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# Introduction to telescope design

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### Introduction to telescope design

1. 1. Introduction to Telescope Design<br />
2. 2.
3. 3. Optics<br />Geometric optics<br />Ray tracing through optical systems<br />Used to calculate important telescope parameters such as length, size of mirrors, and location of eye<br />Physical optics<br />Accounts for the wave nature of light<br />Sources of image disturbance<br />Polarization<br />Interference<br />Diffraction<br />
4. 4. Physical Opticks & Diffraction<br />Images are blurred by diffraction<br />Sets an absolute limit on resolving power<br />Most (84%) of light falls in a small circular region known as an Airy disk<br />Resolution is the smallest angular separation of two point sources (like stars) which allows both to be distinct<br />Center of the Airy disk of one point source just touches the outer edge of the other Airy disk (Rayleigh’s criterion)<br />Limit of resolution:<br />Depends on focal length, wavelength of light, and size of the primary mirror/lens<br />
5. 5. Geometric Optics - Lens<br />Light moves more slowly in lenses (glass, etc.) than air and this causes it to bend<br />Amount of bend is the index of refraction<br />Snell’s law relates indices of refraction and light ray angles<br />Image from http://commons.wikimedia.org/wiki/File:Refraction.PNG<br />
6. 6. Geometric Optics - Mirrors<br />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<br />Mirror shapes can be flat, spherical, or aspherical (hyperbolic, etc.)<br />
7. 7. Geometric Optics – Stops and Pupils<br />Aperture stop determines the amount of light reaching the image (end of the optical system)<br />Can be a diaphragm or the edge of a lens or mirror<br />Determines the total amount of irradiance available<br />In telescopes, this is usually determined by the size of the primary mirror or lens<br />Field stop is an element limiting the angular size of an object being imaged<br />In astronomy this is usually determined by the size of film or CCD when creating astronomical images<br />
8. 8. Geometric Optics – Stops and Pupils<br />Entrance pupil is the image of the aperture stop as seen from the axial point on the object<br />In telescopes this is generally the unobstructed view of the primary mirror or lens<br />In catadioptric telescopes this may be changed slightly by corrective lenses before the primary mirror<br />Exit pupil is the image of the aperture stop as seen from an axial point on the image plane<br />Different eyepieces effect exit pupil size and can cause a loss in available irradiance<br />
9. 9. f/#<br />Focal length is the distance from a mirror or lens where parallel rays meet at a single point<br />f/# (f-number, f-ratio, or relative aperture) is the focal length divided by the diameter of the entrance pupil<br />The entrance pupil for most telescopes is the primary mirror/lens (the objective)<br />Unlike photography, f/# doesn’t effect the irradience at the eye since objects are essentially at infinite distance (only size of the objective matters)<br />Focal length in telescopes determines the field of view and the scale of objects at the eye <br />
10. 10. Optical Ray Tracing<br />Start with parallel rays (point source at infinite distance) and trace the location and direction of rays at key points (edge of aperture, etc.)<br />Trace rays through each element<br />Snell’s law for lenses<br />Use equation for mirror shape (parabola, hyperbola, ellipse, etc.) to determine surface normals<br />Starting point of design is usually to place elements at the focal point of the previous element and adjust to account for aberrations<br />
11. 11. Chromatic Aberration<br />Effects lenses<br />Caused by wavelength dependence of index of refraction<br />Causes different colors to focus at difference points<br />Color blurring<br />Corrected by using mirrors<br />
12. 12. Spherical Aberration<br />Spherical lenses have a different focus on the edges and center of the mirror<br />Causes blurring<br />Can be fixed by using convex and concave mirrors to zero out the spherical aberration<br />Can also be fixed by using aspheric lenses<br />Main cause of early HST problems<br />
13. 13. Comatic Aberration<br />Off axis point sources (located near the edge of the field of view) focus in a different location and on axis point sources<br />Caused by parabolic mirrors<br />Causes a wedge shape<br />Can be corrected with aspheric lenses<br />Image from http://en.wikipedia.org/wiki/File:Lens-coma.svg<br />
14. 14. Gregorian<br />Concave parabolic primary mirror and a concave elliptical secondary mirror<br />Primary focus is before the secondary<br />Eye point is behind the primary<br />Allows the observer to view behind the telescope<br />Has an upright image<br />Useful for solar observation since a field stop can be placed at the primary focus<br />Image from http://en.wikipedia.org/wiki/File:Gregory-Teleskop.svg<br />
15. 15. Newtonian<br />Concave parabolic primary mirror and a flat, angled secondary mirror<br />Eye point is near the top of the telescope and on the side<br />Large telescopes require the observer to sit on a platform<br />Equitorial mounts can make viewing diificult<br />Combined with short f/# can create a very compact telescope<br />Popular with amateur astronomers<br />Simple design<br />Inexpensive for a given aperture<br />Single parabolic mirror is easy to grind by hand<br />Easy to create a short f/# so a wide field of view can be obtained<br />Good for deep sky observation (galaxies, nebulae, etc.)<br />Suffers from coma (serious with f/6 or lower)<br />Secondary mirror causes a central obstruction<br />Requires frequent collimation<br />Image fromhttp://en.wikipedia.org/wiki/File:Newton-Teleskop.svg<br />
16. 16. Cassegrain<br />Concave parabolic primary mirror and a convex hyperbolic secondary mirror.<br />Primary focus is aligned with the secondary’s focus<br />Eye point is behind the primary<br />Long focal length can be achieved with a short tube<br />Suffers from coma and spherical aberrations<br />
17. 17. Schmidt-Cassegrain<br />Catadoptric telescope<br />Cassegrain with a Schmidt corrector plate<br />Aspheric lens which corrects spherical aberration<br />Can also be found in Schmidt-Newtonian<br />Corrector also seals the tube keeping out dust<br />Image from http://en.wikipedia.org/wiki/File:Schema_lame_de_Schmidt.svg<br />
18. 18. Maksutov-Cassegrain<br />Catadoptric telescope<br />A weakly negative meniscus lens corrects coma and spherical aberration<br />Corrector also seals the tube keeping out dust<br />Easier to grind than a Schmidt corrector<br />Secondary is integrated into the corrector (partially aluminized) which lowers manufacture cost<br />Not usually seen in > 7” telescopes as the corrector becomes large (heavy and requires long cool down times)<br />
19. 19. Yolo<br />Off axis telescope<br />Primary and secondary mirrors are concave and have the same curvature<br />Secondary doesn’t cast a shadow<br />Eliminates coma<br />Significant astigmatism<br />Partially corrected by torroidal secondary mirror (different focal distance depending on mirror angle)<br />Creates high contrast images with no obstruction<br />Image from http://en.wikipedia.org/wiki/File:Off-axis_optical_telescope_diagram.svg<br />
20. 20. Dobsonian<br />Alt-az mount often used with Newtonian telescopes<br />Very easy and inexpensive to build<br />Very easy to point by hand, especially for large (> 12”) portable telescopes<br />“Light bucket” telescope with a large objective and low magnification<br />Very good for visual observation of large deep sky objects<br />Large/heavy objective can be easily moved by hand<br />Easy to transport to remote, dark locations<br />Not easy to automatically track<br />Not a good design for camera/CCD use<br />Can be computer assisted with adjustable alt-az marker wheels and/or computer position sensors<br />Can be placed on an equatorial platform for limited clock driven tracking<br />