1. Astronomy 1020-H Stellar Astronomy Spring_2015 Day-19

2. Course Announcements Exam-2 – Fri. 3/6 Chapters 5 & (some of) 6  I will collect the L-T books at this time. Smartworks Chapters 5: Due Fri. 3/6 Smartworks Chapters 6: Due Wed. 3/18 Apr. 2 – Last day to drop a class.

4.  The telescope is the astronomer’s most important tool.  Purpose: to gather light of all kinds.  Two kinds of optical telescopes: reflecting and refracting.  Invented in 1608 by Hans Lippershey.

5. Telescopes Telescopes have three functions: 1. Gather light LGP ∝ Area = πR 2 2. Resolve objects Θ = 2.06 X 10 5 ( λ/D) 3. Magnify EXTENDED objects

6.  When light encounters a new material, it can either experience reflection or refraction.  In refraction, the light will be bent depending on the value of the index of refraction relative to the first material. CONNECTIONS 6.1

7.  Refraction depends on the wavelength— violet light is bent more than red.  Dispersion: the resulting spreading out of the wavelengths of light.  Causes chromatic aberration in lenses, which can be fixed by a compound lens. CONNECTIONS 6.1

8.  Refracting telescopes use lenses.  Objective lens: refracts the light.  Aperture: size of the objective lens (larger aperture gathers more light).  The objective lens is placed in the aperture.

9.  Focal length: distance between lens and the image (longer = larger image).  Aperture sets the light-collecting power.  Focal length determines the image size.

10.  The largest refracting telescope has a 1-meter aperture.  Problems with refractors: Need to be large to have a long focal length. Lenses suffer from chromatic aberration.

11.  Reflecting telescopes use mirrors.  There are primary and secondary mirrors.  Focal length is determined by the path the light takes reflecting off the mirrors.

12.  Reflectors have advantages over refractors.  No chromatic aberration.  Bigger telescopes due to increased focal length in the same amount of physical space and no need for massive lenses.  The largest telescopes in the world are reflectors.

14. A concave mirror focuses light

15. Spherical Aberration

16. Spherical aberration can be eliminated by a parabolic shape or a corrector plate Correcting S.A.

17. There are several types of reflecting telescopes Focal Arrangements

18. Schmidt-Cassegrain

20.  The light-gathering power of a telescope is proportional to the square of the aperture size.  A telescope’s magnification depends on the focal lengths of the objective lens or mirror and the eyepiece. MATH TOOLS 6.1