Asteroid Ceres’ Bright SpotsCourse AnnouncementsAssignmentsWhat is a telescopeRefracting and Reflecting TelescopesRefractionRefractionRefraction to focus lightRefraction to focus lightRefracting TelescopeReflection to focus lightReflecting Telescope TerminologyReflectors versus RefractorsReflectors versus RefractorsReflectors versus RefractorsTelescope Size TerminologyLight-Gathering PowerLight-Gathering PowerLight-Gathering PowerTelescope SizeAngular ResolutionAngular ResolutionAngular ResolutionIncrease Telescope Slide Each SlideIncrease Telescope Slide Each SlideIncrease Telescope Slide Each SlideIncrease Telescope Slide Each SlideModern Imaging – The CCDImages and DetectorsCCD DetectorsImage ProcessingA Problem of the AtmosphereSeeingSolving the problem of SeeingAsteroid Ceres’ Bright SpotsCredit: NASA/JPL-Caltech/MPS/DLR/IDA/PSICourse Announcements•Lots of EC Opportunities this week•Exam 1 will take place Wednesday, 28 Sept.–Will cover Chapters 1 – 4–Study Materials Now Available•“Must Knows”; Chapter Slides; and MA Study Guide–Study sessions next Monday and Tuesday evenings (combined with A151)•A151 Primary - Monday Night at 5:30 – 6:30 PM CANCELLED•A152 Primary – Tuesday Night at 5:30 – 6:30 PM Room 304 in Nielsen Physics–Format:~30 Multiple Choice; 10 Fill-in-the-Blank;handful of ‘short answer’AssignmentsReading Assignments•Chapter 5: 5.5 – 5.8Parallel Lectures•No new parallel lecturesMastering Astronomy•Chapter 4 Homework[Due Monday, 26 Sept. at 11:59 PM EDT]•Chapter 5 Homework[Due Tuesday, 4 Oct. at 11:59 PM EDT]Will appear on MA on Wed.What is a telescopeBasic important facts•A telescope is a tool whose primary function is to capture as much light as possible from a region of the sky or object and concentrate the light (focus) into a narrow beam for analysis.–A ‘light bucket’. Big thing to capture as many photons as possible–Focuses light with lenses or mirrors•A detector of some kind is required to analyze or capture that light.–Your eye, photographic plates/film, CCDs, etc.•Taken together, a telescope is a series of lenses and/or mirrors that focuses (concentrates) light and puts that concentrated light on a detector.Refracting and Reflecting TelescopesTwo means to achieve the same goal: Get light to the eyepiece and detectorRefracting Telescope“Refractor”Reflecting Telescope“Reflector”Uses a series of lenses to focus lightUses a series of mirrors to focus lightRefractionThe bending of light as it passes through a medium (e.g., air, water, glass, etc.)PlasticBlockRefractionRefraction is the concept that makes prisms workPlasticBlock•Refraction is the bending of light as it passes from one medium (i.e., vacuum, air, water, glass, plastic, etc.) to another.•How much light is refracted (bent) depends on the wavelength of light.–The angle of refraction (how much it bends) is inversely proportional to the wavelength of light.–Shorter wavelengths bent more than longer onesi.e., blue and violet bent more than red.Red light bends less entering and exiting the prismViolet light bends more entering and exiting the prismRefraction to focus lightA lens is a specially designed medium (glass or plastic) to bend all incoming parallel light rays to a single point (focus)LensFocusDifferent shapes are used to focus lightRefraction to focus light(1)(2)1) Focus: The point where all light rays converge2) Focal Length: The distance from lens to focus*For lenses, focal length depends on wavelengthA lens is a specially designed medium (glass or plastic) to bend all incoming parallel light rays to a single point (focus)Refracting TelescopeIn simplest form, just two lenses.•Objective Lens:Largest lens that focuses the light•Eyepiece Lens:Small lens that delivers the focused light to a detector (your eye, CCD, etc.)•Telescope Tube:The housing for the lenses. This prevents stray light from getting to your detector.Reflection to focus lightA curved mirror is used to reflect incoming, parallel light rays to a common point (focus)Reflecting Telescope TerminologyPrimary Mirror•Large, curved mirror that collects the incoming (incident) light. Defines telescope sizeSecondary Mirror•Small mirror in telescope to redirect the light prior to reaching prime focus.Prime Focus•The focal point of the Primary MirrorNames associated with optical places and piecesReflectors versus RefractorsProblems with large-scale Refractors1. Chromatic Aberration with lenses. –Recall that refraction is wavelength dependent and so each wavelength (color) is bent differently as it passes through the lens–Thus with lenses, where light passes through a medium, the edges of an image will be color-blurred.Why are almost all large, professional telescopes reflectors?Not a problem with mirrorsReflectors versus RefractorsProblems with large-scale Refractors2. Through-put (or, what percent of light is lost to absorption/scattering within the opitcs): With lenses, the light needs to passes through a medium. This means some of the light will be absorbed, which dims the object of study. While low for visible light, the amount of light absorbed for UV and IR can be quite high•Again, not a problem with mirrorsWhy are almost all large, professional telescopes reflectors?Reflectors versus RefractorsProblems with large-scale Refractors3. Supporting the weight of the primary lens.–Large lenses can be quite heavy. Because a lens can only supported around the edges, a large lens will deform under its own weight (ruining the telescope).–A mirror can be supported over the entirety of it’s surface and from the base of the telescope.4. Polishing: A lens has two surfaces that need polishing versus one surface for a mirror.Why are almost all large, professional telescopes reflectors?Telescope Size TerminologyEdge-to-Edge Length is the Telescope Diameter (Diam. below)•Controls area and resolutionThe area of the mirror is the Telescope Area, or Collecting Area•Controls brightness/exp. time 3.5 metersArea = 9.6 m2Light-Gathering Power•Larger telescope mirror areas (i.e., collecting area) gather more light to focus into an image.–Makes images brighter, i.e., can study fainter objects–Reduces exposure times neededYou have to catch them all… Larger telescopes catch more photons meaning brigther images2x Mirror SizeSame Exposure Time•Brighter Images!–Observed brightness is proportional to telescope area, and therefore the square
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