Astronomy 1020-HSpring_2015

Day-19Stellar Astronomy

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.

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.

Telescopes Telescopes have three functions:

1. Gather light

LGP ∝ Area = πR22. Resolve objects

Θ = 2.06 X 105 (λ/D)

3. Magnify EXTENDED objects

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

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

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.

Focal length: distance between lens and the image (longer = larger image).

Aperture sets the light-collecting power. Focal length determines the image size.

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.

Reflecting telescopes use mirrors. There are primary and secondary mirrors. Focal length is determined by the path the

light takes reflecting off the mirrors.

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.

Correcting S.A.

Focal Arrangements

Schmidt-Cassegrain

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