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ASTR 3130, Majewski [SPRING 2015]. Lecture Notes

ASTR 3130 (Majewski) Lecture Notes


TELESCOPES: INTRODUCTION

Reference: Chapter 6 of Birney, Gonzalez & Oesper.

TELESCOPE MOUNTINGS

Before 1980, nearly all telescopes were mounted with an equatorial mount:

  • Counteract Earth rotation by motion only on one, polar axis.

  • The polar axis of the telescope is always mounted parallel to the Earth's polar axis.

    From Bely, The Design and Construction of Large Telescopes.

  • The equatorial mounting therefore enables a simple, one axis correction of Earth motion with a single rotational speed.

  • In an equatorial mounting, there is no rotation of the image in the focal plane with respect to the telescope.

  • The second, perpendicular axis of motion in the equatorial mounting is the declination axis.

There are a variety of implementations of the polar/declination axis combination:

The McCormick 26-inch, shown here, is built with a German equatorial mounting. So is the 6-inch doghouse telescope. Other types of equatorial mountings are shown below, showing different ways of configuring the polar and declination axes (in the figure below, a "German" equatorial mounting is shown in the "off-axis" figure if one looks only at the solid lines).

From Bely, The Design and Construction of Large Telescopes.

Yet another variation of a yoke-mounted equatorial telescope.
Nowadays most telescopes are built with altitude-azimuth (alt-az) mounts, the same as telescopes used for terrestrial telescopes.

From http://galwayastronomyclub.ie/buying%20a%20telescope-ed3a.htm.
Compare the equatorial and alt-az mount:

From Bely, The Design and Construction of Large Telescopes.

The advantages of alt-az mounts are that:

  • Neither axis changes direction with respect to gravity -- so offer stable platforms for instruments.

  • They are structurally sturdier than equatorial mounts.

  • They are less massive, less expensive to build.

  • They are more compact, requiring smaller enclosures.

Examples of alt-az mounted telescopes, both amateur and professional (WIYN 3.5-m). From http:http://www.comparestoreprices.co.uk/telescopes.asp and http://www.indiana.edu/~rcapub/images/WIYN-hydra.jpg.

BUT, they only became popular for professional telescopes recently because:

  • Three axes of rotation needed: altitude, azimuth, and field rotation (the image in the focal plane rotates about the optical axis with respect to the telescope).

  • All three axes move with variable speed.

  • Could only do this with fast computers, became possible only in 1980s.


THE OPTICS OF A TELESCOPE

In its simplest form, a telescope is a single objective lens or mirror to bring parallel, distant light to focus.

  • REFRACTOR

    • Objective = Lens

    • Hard to make large lens (40" Yerkes is largest; McCormick 26.25") -- hold around edge, increasing diameter requires increasing thickness to support weight, less transparent.

    • If film or other light detector is placed in focal plane, you have a camera

    • Refractors suffer from chromatic aberration, which we discussed in reference to the eye.

      • Cause is the variable index of refraction as a function of wavelength.

      • For a typical lens, more power for bluer light, which focuses before redder wavelengths.

      From http://commons.wikimedia.org/wiki/Image:Chromatic_aberration.svg.

    • A 2-element objective lens (made of a concave flint glass element and a convex crown glass element that "counteract" each other) -- called an achromatic doublet -- is commonly used to reduce (but not eliminate) the chromatic aberration.

      An achromatic doublet, like that in the McCormick 26-inch refractor. From http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html.

      We will return to this point in a few lectures when we talk about "optical aberrations".


    • REFLECTOR

      • Objective is curved mirror.

      • Since light does not pass the glass, no chromatic aberration.

      • Paraboloidal shape is better; spherical surface --> spherical aberration.

        Example of spherical aberration, but in this case, from a lens. From http://commons.wikimedia.org/wiki/Image:Chromatic_aberration.svg.

      • Can make mirrors much larger than lenses (lens supported at edge, mirror across entire back).

      • Different ways to get access to reflector focal plane:
        Note that Nasmyth and Coude foci are similar, but Nasmyth puts beam of light through the altitude bearing of an Altitude-Azimuth-mounted telescope, whereas a Coude puts the beam of light thought the polar axis of an equatorially-mounted telescope.

        Note purpose of having different optical configurations:

        • Multiple reflections shorten telescope length for long focal lengths.

          • Keeps telescope and telescope enclosure smaller, which....

          • ... saves money for telescope construction.

          • ... makes it easier to deal with thermal and mechanical stability.

          • Allows introduction of additional optical surfaces that can be used to modify the beam.

          • Adding optical surfaces allows one to correct out aberrations introduced by the primary objective.

        • Same telescope (with fixed tube) can be reconfigured to different focal lengths.

          • As we shall see, different focal lengths lead to different "plate scales" (image sizes) and allows different fields of view or resolution for the same focal plane area.

        • Can have different instruments mounted at different "ports".


        Traditional reflectors (e.g., Palomar 200", Kitt Peak 4-m) were often designed with at least FOUR configurations possible:

        • Prime focus: Usually a wide field of view camera.

        • Newtonian focus: Spectrograph or higher resolution (smaller field of view) camera.

        • Cassegrain focus: Spectrograph or even higher resolution (even smaller field of view) camera.

        • Coude focus: VERY high resolution spectrograph.



        EXAMPLE: Palomar 200-inch telescope

        • Multiple focus ports:

          Click on the above image to see a cut-away view of the Palomar 200-inch telescope. Four separate optical paths are illustrated (and this does not include the Newtonian focus, which could not be shown in this representation).

          The fifth focus possibility on the 200-inch is a Newtonian focus through the declination axis bearing. The observer can mount a large instrument in the huge yoke arm of the telescope.

        • Note the problem of the Prime Focus: It is in the beam of the telescope and so difficult to access.

          The prime focus was traditionally used with the astronomer riding inside the telescope through the night. Now the astronomers can use cameras that are remotely operable.

        • Click here to see how the prime focus cage is used (photos from the last ever photographic run at the 200-inch prime focus by Mr. Majewski in 1995).


          Note on the drawings shown above: These are part a series of classic drawings of the 200-inch made by Russell Porter, considered a genius of mechanical drawing. These drawings, which are deadly accurate on what the 200-inch looks like, were drawn from the blueprints before the telescope was even built! Click here for the complete set of these drawings, which are considered "masterpieces" of the art. Permission of California Institute of Technology.



THE 3 MAIN FUNCTIONS OF TELESCOPES

  1. GATHER LIGHT - make things appear brighter

  2. RESOLVE - allow finer detail to be seen

  3. MAGNIFY - make objects seem bigger/closer

To keep pace with the topics of the labs in this course, we will start with a discussion of the least important of these properties, magnification, first.


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Refractor and Reflector images from http://astrosun.tn.cornell.edu/courses/astro201/. Other material copyright © 2002, 2006, 2008, 2012, 2015 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 313 and Astronomy 3130 at the University of Virginia.