Optics and Telescopes

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2 Optics and Telescopes

3 Properties of Light Law of Reflection - reflection Angle of Incidence = Angle of 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 - n Speed of light in vacuum Speed of light in a medium Inverse square law - Light intensity diminishes with square of distance from source.

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

5 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

6 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.

7 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.

8 AIR NORMAL GLASS / WATER Slower Propagating Speed

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

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

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

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

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

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

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

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

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

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

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

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23 Geometry of a Converging (Convex) Lens Optical axis Focus 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

24 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

25 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 The solution One focal point for all light rays

26 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

27 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 The problem Focal point for red light Focal point for all light The solution

28 Types of Optical Telescopes

29 Refracting / Reflecting Telescopes Focal length Focal length Refracting telescope: Lens focuses light onto the focal plane Reflecting telescope: Concave mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes.

30 The Focal Length Focal length = distance from the center of the lens to the plane onto which parallel light is focused.

31 Basic Telescope Designs 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

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33 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

34 Basic Telescope Designs 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

35 Basic Telescope Designs Compound Schmidt-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

36 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.

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

38 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.

39 The Powers of a Telescope: 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

40 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.

41 The Powers of a Telescope (II) 2. Resolving power: Wave nature of light => the telescope aperture produces fringe rings that set a limit to the resolution of the telescope Resolving power = minimum angular distance min between two objects that can be separated min = 1.22 ( /D) For optical wavelengths, this gives min = 11.6 arcsec / D[cm] min

42 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

43 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

44 Magnifying Power 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 (F s ) and the eyepiece (F e ): M = F s /F e 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

45 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

46 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

47 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

48 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.

49 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 mounts are motorized, some are computerized!

50 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

51 Finder Protocol Use a star map to define area to observe Use the finder to point the scope to the general area Use your eyepiece with the widest effective field to locate your target. Happy Observing

52 Filters

53 Filter Basics Filters are designed to block light. This inherently darkens the image, so the scope must be able to pull in enough light to still allow you to see the object you are interested in. Due to this fact, small telescope often do not benefit from filters. The Moon looks better through a filter in any size telescope. The Sun can be viewed directly with the proper filter. Most filters are threaded for attaching to the bottom of eyepieces, the front of diagonals or to the visual back of an SCT telescope.

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55 Solar Filters Conventional solar filters come in two varieties (glass and Mylar film) and allow us to see sunspots on the surface of the sun. Most Mylar filters show the sun as a blue disk. Glass filters generally show the sun in yellow. Baader Solar Film (Mylar) show the sun as a white disk and has the best contrast. H-Alpha filters are expensive, but allow us to view the flares and other features in the Sun s chromosphere. These conventional solar filters mount on the front of the scope. Never use a solar filter that mounts on the eyepiece!

56 Moon Filters The Moon is very bright, especially at lower magnifications. This makes it difficult to see fine detail. A standard lunar filter may block 80% or more of all visible light. A polarizing filter uses two polarized elements that can be rotated to vary the amount of light blocked.

57 Color Filters Color filters are mostly used for the planets. By blocking certain wavelengths (colors) of light, they help to bring out faint details. To learn what colors work well for which planets, visit the Learning Center at Other than Jupiter and Venus (two very bright objects) color filters will not provide much benefit for scopes smaller than 4.5.

58 Deep Sky Filters Designed to pass only certain wavelengths of light in order to show faint objects while blocking manmade light and skyglow. Broadband filters allow most light to pass, but block wavelengths commonly produced by exterior lighting. They improve most faint objects. Narrowband filters block much more light, but pass the light emitted by many faint nebulae. Oxygen III (O-III) filters block all but the one specific wavelength common to just a few nebulae (the Veil nebula for example). Hydrogen Beta (H-Beta) filters block all but the one specific wavelength common to just a few nebulae (the Horsehead and California nebula for example). These filters will not provide much benefit for scopes smaller than 6.