Astronomy 1010-HFall_2015Day-22

Planetary Astronomy

Course Announcements• How is the sunset/sunrise observing going?

• SW-chapter 6 posted: due Fri. Oct. 23

• Exam-2 will be returned on Friday

• 1st Quarter Observing night: Tuesday, Oct. 20; 7:30pm

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 6.1MATH TOOLS 6.1

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

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

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.

Concept Quiz—Bigger Telescopes

Why do astronomers want to build bigger telescopes?

A. to eliminate the effects of astronomical seeing

B. to search for life on Mars

C. to observe fainter or more distant objects

Resolution = smallest details that can be separated.

The longer the focal length, the better the separation of two objects or features.

Diffraction, or blurring of an image, sets the best possible resolution.

The diffraction limit depends on the ratio of wavelength-to-telescope aperture.

D

51006.2 Better resolution means smaller angle

Adaptive Optics can clear up most of the distortions caused by the atmosphere

The ultimate resolution of a telescope is set by the diffraction limit.

The angle subtended by the smallest resolution, θ, is determined by the ratio of the wavelength of light being studied to the aperture diameter.

1 arcsecond = 1/3,600 of a degree. Human eye:

MATH TOOLS 6.2MATH TOOLS 6.2

Earth’s atmosphere degrades images.

Astronomical seeing = limit on resolution due to the atmosphere.

Space-based telescopes do not have this problem.

TurbulenceDifferences in the temperature and density of small portions of Earth’s atmosphere cause passing starlight to quickly change direction, making stars appear to twinkle.

Kitt Peak, Arizona

Earth’s atmosphere hinders astronomical research

Image of stars taken with telescope on the Earth’s surface

Same picture taken with Hubble Space Telescope high above Earth’s blurring atmosphere

Adaptive & Active opticsAdaptive optical systems can overcome atmospheric distortionActive = quasi-static; 1 or 2 changes per minute.

Adaptive = rapid changes; tens to hundreds of times per minute

Adaptive optics can help correct for this atmospheric distortion.

Earth-based image quality can compete with the Hubble Space Telescope in the visible.

Photography opened the door to modern astronomy.

Captured images on photographic plates.

Increased integration time comes with longer exposures.

Expensive, slow, and messy.