Lecture 6: Telescopes

1. keywords: clock drive, discovery of the telescope, finderscope optical telescope, primary lens, primary mirror, reflector telescope, refractor telescope, Schmidt-Cassegrain telescope, telescope, etc..
2. human eye: Light is admitted by the pupil, focused by lens (crystalline lens), and the retina processes it into an electrical signal convey by the optic nerve to the brain.
3. refraction:
   1. Optics file: refraction_water.html.
   2. Optics file: prism_animation.html.
5. Ideally, a point source of light at optical infinity sends parallel light rays that are focused by a converging lens to a point on a detector: e.g., human eye retina. An array of point sources of light of light maps to an array of points on the detector giving a optical image of the point sources of light (i.e., a real image since the light rays from the point sources of light actually converge to points). If the light rays do NOT focus to a point, you have an unfocused (i.e., fuzzy) optical image.
6. There is a point inversion from source object to optical image, but psychophysical response in the case of the human eye corrects for this. Saying upside down is NOT a complete description of the optical image.
   1. Optics file: optics_point_inversion.html
   2. A point inversion is 180° rotation about the central light ray.
   3. Optics file: optics_reflection_plane.html.
   4. plane reflection is ordinary mirror reflection. Which is really front-to-back inversion.
   5. Note the psychophysical response in the brain to a thing out there in the world is a representation of the thing, NOT the thing itself. The vision psychophysical response is pretty good at giving us accurate information, but it can be fooled by optical illusions. For example, the spinning dancer illusion: see File:Spinning Dancer.gif. Which way is she rotating? There are actually NO clues, but your psychophysical response locks you into one way or the other. I find I can change the rotation direction sometimes by looking at her feet.
7. charge-coupled devices (CCDs): The modern imaging device for photography. Digital and it allows exact calibration of measured radiant flux. The first CCD was in 1970, but they only came into widespread use in astronomy in the early 1980s when they were a great wonder (Wikipedia: Charge-coupled device: History). Astronomy generally used photographic plate (light sensitive chemicals on glass) rather than photographic film because they do NOT shrink or distort much in processing and are resistant to environmental changes. Vast archives of photographic plate used to exist and some still do, but they are being digitized now. They are useful for studying the past history of variable stars primarily. However, just how long-lasting are digital records. They CANNOTWith digital images you can do all kinds of elaborate digital image processing. The less said, the better.
8. optical telescope characteristics:
   The intrinsic ones that depend on the primary lens, (for reflector telescopes) and primary mirror (for reflector telescopes) are
   1. light-gathering power: which is scales as the area aperture which is usually approximately circular: i.e., A= π*r**2 = π*(d/2)**2 where r is radius and d is diameter.
      Nowadays, all the telescopes used in astronomy are reflector telescopes. The largest primary diameter for segmented mirror reflector telescopes is ∼10 meters, but much larger ones are being planned or built. The Overwhelmingly Large Telescope (OWL, diameter d = 100 m) is planned by European Southern Observatory (ESO). By the by, when you build a bigger telescope, it CANNOT just be bigger. It has to be superior in every way (instrumentation, etc.) or it's NOT worthwhile building it. Note the light-gathering power of OWL is 10**2 = 100 times more than the 10-meter telescopes.
   2. angular resolution: It is limited by diffraction (AKA interference) and seeing (turbulent motions in the Earth's atmosphere).
   3. diffraction limit: See Optics file: optics_rayleigh_criterion.html and Rayleigh criterion. In space astronomy, where seeing is perfect, the Rayleigh criterion sets the limit without special tricks:
      1. Hubble Space Telescope (HST, 1990--2040?, d = 2.4 m, Cassegrain reflector): θ_resolution = ∼ 0.1'' * λ_μm. Note 1 arcseconds ('') = 1/60' = 1/3600°, 1 &mu = 1000 nm, and the specification for the visible band is visible band (fiducial range 0.4--0.7 μm).
      2. James Webb Space Telescope (JWST, 2021--2041?, d = 6.5 m) θ_resolution = ∼ 0.04'' * λ_μm.
   4. seeing:
      1. naked eye astronomy: Best that can be done is probably ∼ 1' = (1/60)°. This what Tycho Brahe (1546--1601) was able to do pre-telescopically. In a superior high altitude site, you can probably do better.
      2. telescopic observering without adaptive optices: The best is ∼ 0.4'' = (0.4/3600)°.
      3. adaptive optics: The best right now is ∼ 0.03'' = (0.03/3600)°. (Wikipedia: Adaptive optics: Wavefront sensing and correction).
      4. space astronomy: The seeing resolution limit is formally zero and you are at the Rayleigh criterion without special tricks. But note that adaptive optics is often limited by faintness. So the HST and JWST have the best angular resolution for faint astronomical objects and the JWST was built precisely for faint astronomical objects: exoplanets and cosmologically remote astronomical objects.
   The main optical telescope characteristics that depend on accessories (i.e., things that can changed as needed) are:
   1. magnification.
   2. field of view (FOV) usually measured in arcminutes ('', (1/60)°) for small telescopes.
9. In geometrical optics you treat light just as consisting of light rays that travel in straight line. But light is a wave phenomenon, and so exhibits interference and diffraction which we do NOT ordinary notice for darn good reasons:
   1. You need to break wavefront with a very small obstacle or aperture to see interference and diffraction.
   2. The effects tend to be washed out by reflections.
   3. The effects are wavelength dependent and so tend to be washed out polychromatic light.
   4. Optics file: diffraction_wavelength_aperture_ratio.html.
   5. But you can see the effects with simple setups: Electromagnetic Radiation file: diffraction_pattern_square.html.
   6. Overexposed stars in astro images often have points. The points are part of diffraction pattern of the stars which are actually almost always effectively point sources of light. 4 points means the camera or secondary mirror is being held by 4 arms and imposes a 4-fold symmetry on the diffraction pattern. For example, see the bright stars in the Pleiades star cluster: see Star file: pleiades.html. The secondary mirror of the Hubble Space Telescope (HST, 1990--2040?, d = 2.4 m, Cassegrain reflector) is evidently held up by 4 arms.
   Actually, interference and diffraction with light were NOT noticed by anyone through all of human history until the 17th century and only generally accepted to exist in the 19th century.
10. telescope: reflector telescope, refractor telescope: All professional and the best amateur telescope nowadays are reflector telescopes. They can just be built bigger more easily. The largest reflector telescopes have a primary lens (see Wikipedia: Yerkes Observatory).
11. There are many special designs of reflector telescopes. The telescopes for our undergrad astronomy lab at UNLV are Schmidt-Cassegrain telescope with 8-in primary mirrors.
12. Actually, Isaac Newton (1643--1727) himself invented in 1668 the Newtonian telescope which was the first working reflector telescope (see Wikipedia: Newton's reflector). The idea of reflector telescopes had been proposed earlier.