Unformatted text preview:

Reminders!Website: http://starsarestellar.blogspot.com/ Lectures 1-3 are available for download as study aids.Reading: You should have Chapters 1-3 read, andprobably Chapters 4-7 by the end of this weekend.Homework: Save the chapter 4 problems for the nexthomework set. Just do the chapters 1, 2, and 3problems on homework #1.Homework #1 is due this Wednesday at theBEGINNING of class.Discussion: Today from 1-2 pm is the last math reviewclass. For Tuesday and Wednesday, we’ll have openoffice hours.Telescopes: Extending Our SensesToday’s Lecture:• Finish discussing lightWhy is the sky blue and sunsets red?• The tools of astronomers (Chapter 3, pages 38-63)Introduction to telescopesDifferent telescopes for different types of light• Phases of the Moon (Chapter 4, pages 64-68)Explain the phases as a geometrical effectShow that the phase is related to the position ofthe Moon in the skyWhy blue sky and red sunsets?Blue light is preferentially scattered. Away from theSun, the sky looks blue.As the light path through the atmosphereincreases, the Sun looks progressively moreyellow, orange, and red and it approachesthe horizon.Telescopes• The primary purpose is to collect more light• Typical large telescope: D ≈ 6 m mirror• Dilated eye pupil: D ≈ 6 mm• A = area = πR2 = πD2/4, since R = D/2• The ratio of areas for two telescopes isD2D1Telescopes (continued)• A 6 meter telescope can see a million times fainter than thehuman eye!• A large telescope also provides higher resolution -- theability to see finer details.• But turbulence in the Earth’s atmosphere blurs the image,and limits the detail that can be seen (especially at opticalwavelengths)If D2 = 6 m andD1 = 6 mm = 6 x 10-3,then the ratio of areasis (1000)2 = 106!Angular ResolutionResolution ≈ 0.002λ/DResolution in arc seconds, λ in Å, and D in cmRefracting Telescopes• Invented in Holland in the 1600s• Light is bent (or “refracted”) by a lens and broughtto a focus.Light raysLensFocusedto a pointLight rays from a star areessentially parallelStars are very, very far away, so the rays reachingus diverge only very slightly. They are PARALLELfor all practical purposes.Refracting Telescopes (cont.)• Simple to make, but they suffer from severalproblems, including chromatic aberration• This is a property of all lenses…including your eyes!• Lenses are also heavy and absorb lightReflecting Telescopes• Use a mirror to bring light to a focus.sphericalmirrorparabolicmirror• A spherical mirror does not focus the light to a singlepoint, which leads to spherical aberration.• A parabolic mirror does focus the light to a single point.• This was in fact the problem that Hubble first had.Common Reflector DesignsTelescopes in space• no distortion (blurring) by the atmosphere• darker sky (especially in the infrared)• be able to see ultraviolet, X-rays, gamma-rays,and infraredozone (O3) blocks UV radiation at altitudeof 20-40 kmwater vapor (H2O) blocks IR at altitude of2-10 kmother atoms/molecules block X-rays and gamma raysTelescopes in space (cont.)• IR: telescopes on top of high mountains, onballoons, in airplanes, and in space.• X-rays, gamma-rays: balloons, rockets,orbiting satellites• UV, optical: orbiting satellites (for example,the Hubble Space Telescope)Twinkling of Stars• Air bends (refracts) starlight. Due to theatmospheric turbulence, the bending varies withtime, so the amount of light hitting your eyes varies.• Stars closer to the horizon => more air, moreturbulence => more twinkling!• This causes blurring of the star’s image, so that itdoesn’t look point-like.Twinkling of Stars (cont.)• Planets don’t twinkle as much. They are closer,and thus disks of light (not point-like) and therandomly varying points average out.• But planets DO twinkle when close to the horizon.• For a telescope larger than 20-30 cm, twinkling isthe limiting factor for resolution (<1’’ resolution).• How do we get rid of the twinkling?Star (far away)Planet (much closer)Adaptive Optics• Used with ground-based telescopes• A method to correct for blurring of images due toturbulence in the Earth’s atmosphere over smallareas of the sky• Monitor the light from a “guide star” or “laserguide star”• Make small changes to the mirror shape manytimes per second, and this cancels the smearingfrom the turbulenceAdaptive Opticscorrects thewavefrontMakes for aMUCH clearerimage.Phases ofthe MoonThe position ofthe Moon in thesky is closelyrelated to thephases!Can you see aFull Moon at 12o’clock noon?What wouldsomeone seefor the phasesof the Earthfrom the


View Full Document

Berkeley ASTRON 10 - Lecture Notes 4

Documents in this Course
Galaxies

Galaxies

26 pages

Lecture 1

Lecture 1

16 pages

Load more
Download Lecture Notes 4
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Lecture Notes 4 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Lecture Notes 4 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?