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Telescopes and Optics

Telescopes and Optics. NHAS Astro 101. Agenda. Optics relating to Telescopes, Lenses and Mirrors Types of Telescopes and their advantages Focal Length and Focal Ratio Types of Eyepieces and their advantages Magnification and Apparent Field of View

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Telescopes and Optics

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  1. Telescopes and Optics NHAS Astro 101

  2. Agenda • Optics relating to Telescopes, Lenses and Mirrors • Types of Telescopes and their advantages • Focal Length and Focal Ratio • Types of Eyepieces and their advantages • Magnification and Apparent Field of View • Types of Astronomical Mounts and their uses • Types of Finders and their uses • Filters and their uses

  3. Lenses and Mirrors

  4. Properties of Light Law of Reflection - Angle of Incidence = Angle of reflection Law of Refraction - Light beam is bent towards the normal when passing into a medium of higher Index of Refraction. Light beam is bent away from the normal when passing into a medium of lower Index of Refraction. Index of Refraction - Inverse square law - Light intensity diminishes with square of distance from source.

  5. Law of Reflection  Normal  Angle of incidence () = angle of reflection () The normal is the ray path perpendicular to the mirror’s surface.

  6. Geometry of a Concave Mirror Focus Principal axis Vertex Focal length Center of curvature - the center of the circle of which the mirror represents a small arc Principal axis - a radius drawn to the mirror surface from the center of curvature of the mirror - normal to mirror surface Focus - the point where light rays parallel to principal axis converge; the focus is always found on the inner part of the "circle" of which the mirror is a small arc; the focus of a mirror is one-half the radius Vertex - the point where the mirror crosses the principal axis Focal length - the distance from the focus to the vertex of the mirror

  7. Index of Refraction As light passes from one medium (e.g., air) to another (e.g., glass, water, plexiglass, etc…), the speed of light changes. This causes to light to be “bent” or refracted. The amount of refraction is called the index of refraction.

  8. Refraction Imagine that the axles of a car represent wave fronts. If the car crosses from a smooth to a rough surface at an angle, one tire of the axle will slow down first while the other continues at normal speed. With one tire traveling faster the other, the car will turn in the direction of the slow tire. This is how refraction works.

  9. AIR NORMAL GLASS / WATER Slower Propagating Speed

  10. AIR Car GLASS / WATER Slower Propagating Speed ( Sand / Gravel )

  11. AIR Car GLASS / WATER Slower Propagating Speed ( Sand / Gravel )

  12. NORMAL LIGHT BENDING TOWARDS THE NORMAL AIR LIGHT RAY GLASS / WATER Slower Propagating Speed

  13. n2 NORMAL LIGHT BENDING TOWARDS THE NORMAL AIR n1 Snell's Law ( Next Slide ) GLASS / WATER Slower Propagating Speed

  14. GLASS / WATER ( Sand / Gravel ) Slower Propagating Speed Car AIR

  15. GLASS / WATER ( Sand / Gravel ) Slower Propagating Speed Car AIR

  16. GLASS / WATER ( Sand / Gravel ) Slower Propagating Speed Car AIR

  17. Slower Propagating Speed GLASS / WATER NORMAL AGAIN, LIGHT BENDS TOWARDS THE NORMAL upon entering a region with slower speed. LIGHT RAY AIR

  18. AIR Car ( Sand / Gravel ) GLASS /WATER Slower Propagating Speed

  19. AIR Car ( Sand / Gravel ) GLASS /WATER Slower Propagating Speed

  20. AIR Car ( Sand / Gravel ) GLASS /WATER Slower Propagating Speed

  21. NOW LIGHT BENDS AWAY FROM THE NORMAL Snell's Law AIR NORMAL LIGHT RAY GLASS /WATER Slower Propagating Speed

  22. Geometry of a Converging (Convex) Lens Focus Optical axis Focal length Optical axis - axis normal to both sides of lens - light is not refracted along the optical axis Focus - the point where light rays parallel to optical axis converge; the focus is always found on the opposite side of the lens from the object Focal length - the distance from the focus to the centerline of the lens

  23. Lens and Mirror Aberrations SPHERICAL (lens and mirror) Light passing through different parts of a lens or reflected from different parts of a mirror comes to focus at different distances from the lens. Result: fuzzy image CHROMATIC (lens only) Objective lens acts like a prism. Light of different wavelengths (colors) comes to focus at different distances from the lens. Result: fuzzy image

  24. Spherical Aberration in Lenses Simple lenses suffer form the fact that light rays entering different parts of the lens have slightly difference focal lengths. This defect is corrected with the addition of a second lens. The problem One focal point for all light rays The solution

  25. Spherical Aberration in Mirrors The Problem Simple concave mirrors suffer from the fact that light rays reflected from different locations on the mirror have slightly different locations on the mirror have slightly different focal lengths. This defect is corrected by making sure the concave surface of the mirror is parabolic The Solution All light rays converge at a single point

  26. Chromatic Aberration in Lenses Focal point for blue light Simple lenses suffer from the fact that different colors of light have slightly different focal lengths. This defect is corrected by adding a second lens Focal point for red light The problem Focal point for all light The solution

  27. Coma • Affects Fast Mirrors with deeply curved reflecting surface • Causes elongation in one axis if the object is not near the center of the FOV • Faster the Mirror the more of an issue. • Its Not a mistake in workmanship • This off-axis distortion is called coma, named after the term for a comet’s head

  28. Types of Optical Telescopes

  29. Refractor Uses a lens to gather the light to a point Most rugged design - easy to care for Gives the sharpest views - especially of planets and the moon Most expensive for any given aperture Usually the tube is quite long, although short tube designs are now available Inexpensive models suffer from chromatic aberration – achromatic vs. apochromatic Basic Telescope Designs

  30. Reflector Uses a mirror to gather the light to a point Open tube collects dust, mirror eventually tarnishes Requires periodic alignment (collimating) of the mirrors Least expensive for any given aperture Available in both long and short tube design Generally no chromatic aberration Most “bang for the buck” Basic Telescope Designs

  31. CompoundSchmidt-Cassegrain, Maksutov Uses mirror and lens to gather the light to a point Sharp views, Maksutov are almost as good as refractors Closed tube protects optics Moderate cost for any given aperture Tube is shortest for any given aperture Most portable for any given aperture Basic Telescope Designs

  32. Refracting Telescope Obj Diam Uses lens to focus light from distant object - the eyepiece contains a small lens that brings the collected light to a focus and magnifies it for an observer looking through it. FL= Focal Length Focal Ratio= FL/Obj Diam

  33. Types of Reflecting Telescopes Each design incorporates a small mirror just in front of the prime focus to reflect the light to a convenient location for viewing.

  34. Focal Length and Focal Ratio • Focal Ratio= Focal Length/Objective Diam • Faster = Shorter= is smaller ratio • Shorter Focal Ratio Optics (F6 and below) • Wider Fields of View • More Compact • More Expensive or More Distorted • Optics must be close to perfect • Fast Optics are difficult to make

  35. Telescope Specs • 100mm F7 Refractor • 100mm F10 Refractor • 200mm F10 Schmidt Cass • 400mm F4.5 Newtonian • 16 inch F4.5 Newtonian

  36. The Powers of a Telescope Light Gathering Power: Astronomers prefer *large* telescopes. A large telescope can intercept and focus more starlight than does a small telescope. A larger telescope will produce brighter images and will be able to detect fainter objects. Resolving Power: A large telescope also increases the sharpness of the image and the extent to which fine details can be distinguished. Magnification: The magnifying power is the ability of the telescope to make the image appear large in the field of view.

  37. Three Fundamental Properties of a Telescope Light-Collecting Area think of the telescope as a “photon bucket” The amount of light that can be collected is dependent on the mirror area A =  (D/2)2 Resolution smallest angle which can be seen  = 1.22  / D The angular resolution of a reflecting telescope is dependent on the diameter of the primary (D) and the wavelength of the light being viewed () These properties are much more important than magnification which is produced by placing another lens - the eyepiece - at the mirror focus.

  38. Light Gathering Ability: Size Does Matter 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D A = (D/2)2

  39. Angular Resolution • The ability to separate two objects. • The angle between two objects decreases as your distance to them increases. • The smallest angle at which you can distinguish two objects is your angular resolution.

  40. Eyepieces • Used to magnify the image at the focal plane for viewing by the naked eye • Your image will only be as good as the weakest chain in your optical system • Many Different designs • All specified with an Eyepiece FL and an AFOV

  41. Types of Eyepieces • Old designs (limited use) • Huyghenian, Ramsden, Kellner, Erfle • Low Cost, with distortion • Gold Standards, (52deg AFOV) • Plossl, Orthoscopics • Med Cost, without distortion • Widefields, ( up to 82deg AFOV) • Naglers, Panoptics, Radians, Swans • High Cost: Distortion Free AFOV correlates to Cost • More money vs more distortion

  42. MagnifyingPower Magnifying Power = ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fs) and the eyepiece (Fe): M = Fs/Fe A larger magnification does not improve the resolving power of the telescope! Rule of Thumb- Maximum useful Mag is 50x per inch of Objective diameter under ideal seeing - 20x to 30x per inch of Objective is more common in NE

  43. Field of View: FOV • Each eyepiece design has a specified Apparent Field of View, AFOV • AFOV/ Magnification = effective FOV • Expressed in angular degrees Ex… A 25mm Plossl with 52deg FOV is being used on a refractor with a 1000mm FL. What is the magnification and FOV: 1000mm FL/25mm Ocular= 40x mag 52deg AFOV / 40 Mag = 1.3 deg effective FOV

  44. Examples • 100mm F7 Refractor, w 32mm Plossl (52deg AFOV) • Mag • FOV • 200mm F10 Schmidt Cass, w 32mm Nagler (82deg AFOV) • Mag • FOV • 400mm F4.5 Newtonian, w 32mm Widefield (66deg AFOV) • Mag • FOV

  45. Eye relief • The distance from the last surface of the eyepiece eye lens (the lens closest to your eye) to where the image is formed. • Eye relief should be fairly long for comfortable viewing, • if you must wear eyeglasses, you will need a minimum of 15mm of eye relief to see the entire field of view • Eye relief usually decreases as eyepiece focal lengths get shorter • More $$ for more eye relief

  46. Barlow Lens • x2 or x3 increase in your mag or a /2 or /3 decrease in your eyepiece FL. • Using a x2 barlow you can make a 32mm eyepiece also serve as a 16mm eyepiece. • (But you keep the 32mm eye relief) • Slight decrease in image brightness due to extra elements.

  47. Altitude-Azimuth (Alt-Az) Simple, easy to use Inexpensive Most portable Equatorial Easy to keep objects in the field of view More difficult to setup Usually heavy Usually driven Dobsonian (Dob) Very easy to use Least expensive ?? Very stable Most important: Stability! Telescope Mounts Many mountsare motorized,some arecomputerized!

  48. Finders • Why? most telescopes have a 1 to 2 deg FOV at their lowest magnification • Types • Reflex Sight: • Zero Power • dovetail, red dot, telrad (concentric circles) • Magnifying 30mm, 50mm and 70mm • Correct view • Telescope view

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