March 30, 2005 Physics for Scientists&Engineers 2 1Physics for Scientists &Physics for Scientists &EngineersEngineers 22Spring Semester 2005Lecture 40March 30, 2005 Physics for Scientists&Engineers 2 2ReviewReview! To correct near-sightedness, a corrective lens (a diverging lens) mustform a virtual, upright image at the far point, dfar, in front of the lensof an object located at infinity as shown below! To correct far-sightedness, a corrective lens (a converging lens) mustproduce a virtual, upright image of the newspaper at the near point,dnear, of the person’s vision as shown below1!+1"dfar=1f10.25 m+1!dnear=1fMarch 30, 2005 Physics for Scientists&Engineers 2 3The TelescopeThe Telescope! Like the microscope, telescopes come in many forms! First we will discuss the refracting telescope and then we will delve into thereflecting telescope! The refracting telescope consists of two lenses• the objective lens and the eyepiece! In our example we represent the telescope using two thin lenses! However, an actual refracting telescope will use more sophisticated lenses! The objective lens forms a real image of the distance object at distance fo! The eyepiece is placed so that the image formed by the objective is a distancefe from the eyepiece! The eyepiece forms a virtual, magnified image of the image formed by theobjective! Because the object to be viewed is at a large distance, the incoming light rayscan be thought of as being parallel! The eyepiece forms a virtual image at infinity, again producing parallel raysMarch 30, 2005 Physics for Scientists&Engineers 2 4Geometry of the TelescopeGeometry of the Telescope! In the drawing of the geometry of a refracting telescope below, theparallel light rays from the object are depicted by a single ray! A red-black dashed line depicts the parallel light rays forming thevirtual imageMarch 30, 2005 Physics for Scientists&Engineers 2 5Magnification of a TelescopeMagnification of a Telescope! The magnification of the telescope is defined as the angle observed in theeyepiece, !e, divided by the angle subtended by the object being viewed, !o! Because the telescope deals with objects at large distances, we cannotcalculate the magnification of the telescope using the lens law! For example, one might try to express the magnification of the objective lensusing the lens equation! We can get the angular magnification from the geometric drawing on the nextslidem!= "!e!o= "fofem = !dido= !di"= 0 (bad)March 30, 2005 Physics for Scientists&Engineers 2 6Calculation of Magnification of TelescopeCalculation of Magnification of Telescope! Let’s calculate the angular magnification of a refracting telescope! The angle !o is the angle subtended by a distant object! The angle !e is the apparent angle seen in the eyepiece! The magnification is!o" tan!o=dfo!e" tan!e=dfe!e!o=d / fed / fo=fofe " m!= #fofe (inverted)March 30, 2005 Physics for Scientists&Engineers 2 7Example: Refracting TelescopeExample: Refracting Telescope! The world’s largest refracting telescope wascompleted in 1897 and installed in Williams Bay,Wisconsin (between Chicago and Milwaukee). Ithad an objective lens of diameter 40 inches (1.0m) with a focal length of 62 feet (19 m). Whatshould the focal length of the eyepiece be togive a magnification of 250?fo= 19 m m = 250m =fofefe=fom=19 m250= 0.076 m = 7.6 cmThe 40 inchrefracting telescopeat YerkesObservatoryMarch 30, 2005 Physics for Scientists&Engineers 2 8Problems with Refracting TelescopesProblems with Refracting Telescopes! The objective lens of a refracting telescope is large andheavy• The 40 inch refractor at Yerkes weighed 500 pounds! Supporting a large glass lens was difficult• Must be supported by its edges! Constructing the large glass lens was difficult! The glass lens was thick and absorbed light! A glass lens has chromatic aberration• Different focal lengths for different colors! The solution was to replace the objective lens with a mirrorMarch 30, 2005 Physics for Scientists&Engineers 2 9The Reflecting TelescopeThe Reflecting Telescope! Most large astronomical telescopes are reflectingtelescopes with the objective lens being replaced with aconcave mirror! Large mirrors are easier to fabricate and position thanlarge lenses! The eyepiece is still a lens! Various types of reflecting telescopes have been developed! We will discuss three examples of the geometries ofreflecting telescope• Reflector• Newtonian• CassegrainMarch 30, 2005 Physics for Scientists&Engineers 2 10Basic Reflecting TelescopeBasic Reflecting Telescope! Basic reflector! Replace the objective lens with a parabolic mirror! This design is impractical because the observermust be in the line of the incident lightMarch 30, 2005 Physics for Scientists&Engineers 2 11Newtonian Reflecting TelescopeNewtonian Reflecting Telescope! In 1670 Newton presented his design for a reflecting telescope to theRoyal Society• The idea for a reflecting telescope came from James Gregory! Newton solved the observer problem by placing a small mirror thatreflect the light out to an eyepiece! This mirror is small compared with the objective mirror and causes onlya small loss of light from the imageMarch 30, 2005 Physics for Scientists&Engineers 2 12Cassegrain Cassegrain Geometry for Reflecting TelescopeGeometry for Reflecting Telescope! A further improvement on the geometry of the reflecting telescope isthe Cassegrain geometry (named for the French sculptor SieurGuillaume Cassegrain) first proposed in 1672! Here a small mirror is used to reflect the image through a hole in thecenter of the objective mirror! This design and many improvements this basic idea are the basis ofmodern astronomical telescopesMarch 30, 2005 Physics for Scientists&Engineers 2 13The Hubble Space TelescopeThe Hubble Space Telescope! The Hubble Space Telescope (HST) was deployed April 25, 1990 fromthe Space Shuttle mission STS-31! The HST orbits the Earth 590 km above thesurface of the Earth, far above theatmosphere that disturbs the imagesgathered by ground based telescopes! The HST is a Ritchey-Chrétien reflectingtelescope arranged in a Cassegrain geometry! This type of telescope uses a concave hyperbolic objective mirrorrather than a spherical mirror and a convex hyperbolic secondary mirror! This arrangement gives the HST a wide field of view and eliminatesspherical aberration! The objective mirror is 2.40 m in diameter and has an effective focallength of 57.6 mMarch 30, 2005 Physics for
View Full Document
Unlocking...