Astronomy 1010 Planetary Astronomy Fall_2015 Day-21

Course Announcements SW-chapter 6 posted: due Fri. Oct. 23 Exam-2 will be returned on Friday 1 st Quarter Observing night: Tuesday, Oct. 20; 7:30pm

Course Announcements CHECK YOUR LAB GRADES IN D2L. If you are missing grades, and have gotten the lab back, return it so we can enter the grade. (mostly Scientific Methods, but might be others as well.)

Telescopes & Instruments

 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 come in three general types Refractors use lenses to bend the light to a focus Reflectors use mirrors to reflect the light to a focus Catadioptric telescopes use both lenses and mirrors

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

The most important property of any telescope is to gather large amounts of light and concentrate it to a focus.

Refraction is the bending of light when it goes from one medium to another “n” is the index of refraction. Refraction is governed by Snell’s Law:

If we curve the surface and make a lens, we can get the light to concentrate to a point

 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.

The refracting telescope uses two lenses Since the eye already has a lens, the eyepiece is needed to bring the light rays back to parallel for the eye to see

Large refractors can be very long and bulky

The Largest Lens is 40” Built in the late 1890’s, it is the last great refracting telescope.

Lenses and refractors suffer from Chromatic Aberration This applies to camera lenses, your eye, telescopes and anything else that uses a lens to focus light

Correcting for Chromatic aberration can be expensive The compound lens takes two lenses of different materials and combines them to correct for color distortion

Color separation is useful in a prism so that we can obtain a spectrum of light Since it is meant to be separated we don’t call it an aberration. Instead, it is called dispersion

A diffraction grating works on interference of light waves Diffraction is much more efficient at separating light into its colors than dispersion

Unfortunately, diffraction also leads to problems Look closely enough at stars and they aren’t just points of light but rings, too

i_Clicker Question Telescopes and Astronomical Instruments: Refraction – Option 1 Lenses – Option 1

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

Reflection is the bouncing of light off a surface Mirrors do not suffer from chromatic aberration and they do not cut off long or short wavelengths

A concave mirror focuses light to a focal point Telescope mirrors are made so that the focus is a plane instead of a point

Reflecting Telescopes Suffer From Spherical Aberration

There are several types of reflecting telescopes

i_Clicker Question Telescopes and Astronomical Instruments: Reflection – Option 1 Spherical Aberration

LSST Site – Artist Concept

LSST Site – 1 st Blast – March 2011

Steward Observatory Mirror Lab

LSST M1 in polishing build-up

Coming “soon” The 30m Tele.