Announcements Lab tonight: planetarium Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6 If you haven’t finished the Telescopic Observations of the Moon lab you have one more week.

Telescopes Newton’s original telescope with mirror 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:

If the surfaces of a piece of glass are curved, they will focus light to a point 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. Assumes lens is used in air 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

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

For any telescope, the most important property is the Light Gathering Power (LGP) 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 f is the focal length of the objective and d is its diameter

Magnification is determined by the focal lengths When using an eyepiece When using a CCD camera, the image scale determines the magnification 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 l is the wavelength of the light being used and d is the diameter of the aperture. q is the smallest resolvable angle of the telescope. The angle will be in radians

All three of these aberrations apply to both lenses and mirrors Spherical 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 Invented by Laurent Cassegrain in 1672 Classic Cassegrain…Parabolic Mirrors Ritchey-Chretien (RC)…Hyperbolic Mirrors

Examples of Cassegrain’s Keck 1 Most professional telescopes use a Cassegrain focus with hyperbolic mirrors: an RC Gemini North

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 Mount Wilson, CA

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 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 60 ° fov 68 ° fov 82 ° fov