What we call “light” is only one type of …• Electromagnetic Radiation – a way in which energy moves

through space.Do not confuse EM radiation with Particle radiation.

We’ll start by discussing the visible light spectrum.

When white light is passed through a prism, a “rainbow”-like band of color is seen.

ROY G. BIV … Ever met him?

RedOrangeYellowGreenBlue

IndigoViolet

This is the order from longest to shortest wavelength. This order of colors never changes!

What is wavelength?…

Wavelength (λ) – length over which the wave repeats.Or…

Wavelength (λ) – Distance between two crests of a wave.

   Light wavelengths are very short!

λ measured in Angstroms (Å). Named after 19th century Swedish physicist A.J. Angstrom. 1 Å = 1x10-10 m

Approximate sensitivity limit of the human eye.4000 Å - 7000 Å (or… 400 nm - 700 nm)

λ > (longer than) 7000 Å - Infraredλ < (shorter than) 4000 Å - Ultraviolet

• Electromagnetic Radiation – a way in which energy moves through space.

Speed of light in a vacuum c = 3 x 108 m/s= 186,000 miles/sec

7 times around the Earth in 1 second!!

“Electromagnetic Radiation” gets its name from the fact that it is a manifestation of rapidly varying Electric & Magnetic fields.

• There are seven different regions of the E-M Spectrum

They are, in order of increasing wavelength;Gamma Rays

X-RaysUltraviolet

Visible Infrared

MicrowaveRadio wave

(Be able to describe applications of each region, or where we encounter them in life.)

All of these regions can be used in astronomy to measure objects in the universe.

Different methods are needed to detect different wavelengths of radiation.

Only certain parts of the E-M spectrum can penetrate the Earth’s atmosphere.

• The parts that get through are called windows of transparency.• Earth’s atmosphere has two main windows in visible and radio.

TELESCOPES

For this discussion, we will use the word telescope to mean Optical Telescope

These are the types of telescopes you (the student) are most likely to use.

What are the most important things telescopes do in astronomy?1. Gathering light

(Our eye’s pupil is only about 8 mm wide)

2.Magnification

3. Increasing light-gathering timeUsing CCD’s or film

Our brain “refreshes” about 30 times/sec.No such thing as a “long-exposure” brain images!

4. Increase resolution or resolving power.Resolution [1/(diameter of optical element)] (see Dawes Limit)

Resolution inversely proportional to diameter – The larger the optical element, the smaller the features that can be resolved.

• Gathers light – it’s most important job!– Allows us to see faint,

distant objects.– Doubling the diameter

quadruples the light gathered.

Since A = r²

(1)² / (2)² = ¼– Thus, the bigger one

gathers 4x the light!

Radius = 1 Radius = 2

So What Does a Telescope So What Does a Telescope Do?Do?

• Magnifies – Important when

viewing planets,

nebulae, and

other objects in space.

Magnification equals the focal length of the telescope divided by the focal length of the eyepiece. Or, in equation form…

Mag = FLT ÷ FLE

The longer the eyepiece focal length,

the less the magnification.

What a Telescope Does:

What a telescope does:• Resolves

– Separates two objects at a distance so they appear separate.

ResolutionIn the figure right, two lights are imaged by telescopes of different apertures. Apertures are decreased by a factor of two each step to the right. •Notice the decrease in the ability to resolve the two lights as separate objects. Bigger aperture = Better resolution

•Note also the wavelength dependence.

Bluer light = Better resolution.

This is due to the Dawes Limitr (arcsec) = 2 x 10-3 λ/d

λ - wavelength in Angstromsd - diameter of the aperture in cm.

Important: Because of air turbulence, the best resolution from ground-based telescopes is limited to ½ arcsec.

1degree = 60 arcmin1 arcmin = 60 arcsec

½ arcsec = the width of a human hair across two football fields (~ 200 yards).

The limiting factor of air turbulence in Earth’s atmosphere is why the

Hubble Space Telescope (and other orbiting observatories) are placed into space.

Refractors vs Reflectors

• Primary/Objective lens• Secondary lens or

eyepiece

• Primary/Objective mirror

• Secondary mirror• Eyepiece

RefractingTelescopeRefraction – the bending of light, as when it travels

from one medium to another.

Focal Length – Distance at which the light rays

converge.

Chromatic Aberration – Different colors are focused at different points. This causes

blurry images.

Chromatic Aberration can be solved using compound lenses, but this costs light and makes images

dimmer. It also makes telescopes heavier and more expensive.

Reflectors RuleBut Why??????

• Large diameter refractors are expensive to make.

• Mirrors are easier to mount. Large mirrors can be supported from behind.

• Refractors suffer from chromatic aberration.

• Also, large lenses sag under gravity.

However, reflectors can suffer from Spherical Aberration

Parallel light rays do not image at the same point.

Parabolic mirrors solve

Spherical Aberration

Types of Reflectors

Newtonian – Eyepiece on the side of the tube near the front aperture or opening.

Types of Reflectors• Cassegrain – Eyepiece at the rear of telescope.

Light travels through a hole in the primary mirror.

Types of ReflectorsSchmidt-Cassegrain – Similar to a Cassegrain, but

with a spherical mirror and a lens on the front called a corrector plate. Maksutov telescopes are also similar.

Telescope Mounts• Altitude-azimuth mounts

Simple up-down, side-to-side motion• Equatorial mounts

Oriented to track the stars

Classic Alt-Az

Dobsonian

Fork Equatorial

German Equatorial

Manual Telescopes

• More mirror for the $$$$$.

• Suitable for all ages.

• You must learn your way around the night sky.

Automated Telescopes

• Ease in finding objects.

• A necessity for astrophotography.

• Expensive.

• Not suitable for children.

Manual star alignments

GPS system alignments