Solar Telescope

From AstroEd

A vertical solar telescope feeds a white-light image of the Sun into the undergraduate astronomy laboratory. The coelestat mirror system shown here tracks the daily motion of the Sun and directs light downward into the telescope supported from the ceiling of the astronomy lab.

Solar ferland 24apr2001.jpg

Professor Gary Ferland of the University of Kentucky at the Solar Telescope Coelestat in April 2001.

Students also use a solar flare monitor at Moore Observatory which records very low frequency (VLF) radio reception from a stable transmitter in Cutler, Maine. Conditions in the ionosphere responsible for the strength of the signal are very sensitive to X-ray flux from solar flares and sudden x-ray emitting events outside the solar system, and rapidly alter the received signal.

Design of the Solar Telescope

Light from the Sun is directed vertically downward through an opening in the roof of the Natural Science Building after reflecting off two mirrors in the coelestat shown in the picture above. The mirror on the right (north) side turns at half the sidereal rate to keep the reflected beam on the mirror over the vertical light path. This mirror is parallel to the Earth's axis, and is on wheels that allow it to move north-south during the year. When the Sun is above the celestial equator in the summer, the mirror is moved north, away from the tower. When the Sun is below the equator in the winter, the mirror is moved south, toward the tower. It is not necessary to make daily adjustments of the mirror's north-south position, but it is easy to do whenever necessary.

The mirror at the top inside of the tower is adjustable about a horizontal axis to provide small corrections of the image north-south. It must be changed by larger angles whenever the sidereal mirror cart is translated. Both mirrors are driven by worm gears and stepping motors. The stepping motor controllers are in the lab under the roof, and cables pass through the optical opening directly to the motors.

These mirrors are fused silica flats, nominally 30 cm in diameter and 5 cm thick. They have a surface flatness of 1/10th wave for visible light, and are coated with aluminum. Normally these mirrors are covered, and in severe weather the sidereal mirror cart is rolled forward to protect its mirror for rain and wind.

Sunlight from these mirrors passes into a vertical Dall-Kirkham Cassegrain telescope that is mounted under roof, that is, on the ceiling of the classroom below. The telescope is a 20 cm diameter f/20 (4 meter focal length) telescope that forms an image of the Sun approximately 2 meters below the ceiling in the classroom. The long focal length and high precision of the mirrors in this telescope produce excellent images free of chromatic and spherical aberration. The image of the Sun is approximately 3.5 cm in diameter at the focal plane of the telescope. It is projected with a plano-convex two-element achromatic lens to form a solar image approximately 40 cm in diameter on the floor below. At this size, the image is about 1/4 the brightness of direct daylight, and is an darkened corridor where it is easy to view. Under usual seeing conditions features as small as 2 arcseconds are visible. Students will see solar granulation, limb darkening, faculae, and sunspots on almost any day the Sun is visible.

The drive system on the mirrors does not operate continuously. Students must interact with the telescope to place the image in the center of the field. If they do not have the motors running, then the Earth's rotation causes the image to drift quickly out of view. However, the sidereal motion is slow enough that they can follow detail, and it helps to separate what is on the Sun and what is dust or a mark on the screen.


Keys to the telescope are in the center desk drawer. Students should not go to the roof unaccompanied, and usually the teaching assistant alone is responsible for uncovering the optics and the initial setting of the coelestat. Occasional visits to the roof may be neccessary if the Sun is completely lost from view.

To open the telescope:

  • Go through the mechanical space in the penthouse
  • Open the north roof door
  • Remove the mirror covers (bungee cords hold the covers in place)
  • Remove the weight that retains the cover over the opening into the classroom
  • Store the weight, mirror covers, and bunji cords so that wind will not send them off the roof
  • Rotate the sidereal mirror by hand (it is clutched) to send light to the tower mirror
  • Roll the cart if needed to center the light on the tower mirror
  • Remove the tower side panels if they are blocking sunlight (usual in the winter)

Before you leave the roof make sure that everything you have removed is secure from the wind. Remember that the opening to the vertical telescope looks down on its mirror, so take care not to drop anything in the hole.

From the lab:

  • Use a stepladder if needed to remove the cover over the primary mirror of the telescope
  • Place a clean white poster board on the floor under the telescope
  • Turn on the motor controllers
  • If you do not see sunlight on the secondary, run the motors to center the reflected light
  • Adjust the mirrors so that the image of the Sun is centered in the field on the floor

If you cannot see the Sun on the secondary mirror, look for bright light on floor around the shadow of the telescope. This would be from sunlight that is coming in at the wrong angle, and you can use it to bring the solar image into the field.

Since the telescope only tracks when you are running the sidereal motor, usually the Sun's image will drift across the field as you watch. You can adjust it in small steps, or try a very slow step rate and let it run constantly. The image is best if the telescope is not tracking since there are no stepper vibrations to jitter the view.

It's possible to recenter the image without going back to the roof if you check on it frequently during the lab. However, if it is off by a hour or more, you may have to go back up to make a coarse adjustment of the sidereal mirror.

There are no focus adjustments. The telescope is permanently focused.

Never clean the optics. Ask for help if you find them very dirty, as may happen if the covers are left off for an extended period.

To close the telescope:

  • Turn off the motor controllers
  • Cover the telescope primary mirror (before going to the roof)
  • On the roof, cover the flat mirrors and secure the bungee cords
  • Place the cover over the opening in the roof, and put the weight on the cover
  • Return the sliding panels to their places on the sides of the tower
  • If severe weather or a long period of storage is anticipated, roll the cart south to protect the mirror