Announcements•No lab tonight due to Dark Sky Observing Night last night

•Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6

•First Quarter Observing Night next Wednesday. Set-up starts around 6:45pm

•Due in two weeks: Telescopic Observations of the Moon. If you want to check out a Dobs for the weekend, you can get it tonight or tomorrow before 4:30pm.

Telescopes

Galileo’s early telescope and lens

Newton’s original telescope with mirror

Telescope basics

Telescopes either use refraction or reflection to focus light to a point. For refraction, the basic law is Snell’s Law: n1sinq1 = n2sinq2. The law of reflection is a much simpler: ri

If the surfaces of a piece of glass are curved, they will

focus light to a point

The focal length, f, is the distance from the lens axis to the focal point or focal plane. The focal length of a lens depends on the material of the lens and the curvature of the surfaces

2121

1111

1

RnR

dn

RRn

f

R1 and R2 are the radii of curvature of the two faces, n is the index of refraction of the glass and d is the center thickness of the lens

A concave mirror will also focus light to a point

For a spherical mirror the focal length is just half the radius of curvature of the mirror. For other shapes the formula is somewhat more complicated

2

rf

For any telescope, the most important property is the

Light Gathering Power (LGP)

49

20dLGP

do is the diameter of the objective in mm.

Compares the light gathering

power to that of the human eye

Telescopes are often referred to by their f-ratio

d

ffratiof / f is the focal length of the

objective and d is its diameter

Magnification is determined by the focal lengths

primary

eyepiece

fM

f

When using an eyepiece

When using a CCD camera, the image scale determines the magnification

206.265I

primary

Sf

m is the size of a

single pixel in micrometers

The ability of a telescope to resolve fine detail is given by

the Rayleigh Criterion

d

22.1sin

l is the wavelength of the light being used and d is the diameter of the aperture. qis the smallest resolvable angle of the telescope

AberrationsSpherical aberration can be corrected by using parabolic or hyperbolic surfaces

Coma is worse for parabolic surfaces than spherical ones. Correction is to use hyperbolic surfaces

Astigmatisms are the result of a non-axially symmetrical lens.

All three of these aberrations apply to both lenses and mirrors

The Refracting Telescope

Examples of Refractors

Problems With Refractors:Chromatic Aberration

Achromatic Doublet

Provides some color correction but doesn’t completely remove all chromatic aberration.

Apochromatic Design

The best apochromats use a three element design with low dispersion glass to reduce chromatic aberration to a minimum. Of course, the more glass the light goes through, the greater the loss.

Newtonian Reflector

Still a popular design among amateurs but not widely used by professionals. The focal plane is not large and there are lots of off-axis distortions.

Examples of Newtonian Reflectors

Invented by Isaac Newton in 1668

Cassegrain Reflector

Classic Cassegrain…Parabolic MirrorsRitchey-Chretien (RC)…Hyperbolic Mirrors

Invented by Laurent Cassegrain in 1672

Examples of Cassegrain’s

Gemini North

Keck 1Most professional telescopes use a Cassegrain focus with hyperbolic mirrors: an RC

Schmidt-Cassegrain Design

Primary mirror is spherical instead of parabolic. The “correcting lens” corrects for spherical aberrations.

Examples of Schmidt-Cassegrain telescopes

Maksutov-Cassegrain Design

The correcting lens is a meniscus shape. The Maksutov-Newtonian is also a popular design

Examples of Maksutov Telescopes

Mounts

Alt-Az Mounts

The Dobsonian Mount is an Alt-Az mount

Equatorial Mounts

German Equatorial Mount

Fork Equatorial Mounts

Other Types of Equatorial Mounts

Cross-Axis Equatorial English Yoke Equatorial

Example of a Yoke mount

Hooker Telescope

Eyepieces come in a variety of different optical designs

The magnification of a telescope is just the ratio of the focal length of the objective to the focal length of the eyepiece

eyepiece

objective

f

fmag

Since the light is passing through glass, eyepieces suffer from chromatic aberration

My personal favorites are Tele Vue eyepieces

Field of View depends on the eyepiece

objective

eyepieceeyepiece

eyepiece

objective

eyepieceeyepiece

f

fovf

ff

fov

mag

fovfov

82 ° fov

68 ° fov

60 ° fov