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The McCormick Observatory 26-inch Clark Refractor

Revised September 2010

The 26-inch refracting telescope was built by Alvan Clark Sons and completed in 1875. It is housed in the dome on Mount Jefferson; the observatory was completed in 1884, and dedicated on April 13th, 1885. The Clark refractor has a 26-inch diameter lens with a focal length of 32.5 ft (9.9 m). Its field of view is about 0.75 of a degree with photographic plates. This telescope has been used primarily for astrometric observations and the observatory plate file contains over 140,000 plates which were taken between 1914 and 1995 for the purpose of determining stellar distances and motions.

For a full history of the telescope see:


Figure 1: The 26-inch Clark Refractor in proper stow position.

A Tour of the Telescope

The dome is entered via an entrance to your left as you come in the front door of the observatory building. There is an additional entrance located on the south side of the dome, which is not normally used (and should not be used for public access since it requires people to pass by mechanical equipment in the dark).


Figure 2: The dome entrance as viewed from inside the dome. The green box indicates the location of the light switches and plug for the dome charger.

There are three dome slits which are opened and closed by ropes which hang along the dome wall. There are two floor mats near the entrance which are to collect water from known leaks in the dome slits. The leaking water must be kept off of the original observatory floor. There is no need to move these mats (e.g. the observing chair will move over them), but if they need to be moved, they must be replaced in their original location.

There are two observing chairs. The larger of the two runs on a track around the edge of the dome. This track must be clear for the chair to move safely. Damage or injury may result if the track is not clear. Please note that the chair will move freely with the floor mats in place. A smaller chair is located opposite the larger chair. This chair can move freely and the wheels can be locked when in use. Never lock the wheels when stowing the chair at the end of the night. There is a small box which can also be used for observing; care should be exercised, since the box can damage the floor.

There are two sets of telescope controls. One is located on the pier and the other on a hand paddle attached to the telescope itself. The sidereal time can be read from a digital clock on the far wall and the local time is read from the analog clock on the pier.

Equipment for maintaining and repairing the telescope is stored in the dome at the South end of the pier. This equipment should not be touched or moved.


Figure 3: An observer adjusts the location of the telescope from the pier controls. Also highlighted are the locations of local and sidereal clocks in the dome.

Opening Procedure

  1. Allow at least one hour of preparation time before your planned observations.

  2. Turn on the wall switch lights which are to the left of the dome entrance (see green box in Figure 2). Directly adjacent to the switch is a small toggle that will adjust the intensity of the lights.

  3. Unplug the dome motor battery charger (hanging cord) from the wall outlet above the light switch, remove the extension cord and place it on the shelf in the alcove to the right of the plug. Never leave this extension cord attached when the dome motor is unplugged.

  4. Open the dome shutters by pulling on the appropriate ropes.

    The dome itself can be moved from both the console and the paddle using the Dome Left or Dome Right toggle (See Figure 4 (2) or Figure 5 (4)). The dome can also be moved from the hanging plug by holding down the left or right buttons. Always allow the dome to come to a complete stop before changing the direction of dome motion.

    Always inspect the wall of the dome to be sure that no objects will be caught by the dome ropes as the dome rotates. Always use caution when rotating the dome, and watch for any potential problems. Never store objects by leaning them against the wall of the dome.

    \epsfig{file=26inch/PierPanelFigure.eps} Figure 4: Pier Control Panel

  5. Turn on the telescope power from the console (top left button, see Figure 4 (1)). This will activate all console controls, paddle controls and console lights. To do this push the Power On button on the console (note there are also power controls on the top of the hand paddle). The red indicator light on the bottom left will illuminate when the power is on (Figure 4 (3)).

  6. To unclamp the Right Ascension (RA) or Declination (Dec) motions, press the RA Clamp and/or Dec Clamp button on the control console (see Figure 4 (4)). This will release the telescope clamps. Red indicator lights will illuminate when the telescope is unclamped (see Figure 4 (4)).

    To slew the telescope down into position for observing, unclamp the RA Clamp, press the West (W) slew button (see Figure 4 (8)). This will move the objective toward the west and the tailpiece down toward the pier. Releasing the button will stop the motion.

    Care must be exercised whenever the telescope is being slewed. Make sure that the telescope's path is clear, and always watch it as it moves. Particular care should be taken with any movements of the tailpiece near the pier.

    Release the W button to stop slewing once the tailpiece is within easy reach. Again, use caution to avoid hitting the pier, or anything else.

    Motion in declination works in a similar fashion (although the Dec slew motor does not work consistently). Unclamp the Dec Clamp to allow movement in Declination.

  7. To open the lens cover, flip the objective cover toggle down (see Figure 4 (6)). The green indicator light will turn red when the objective is opened.

    Note: Always open the objective cover before closing the dome shutters. Sometimes paint chips or other materials on the dome slits can fall onto the objective while the dome is in motion or while closing the dome shutters. This can cause serious damage to the objective.

  8. To rewind the RA drive sector, press the Sector Reset button on the console or hand paddle (see Figure 4 (7) or Figure 5 (2)). The indicator light will turn red as this drive is reset. When the light turns off then the drive is reset and the observer has about 1.5 hours of motion available before needing to reset.

  9. To turn on the clock drive, press toggle the Track switch on either the console or hand paddle (See Figure 4 (5) or Figure 5 (5)). The indicator light will turn red when the tracking is on.


    Figure 5: Hand Paddle Control Panel

Locating an Object

  1. Pick the object that you want to observe.

    Note: Any declination greater than 24 degrees will cause the telescope to hit the pier soon after the object passes the meridian. Although it may not appear obvious from ground level, the declination periscope (brass tube) will hit the shelf on the top of the pier starting at +24 degrees even though the tailpiece will clear the pier.

    Amount of Time after Transit before Telescope Hits Pier

    Declination Time (minutes)
    +23:40 Will Pass
    +24:00 19
    +25:00 18
    +26:00 10
    +27:00 8
    +30:00 6
    +34:00 6
    +35:00 6
    +40:00 5
    +45:00 6
    +50:00 9
    +55:00 9
    +60:00 10
    +65:00 12
    +70:00 21
    +75:00 34

  2. Set the declination.

    The current declination of the telescope can be read off of the declination periscope on the side of the telescope opposite the finding scope. Gently move or slew the telescope to the correct declination.

    Clamp the telescope in declination while holding the telescope in place with the silver metal bar around the tailpiece. Never push against the tailpiece itself. After clamping small adjustments can be made with the slew buttons. While clamping the telescope will move slightly.

  3. Set the Hour Angle (HA).

    Compute the HA: HA = Sidereal Time – RA

    (A positive value indicates minutes to the west, negative indicates minutes east, i.e., if the RA is greater than the sidereal time the object is east of the meridian.) Read the hour angle from the large circle on the polar axis (1 division = 5 minutes).

    Clamp the telescope in Right Ascension.

  4. Position the dome with an open slit over the objective lens.

    Declination Coverage of the Dome Slits when Placed Along the Meridian

    Shutter To South To North  
    Top +6 to +45 +35 to +73  
    Middle -24 to +14 +67 to pole  
    Bottom -18 to Horizon    

  5. Use the track toggle to initiate the clock drive. The tracking only functions when the telescope is clamped in RA.

  6. Center the object of in interest in the finder scope.

    Compare the star field through the finder with your finding charts. Center your object on the cross hairs using the fine adjust slew motions on the paddle.

  7. Clamp the telescope in RA. Tracking will commence as soon as the telescope is clamped in RA. Periodically check the finder to ensure that the object has not drifted out of view.

Telescope Tailpiece

The telescope tailpiece has been designed to allow for simultaneous use of the eyepiece, Astrovid 2000 camera, and an SBIG CCD. Flip mirrors located at the back end of the telescope allow one to change between instruments. The tailpiece uses the 2 inch eyepieces found in the metal cabinet in the Observer's Room.

We currently have three eyepieces for use on the 26-inch. Great care should be exercised in handling all of the eyepieces and filters.

Available Eyepieces:

  1. 20 mm Nagler Type 2 (great for use with planets!)
  2. 35 mm Panoptic
  3. a 55 mm Plössl

\epsfig{file=26inch/eyepieces.eps} \epsfig{file=26inch/backendfigure.eps}

Figure 6:

Left: Eyepieces for the 26-inch are located in the metal instrument cabinet in the Observer's Room.

Right: The backend of the telescope with major features boxed in red. The metal bar around the tailpiece is used to move the telescope. The declination periscope is located on the opposite side of the telescope from the finder scope.

Visual Observations

Visual observations are conducted using a set of 2-inch eyepieces kept in the Observer's Room (See Figure 6).

  1. Loosen the screws on the eyepiece holder on the back end of the telescope and remove the stop, storing it on the desktop. Slide in the desired eyepiece and tighten the screws to lock in the eyepiece. Stow both the eyepiece cap and the stop on the desktop for the remainder of the night.

  2. The rough focus for the backend can be turned with the crank on the pier table. A rough guide to where the focus has been historically is located 45 degrees counter clockwise from the location of the rough focus. Return the focus crank to the pier table.

  3. Fine focus knobs are located on the eyepiece holder on the backend of the telescope (see Figure 6).

  4. Make observations!

  5. When finished, remove the eyepiece and return it to the case. Replace the stop. Then, follow the close down procedures in Section VIII.

Basic CCD Observations

  1. Carefully roll the PC cart from the observer's room into the dome.
  2. The powerstrip on the PC cart can be plugged into the outlet on East side of the pier.
  3. Connect the male end of the ribbon cable into the parallel port on the PC. Connect the female end of the ribbon cable to the parallel port on the CCD Head.
  4. Connect the 5-pin power cable from the CCD power supply to the CCD head. Plug the power supply into the power bar on the telescope.

    If everything is connected properly then, the unit now has power. The red LED on the head of the CCD head will glow and the fan will being spinning.

  5. Plug the PC and the PC monitor into the power strip on the PC cart. Start the PC by pressing the power button.
  6. Insert/Check the filter. The filter is mounted in a holder just before the instrument. A set of BVR filters are stored in the Observers Room.
  7. The rough focus for the back end can be turned with the crank on the pier table. A rough guide to where the focus has been historically is located 45 degrees counter clockwise from the location of the rough focus. Return the focus crank to the pier table. Fine focus knobs are located on the backend of the telescope.
  8. Center the object of interest in the eyepiece as described in Section III.
  9. Fine focus for the CCD camera is located on a knob near where the CCD attaches to the tailpiece.
  10. Flip the mirror to send the light into the instrument.
  11. Image the Object. A PC is required to control the CCD. The CCD is controlled by the Maxim DL software, which operates in the Windows environment.

Warnings and Additional Instructions

  1. Do not leave the telescope unattended with the drive running. If leaving the dome for any length of time close down the telescope.
  2. The sector is located behind the Right Ascension drive. For historical reasons (the original drive was a weight driven governor clock drive, with a weight that had to be reset every 90 minutes), the main gear for the RA drive is only a small section of a full circle. This will drive the telescope for about 1.5 hours, after which the sector must be reset. You are advised to reset the sector
  3. Remember that the 26-inch can not point to all places in the sky. The telescope is always on the West side of the pier, never on the East. This means that you should try to observe objects before they transit, especially northern ones. You will have to be mindful in your observing plans in order to ensure you can observe the desired objects within these limitations.
  4. The Observer's Room contains the Astronomical Almanac, Norton's Star Atlas and the SAO Catalog. The computer in the Observer's Room is connected to the department's network. The 26-inch observing logbook is also located here. Please do not forget to fill out the log when you observe.
  5. There is a phone on the east side of the pier and a phone in the Observer's Room. There is a list of departmental phone numbers on the wall in the Observer's Room as well as a list of the emergency contacts.
  6. There is a First-Aid kit in the Observer's Room in the built in bookshelf.
  7. There are pressurized water fire extinguishers in the entry hall and the lecture room. These should not be used on electrical or chemical fires. A small extinguisher for all types of fires is located downstairs in the room outside of the bathroom.

    Call 911 (9-911 from a University phone) before attempting to put out the fire by yourself.

Closing Down

  1. Turn off the RA Drive and reset the sector. The reset is complete with the red indicator light turns off (see Figure 4 (7) or Figure 5 (2)).

  2. Close the objective cover using the console (see Figure 4 (6)). The indicator light will turn green when the cover is completely closed.

    Note: Always close the objective cover before closing the dome shutters. Sometimes paint chips or other materials on the dome slits can fall onto the objective while the dome is in motion or while closing the dome shutters. This can cause serious damage to the objective.

  3. The telescope stow position is: +38 degrees declination and HA of 4 hours east. Bring the telescope down so that the objective cover points at the zenith. The telescope tube should be parallel to the pier. Then, use the RA slew to move the telescope 4 hours east.

    If you are unable to safely stow the telescope, call for help immediately.

  4. Clamp the telescope in RA and Dec. The red indicator lights will turn off when the telescope is clamped (see Figure 4 (4)). If using the CCD, shut down the CCD before turning of telescope power.

  5. In general, green indicator lights mean that it is okay to leave the telescope and the dome (except for the on red light which simply indicates that the power is on). Red indicator lights mean that some aspect of the telescope is not in the proper mode for telescope storage. If the objective cover light is green and no other indicator lights are illuminated aside from the red power light, then turn off power to the telescope with the button on the pier. The red indicator light will turn off when the power is turned off (see Figure 4 (1)).

  6. Close the dome shutters completely. Close the shutters by using the ropes. Often the shutters will stick when very close to being closed. Be sure to give a strong hard tug to make sure the shutters are completely closed. You must visually inspect the dome shutters to ensure they are completely closed. The dome shutters will make a sound even when they are not completely closed, thus only hearing the shutters close is not enough to confirm they are closed.

  7. Rotate the dome so that the charger cord is hanging just to the right of the outlet. Plug in the extension cord, and plug in the dome charger (See Figure 2).

  8. Clean up the area before leaving. Be sure to return all objects to their proper storage locations.

    (1) Carefully push the large observing chair past the second window on the left side of the room. Return the small observing chair to the west side of the room against the track and next to the historic tailpiece jack. Do not lock the wheels when stowing the small chair. Make sure the box is against the pier, being careful to not damage the floor.

    (2) Return ladders or any other equipment to its location against the pier.

    (3) Ensure that the focus crank is on the desktop on the north side of the pier.

    (4) Return all reference materials to their proper locations in the Observer's Room.

    (5) Return the computer and other equipment to their proper locations in the Observer's Room.

    (6) Return the eyepieces to the closet (stowed in their case).

  9. Turn off lights in the dome and close the door to the dome.

  10. Observer's Room:

    (1) Record use of the telescope in the log. Please record ONLY your name, purpose of telescope use (e.g. Astr3130) and the number of visitors.

    (2) Log out of the computer in the observer's room and return books and other equipment to its proper place. Turn off the monitor.

    (3) Report any and all problems to:

    Immediate, serious problems must also be reported to the TA or Ricky Patterson (214-0414; lives in Vyssotsky Cottage beside the observatory; available at night) or Ed Murphy (293-5634; available at night).

    The TA is your first access point for help. If you are unable to reach either TA, then contact your professor or another faculty member at the department. Phones are located both in the dome and in the Observer's Room. In the Observer's Room is a list of phone numbers for members of the Astronomy department. It is critical that problems be reported in a timely manner.

  11. Secure the observatory and grounds.

    (1) Lights in most of the observatory are on motion sensors and will turn themselves off after you leave. It may seem disconcerting to leave the observatory with lights on, but do not turn off the motion sensors; the next person who arrives after dark will thank you!

    Lights in the dome room and entrance are not on motion sensors. Be sure to turn off these lights when leaving.

    (2) Leave the door to the Observer's Room open and leave the door to the closet (with the security light switches) slightly ajar to control humidity.

    (3) Turn on the outdoor security lights in the closet.

    (4) Make sure the inner door locks behind you as you leave.

    (5) Close the outer doors. Make sure that the left-hand outer door is bolted. If you close both doors without bolting the left-hand door, it can be very difficult to open the doors.

    (6) Close the gate. The gate should be closed at all times (at night) except during public events (e.g. public night). It should be closed while you are observing at the telescope, and should only be opened when driving in or out. Never leave it open for someone who is due at any minute; they will have to open and close the gate for themselves).

Opening Checklist

  1. Turn on Lights.
  2. Open Dome Slits. Unplug Dome Charger and stow extension cord.
  3. Turn on telescope power and drive.
  4. Unclamp the telescope, slew it so the tailpiece is accessible, reset the sector.
  5. Open the objective cover.
  6. Turn off main lights, prepare to observe.

Closing Checklist

  1. Turn off the RA drive. Reset the sector.
  2. Close the Objective Cover.
  3. Stow the telescope at +38 degrees, and HA = 4 hrs east. Clamp the telescope in both RA and DEC.
  4. If using the CCD, turn off power to the CCD. Turn off power to telescope.
  5. Close the dome shutters completely, visually inspecting the slits. Do not rely on the feel of resistance on the dome pulley rope as an indication that the slit is completely closed.
  6. Align the dome correctly and plug in the dome motor.
  7. Clean up the area before leaving. Be sure to return all equipment to its proper storage location within the dome or Observer's Room.
  8. Turn off light in the dome. Close the door to the dome.
  9. Sign the log book and clean up the Observer's Room.
  10. Secure the Observatory and Grounds. Be sure that the security lights are on at the observatory, the observatory is locked and that the gate is closed.
  11. Contact someone immediately if you are unable to stow the telescope safely. Report all problems to

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