University of Michigan - Department of Astronomy

Version: detroit

Timekeeping and Telescopes at the Detroit Observatory

A telescope that has a lens as its primary optical element, or objective, is a refracting telescope. A telescope that has a mirror as its objective is a reflecting telescope. Telescopes are described in more detail in the Daytime Observing at Angell Hall activity.

As you enter the Detroit Observatory, take note of the massive central telescope pier in the center of the first floor. The Fitz Telescope is resting on this pier, whose foundation is 15 feet below grade, and has a total height of roughly 60 feet. The pier is physically isolated from the rest of the building, so that vibrations in the building do not disturb the telescope pointing. All professional astronomical telescopes are built on similar piers.

Part 1: The 12.6-inch Fitz Telescope

  1. Is this telescope mounted for the altitude-azimuth or equatorial coordinate system? How can you tell?

  2. The telescope can be pointed with the aid of the two setting circles. One measures position N-S, the other measures position E-W. The E-W circle measures position in hours from the meridian. Toward what declination is the telescope pointing? Can you also determine the right ascension? Explain.

  3. Can this telescope point to any position on the sky? Can the dome provide a window to any position on the sky?

  4. Is this telescope a refracting or reflecting telescope? How can you tell?

  5. Focal lengthIf a telescope's focus is close to the objective lens or mirror, its surfaces must be highly curved. Large lenses are difficult to manufacture accurately, especially if the surfaces have a high degree of curvature. Furthermore, the glass must be of superb optical quality, pure and uniform. Even with the best glass, much of the precious light is not transmitted. For these reasons, telescope objective lenses are almost all thin lenses, which focus light at a relatively large distance from the lens, i.e., they have a long focal length. What is the focal length of the Fitz Telescope? Ask the GSI or Docent for this information.

Part 2: the Meridian Circle Telescope

This telescope is designed to accurately observe the transit of stars across the meridian. Note that the shutters open up to give a view of the meridian. The telescope can only point in declination, and not in right ascension. However, its mount is aligned north-south with the highest possible accuracy, and the eyepiece has fine lines to further note the crossing times of stars with exquisite precision.

  1. Is this telescope a refractor or a reflector? How can you tell?

  2. What is the diameter of the objective? Ask the GSI or Docent for this information.

  3. Can this telescope point in altitude? Can it point in azimuth? Explain.

  4. If you are standing in the doorway facing the meridian circle telescope, which direction are you facing? Which direction is north, standing in this position?

  5. This telescope can be used to measure the equatorial coordinates of transiting stars. How would you measure the RA of a star?

    Knowing the altitude of the north celestial pole, how would you measure the Dec?

    A SextantFigure 1: A sextant.
    A quadrant and a sextant are instruments that measure angular distances of up to 90 degrees and 60 degrees, respectively (one quarter and one sixth of a circle). They are essentially truncated protractors. Ancient astronomers measured the positions of stars at transit, using the same principles that you just described for the Meridian Circle Telescope. (Click the image for more on navigating with a sextant)
  6. Draw the pattern of the cross-hairs (reticle) in the eyepiece. You may use the enlargement provided by the docent. Label the line corresponding to the meridian.

  7. The eyepiece has a cross-hair with many lines instead of a simple cross. Explain why.

  8. How can this telescope can be used to measure the length of a sidereal day?

Concluding Questions

  1. Compare the objective diameters of the Fitz Telescope and the Meridian Circle Telescope. Why are they so different?

  2. How can you use the Meridian Circle Telescope to set a clock's rate measuring Local Sidereal Time?

  3. How can you use the Meridian Circle Telescope to set a clock's time (hour, min, sec) showing the Local Sidereal Time?

  4. You observe a star crossing the meridian at 10:15 EDT (Eastern Daylight Time) on October 1. What time (EDT) does the same star cross the meridian on the following night, Oct 2? The following week, on Oct 8? Explain.

  5. Using the Meridian Circle Telescope, you see that Star A, whose RA = 0.0 hrs, crosses the meridian at 10:33 PM. Later, you see Star B crossing the meridian at 2:47 AM. What is the RA of Star B?

Updated: 9/9/10 by MSO & SAM

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