Transcript Day 5
Announcements •Lab tonight: planetarium •Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6 •First Quarter Observing Night Wednesday after fall break. Set-up starts around 6:45pm 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: i r If the surfaces of a piece of glass are curved, they will focus light to a point 1 1 n 1d 1 n 1 f R R nR R 2 1 2 1 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 r f 2 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) 2 0 d LGP 49 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 f ratio f / d f is the focal length of the objective and d is its diameter Magnification is determined by the focal lengths When using an eyepiece M f primary f eyepiece When using a CCD camera, the image scale determines the magnification 206.265 is the size of a single S I f primary pixel in micrometers The ability of a telescope to resolve fine detail is given by the Rayleigh Criterion sin q q 1.22 d 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 Aberrations 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 nonaxially 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 mag f objective f 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 fov foveyepiece mag foveyepiece f objective f eyepiece 60 ° fov 68 ° fov f eyepiece foveyepiece f objective 82 ° fov