be e ci ed. The efo e, he ene g ha he a om ab o b doe ge e en all elea ed, b he ligh i gi en off in andom di ec ion . The ca e of an ab o p ion pec m migh be ho gh of a a "deflec ion pec m" ince he ligh i mainl j deflec ed off in andom di ec ion b i in e ac ion i h he a om . In he ca e of he emi ion pec m, ome hing i ca ing he ligh o be p od ced b he ga i elf all hea - and hi ca e he ligh o come o ba ed pon he c e of he a om. B making he ga hicke and hicke , o o ld ge hicke and hicke emi ion line n il o ge o he poin he e he hing i olid and no o a e looking a a black bod and ha e a con in o pec m. In he ca e of Ki chhoff' la 2 and 3, he diffe en a om in he ga gi e diffe en pa e n of line co e ponding o he pe of ligh he ab o b o emi . The pe of ligh an a om ab o b o emi depend pon he pecific c e of he a om (ho fa apa he ene g le el a e). In o he o d , a ce ain pe of a om ill p od ce a ce ain pa e n of line (da k ab o p ion line o b igh emi ion line ) and each pe of a om make i o n di inc pa e n. An a om of h d ogen i diffe en f om an a om of heli m, hich i diffe en f om a ca bon a om, hich i diffe en ... ell, o ge he pic e. See Fig e 7 fo a a ie of pec a f om ome ga e . Fi g re 7.The ab o p ion and emi ion pec a f o dif f e en ga e a e ho n. No ice ho he pec a a e o of nega i e of one ano he - ha i ab o bed in one pec a f o a pa ic la elemen i emi ed in he emi ion pec a f o ha elemen . If he pa e n p od ced b he emi ion o ab o p ion pec m i diffe en fo each elemen (and i i ), hen b looking a a pec m, o can iden if he ga ha i in ol ed in he p od c ion of he pec m. Thi i j like finge p in ing he a om - he e' a me hod of ob aining elemen iden ifica ion ! When a onome o de e mine he compo i ion of a clo d of ho ga , a a o a gala , he e he pec a o p o ide info ma ion abo he objec . We don' e en ha e o o ch he a o fig e o ha i i made of, hich i a good hing, ince ho e c i e a e eall ho . Thi i ho a onome can alk abo ha a di an gala i made of, ho gh he ha e ne e had he chance o ample i di ec l - b j had he oppo ni o look a i pec m. In eali i i a bi mo e comple ince objec a e gene all made p of a b nch of diffe en elemen - o all of ho e elemen o ld be i ible in he pec m. In ome ca e i i e en po ible ha he pec a ha e bo h ab o p ion fea e and emi ion fea e - hich i eall complica ed. An e ample i ho n in Fig e 8. www.uni.edu/morgans/astro/course/Notes/section1/new4.html 8/13 12/24/11 Astronom Notes 4 - Light and Telescopes Fi g re 8. Emi ion pec a f o h ee elemen a e ho n (H d ogen, Ca bon and O gen). If all of he e elemen e e in a ho ga clo d, he co ld p od ce he pec m a he bo om hich i a combina ion of he h ee indi id al pec a. More Radiation la s T .T , .I , , .A I , ' , .O .T F 9. Fi g re 9. Se e al ene g o p c e f o black bodie . The amo n of ene g gi en of f a dif f e en a eleng h i ho n. No e ho he c e all peak a j one a eleng h. Thi peak co e pond o he dominan pe of ligh ha i gi en of f b he black bod . Y .T .T , .H , , .Y , .F , , , , , - - .T IR .O , .I UV , ( - , , .T ) Wie n's La , ma K www.uni.edu/morgans/astro/course/Notes/section1/new4.html - = 0.0029/T me te rs .T 9/13 12/24/11 Astronom Notes 4 - Light and Telescopes , ( , ). B , .S , " " .I , . I .T F , .I 9 ' , - , . N F 9 .Y .T - .T .H .H ' E= T4 W Ste fan-Bolt mann Law .A ' .Y , .T , , .T .A ( !). W ( ) (1/3 ?T 3 3 3 3 = 81 ), !A ! Doppler Effect A .T .T Dopple r e ffe ct. T , .Y ; T W , , . , - ( , ( , ?). , ), ( F ' ). T 10. I . www.uni.edu/morgans/astro/course/Notes/section1/new4.html 10/13 12/24/11 Astronom Notes 4 - Light and Telescopes Fi g re 10. A light source is mov ing t owards t he lef t . The peak s of t he light wav es are shown as circles t hat are mov ing awa f rom t he locat ion t he where t he were f irst giv en of f b t he light source. So t he largest circle represent s t he f irst light emit t ed b t he source. In t he direct ion of it s mot ion, t he light wav es are compressed, so t hat Observ er A would see t he light source bluer t han it act uall is, while Observ er B would see t he light source redder t han normal (since t he light in t hat direct ion is st ret ched out t o longer wav elengt hs). Astronomers describe the spectrum as being either e d hif e d or bl e hif e d. This just means that the light has been shifted due to, respectivel , relative motion awa from ou or motion towards ou. The degree of the shift depends upon the speed of the motion. You can determine the speed using this formula V =c , where V is the velocit (in km/s), c is the speed of light (=300,000 km/s), is the change in wavelength of the spectral features from the normal wavelengths and is the normal wavelength. A larger shift results from a large velocit . Figure 11 shows how the Doppler shift would cause the absorption features in a spectra to appear at the "wrong" wavelengths. Fi g re 11. Absorpt ion spect ra shown under t hree dif f erent circumst ances. At t he t op, t he normal nonmov ing spect ra wit h t he f eat ures appearing at t heir appropriat e wav elengt hs. In t he middle, t he obj ect is mov ing awa f rom t he observ er (t he person who obt ained t he spect rum) so t hat t he absorpt ion f eat ures appear at slight l longer wav elengt hs t han normal. The bot t om spect rum is t he case where t he spect ral source is mov ing t owards t he observ er so t he f eat ures are blueshif t ed - occurring at slight l short er wav elengt hs t han normal. Tele cope Telescopes are the main tools of astronomers and have been used since the da s of Galileo to stud objects in the sk . Of course since there are different t pes of light, there are also different t pes of telescopes. Regardless of the differences, telescopes have three purposes: 1. To colle c ligh . Believe it or not, this is the most important purpose. As we like to sa in astronom "Si e does matter - the bigger, the better." In fact, si e can make all the difference. You ma want to think of a telescope as a light bucket and the more light it collects, the better the view. The amount of light a telescope collects depends directl upon the si e of the light collecting surface - the part of the telescope that gathers up the light. The light gathering abilit goes as the radius of the light gathering surface squared, since telescopes usuall have a circular light gathering surface and the si e of this surface is a function of radius squared (Area of a circle= R2). If ou were to double the radius, ou would increase the light gathering area b a factor of four - ou would gather four times more light. However, bigger also means that ou have to have some prett big machiner to operate the telescope, as well as a prett big wallet to cover the cost of the thing. 2. To e e fine de ail - o e ol e fe a e . The abilit to see things more clearl , or to have better e ol ion, is usuall measured in arcseconds, and the smaller the resolution, the better. This just means www.uni.edu/morgans/astro/course/Notes/section1/new4.html 11/13 12/24/11 Astronom Notes 4 - Light and Telescopes that ou can see things that are prett close together or that ou can see small details clearl . Resolution is important since it helps astronomers get a better idea about what is going on out there - small details could provide a great deal of information about objects and the processes that affect them. Unfortunatel , the resolving abilit of a telescope depends upon the t pe of light (wavelength) ou are using and the si e of the telescope ou are using. For the best resolution, a ver large telescope is needed, and in man cases this is difficult to manufacture (or afford). In some cases having a large telescope doesn't reall help - that's because the Earth has this anno ing atmosphere that not onl blocks our view of some things (especiall on cloud da s) but also blurs our views of objects. There are some wa s to get around these problems, as ou'll see. 3. To magnif obje cts . While man people think that this is what telescopes mainl do, it is usuall not a major feature. If ou were to sit two inches from our television ever thing would look bigger, right? How is the qualit of the picture? Prett bad, isn't it? Often when the magnification is increased the image looks prett crudd , so sometimes a lower magnification is better. Usuall onl a high magnification is used when looking at nearb object like planets or the Sun. T pes of Teles copes The first telescopes built were made up of a series of lenses, sort of like a bunch of e eglasses lined up. These lenses would bend the light to bring it to a focus at one location. I suppose we should call these telescopes benders, hmm? No, that's not ver scientific. When light is bent b passing through something like glass, it is re fracte d, so these earl telescopes are known as re fractors . Figure 14 shows ou a refracting telescope design. You probabl have a refracting telescope in our home even if ou don't know it - that is if ou own a set of binoculars, since the operate mainl b using this method. The old fashioned refractors used to be fairl popular, since it was sort of eas to make lenses. Fi g re 14. A ref ract ing t elescope operat es b bringing light t o a f ocus using a lens or sev eral lenses. The light collect ing area of t hese t pes of t elescopes is limit ed. Here's a good question - wh do we want to bring light to a focus? Let's sa ou have a refracting telescope that has a 3 inch wide lens. All of the light falling in that 3- inch wide lens is being gathered b our telescope. How are ou going to put all that light into our e e? Your pupil is prett small, onl a few millimeters wide at most. It's a waste of time if ou can't get all of that light ou are gathering into a convenient package that fits into our e e. The light needs to be gathered or focused so that it will fit into our pun little pupil. That's wh we use the lenses to bend the light into a focus. Unfortunatel , there is a limit to the si e of a refracting telescope, since the onl support of the lenses is on the edges, just like e eglasses. If a lens was ver large, its weight would cause the glass to sag and the images produced b the refracting telescope would be distorted. Refracting telescopes had a si e limitation that could not be overcome, at least not without sacrificing the light gathering area. Since si e is so important (remember, s i e doe s matte r), this is a prett bad situation. One of the largest refracting telescopes is at the Yerkes observator in Wisconsin (see Figure 15). The lens in this telescope is 40 inches wide - about as big as ou can get. Fi g re 15. Yerk es observ at or ref ract ing t elescope. Not e how long t he scope is compared t o t he amount of light t hat it gat hers. This long lengt h is needed t o f ocus t he light properl . (Phot o f rom t he Yerk es v irt ual t our) www.uni.edu/morgans/astro/course/Notes/section1/new4.html 12/13 12/24/11 Astronom Notes 4 - Light and Telescopes To overcome the si e limits, another telescope design is usuall used. If ou have a curved surface which is highl reflective (like a mirror), the light could be brought into focus b having it bounce off the mirror at a certain angle. Since a mirror could be supported from behind, ou can make a ver large mirror! To focus the light, the best surface to use is a parabola (Figure 16), and this scheme is used in most large modern telescopes, not just those that ou look through with our e e, but also radio, IR, etc. Currentl most modern telescopes are of this t pe e fle c ing e le cope . Fi g re 16. Ba ic de ign f o a ef lec ing ele cope. The c ed mi o i in he hape of a pa abola and hi allo i o b ing all of he ligh o a f oc in one loca ion. You might have noticed that the light in a reflecting telescope is focused at a point in front of the mirror. If ou wanted to look at the gathered light, ou would have put ourself in front of the mirror. This doesn't www.uni.edu/morgans/astro/course/Notes/section1/new4.html 13/13 ...
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This note was uploaded on 12/25/2011 for the course ASTRONOMY 086 taught by Professor Klazner during the Fall '09 term at Everglades.

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