Asteroid Ceres Bright Spots Credit NASA JPL Caltech MPS DLR IDA PSI Course 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 Assignments Reading Assignments Chapter 5 5 5 5 8 Parallel Lectures No new parallel lectures Mastering 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 telescope Basic 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 Telescopes Two means to achieve the same goal Get light to the eyepiece and detector Refracting Telescope Reflecting Telescope Refractor Reflector Uses a series of lenses to focus light Uses a series of mirrors to focus light Refraction The bending of light as it passes through a medium e g air water glass etc Plastic Block Refraction Plastic Block Refraction is the concept that makes prisms work 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 ones i e blue and violet bent more than red Red light bends less entering and exiting the prism Violet light bends more entering and exiting the prism Refraction to focus light A lens is a specially designed medium glass or plastic to bend all incoming parallel light rays to a single point focus Different shapes are used to focus light Lens Focus Refraction to focus light A lens is a specially designed medium glass or plastic to bend all incoming parallel light rays to a single point focus 1 Focus The point where all light rays converge 2 Focal Length The distance from lens to focus For lenses focal length depends on wavelength 1 2 Refracting Telescope In 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 light A curved mirror is used to reflect incoming parallel light rays to a common point focus Reflecting Telescope Terminology Names associated with optical places and pieces Primary Mirror Large curved mirror that collects the incoming incident light Defines telescope size Secondary Mirror Small mirror in telescope to redirect the light prior to reaching prime focus Prime Focus The focal point of the Primary Mirror Reflectors versus Refractors Why are almost all large professional telescopes reflectors Problems with large scale Refractors 1 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 Not a problem with mirrors Reflectors versus Refractors Why are almost all large professional telescopes reflectors Problems with large scale Refractors 2 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 mirrors Reflectors versus Refractors Why are almost all large professional telescopes reflectors Problems with large scale Refractors 3 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 Telescope Size Terminology Area 9 6 m2 3 5 meters Edge to Edge Length is the Telescope Diameter Diam below Controls area and resolution The area of the mirror is the Telescope Area or Collecting Area Controls brightness exp time Light Gathering Power You have to catch them all Larger telescopes catch more photons meaning brigther images 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 needed Same Exposure Time 2x Mirror Size Light Gathering Power Bigger means brighter Brighter Images Observed brightness is proportional to telescope area and therefore the square of the telescope diameter Area x Radius 2 Diameter is twice the radius Double telescope size and get 22 4 times brighter images or same brightness in the exposure time Make a telescope 10x bigger and capture 10 2 100 times more photons Same Exposure Time 2x Mirror Size Light Gathering Power Brighter means less exposure time Shorter Exposure Times With brighter objects exposure times are reduced A 5 m telescope will require 52 25 times shorter exposure time than a 1 m telescope If a dim object required 25 hours of exposure in the 1 m telescope you can get the same observation done in 1 hour with a 5m telescope Same Exposure Time 2x Mirror Size Telescope Size Resolving Power Larger telescopes give a increased angular resolution i e ability to form distinct separate images of two objects lying close together in the field of view or sky Can you see a single blur or two distinct objects Angular Resolution Ability to see two points as distinct points At
View Full Document
Unlocking...