3. Optics Geometric optics Ray tracing through optical systems Used to calculate important telescope parameters such as length, size of mirrors, and location of eye Physical optics Accounts for the wave nature of light Sources of image disturbance Polarization Interference Diffraction
4. Physical Opticks & Diffraction Images are blurred by diffraction Sets an absolute limit on resolving power Most (84%) of light falls in a small circular region known as an Airy disk Resolution is the smallest angular separation of two point sources (like stars) which allows both to be distinct Center of the Airy disk of one point source just touches the outer edge of the other Airy disk (Rayleigh’s criterion) Limit of resolution: Depends on focal length, wavelength of light, and size of the primary mirror/lens
5. Geometric Optics - Lens Light moves more slowly in lenses (glass, etc.) than air and this causes it to bend Amount of bend is the index of refraction Snell’s law relates indices of refraction and light ray angles Image from http://commons.wikimedia.org/wiki/File:Refraction.PNG
6. Geometric Optics - Mirrors A light ray strikes a mirror at an angle relative to the surface normal and reflects at the same angle relative to the other side of the normal Mirror shapes can be flat, spherical, or aspherical (hyperbolic, etc.)
7. Geometric Optics – Stops and Pupils Aperture stop determines the amount of light reaching the image (end of the optical system) Can be a diaphragm or the edge of a lens or mirror Determines the total amount of irradiance available In telescopes, this is usually determined by the size of the primary mirror or lens Field stop is an element limiting the angular size of an object being imaged In astronomy this is usually determined by the size of film or CCD when creating astronomical images
8. Geometric Optics – Stops and Pupils Entrance pupil is the image of the aperture stop as seen from the axial point on the object In telescopes this is generally the unobstructed view of the primary mirror or lens In catadioptric telescopes this may be changed slightly by corrective lenses before the primary mirror Exit pupil is the image of the aperture stop as seen from an axial point on the image plane Different eyepieces effect exit pupil size and can cause a loss in available irradiance
9. f/# Focal length is the distance from a mirror or lens where parallel rays meet at a single point f/# (f-number, f-ratio, or relative aperture) is the focal length divided by the diameter of the entrance pupil The entrance pupil for most telescopes is the primary mirror/lens (the objective) Unlike photography, f/# doesn’t effect the irradience at the eye since objects are essentially at infinite distance (only size of the objective matters) Focal length in telescopes determines the field of view and the scale of objects at the eye
10. Optical Ray Tracing Start with parallel rays (point source at infinite distance) and trace the location and direction of rays at key points (edge of aperture, etc.) Trace rays through each element Snell’s law for lenses Use equation for mirror shape (parabola, hyperbola, ellipse, etc.) to determine surface normals Starting point of design is usually to place elements at the focal point of the previous element and adjust to account for aberrations
11. Chromatic Aberration Effects lenses Caused by wavelength dependence of index of refraction Causes different colors to focus at difference points Color blurring Corrected by using mirrors
12. Spherical Aberration Spherical lenses have a different focus on the edges and center of the mirror Causes blurring Can be fixed by using convex and concave mirrors to zero out the spherical aberration Can also be fixed by using aspheric lenses Main cause of early HST problems
13. Comatic Aberration Off axis point sources (located near the edge of the field of view) focus in a different location and on axis point sources Caused by parabolic mirrors Causes a wedge shape Can be corrected with aspheric lenses Image from http://en.wikipedia.org/wiki/File:Lens-coma.svg
14. Gregorian Concave parabolic primary mirror and a concave elliptical secondary mirror Primary focus is before the secondary Eye point is behind the primary Allows the observer to view behind the telescope Has an upright image Useful for solar observation since a field stop can be placed at the primary focus Image from http://en.wikipedia.org/wiki/File:Gregory-Teleskop.svg
15. Newtonian Concave parabolic primary mirror and a flat, angled secondary mirror Eye point is near the top of the telescope and on the side Large telescopes require the observer to sit on a platform Equitorial mounts can make viewing diificult Combined with short f/# can create a very compact telescope Popular with amateur astronomers Simple design Inexpensive for a given aperture Single parabolic mirror is easy to grind by hand Easy to create a short f/# so a wide field of view can be obtained Good for deep sky observation (galaxies, nebulae, etc.) Suffers from coma (serious with f/6 or lower) Secondary mirror causes a central obstruction Requires frequent collimation Image fromhttp://en.wikipedia.org/wiki/File:Newton-Teleskop.svg
16. Cassegrain Concave parabolic primary mirror and a convex hyperbolic secondary mirror. Primary focus is aligned with the secondary’s focus Eye point is behind the primary Long focal length can be achieved with a short tube Suffers from coma and spherical aberrations
17. Schmidt-Cassegrain Catadoptric telescope Cassegrain with a Schmidt corrector plate Aspheric lens which corrects spherical aberration Can also be found in Schmidt-Newtonian Corrector also seals the tube keeping out dust Image from http://en.wikipedia.org/wiki/File:Schema_lame_de_Schmidt.svg
18. Maksutov-Cassegrain Catadoptric telescope A weakly negative meniscus lens corrects coma and spherical aberration Corrector also seals the tube keeping out dust Easier to grind than a Schmidt corrector Secondary is integrated into the corrector (partially aluminized) which lowers manufacture cost Not usually seen in > 7” telescopes as the corrector becomes large (heavy and requires long cool down times)
19. Yolo Off axis telescope Primary and secondary mirrors are concave and have the same curvature Secondary doesn’t cast a shadow Eliminates coma Significant astigmatism Partially corrected by torroidal secondary mirror (different focal distance depending on mirror angle) Creates high contrast images with no obstruction Image from http://en.wikipedia.org/wiki/File:Off-axis_optical_telescope_diagram.svg
20. Dobsonian Alt-az mount often used with Newtonian telescopes Very easy and inexpensive to build Very easy to point by hand, especially for large (> 12”) portable telescopes “Light bucket” telescope with a large objective and low magnification Very good for visual observation of large deep sky objects Large/heavy objective can be easily moved by hand Easy to transport to remote, dark locations Not easy to automatically track Not a good design for camera/CCD use Can be computer assisted with adjustable alt-az marker wheels and/or computer position sensors Can be placed on an equatorial platform for limited clock driven tracking
