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The Doghouse Observatory 6-inch Clark Refractor


6-INCH CLARK REFRACTOR

(Rev. August 09, 2011)

Introduction

The 6-inch Clark refractor is housed in the Doghouse at the McCormick Observatory. It has an equatorial mount and is equipped with a Saegmuller weight-driven mechanical clock drive. It is a simple and reliable telescope to use if the proper care is exercised. It is also an irreplaceable piece of equipment; treating it with care will allow it to be used for many, many years to come. The Doghouse telescopes should not be used until you have been shown how to use these telescopes and have been given permission by a TA or faculty member to use them on your own (i.e., checked out).

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Operation of the 6-inch Clark Refractor

Roof: The roll-off roof is operated manually with the winch beside the door. There is a vice clamp holding the chain in place which must be loosened before opening the roof. Be sure to reclamp the chain TIGHTLY after closing up for the night. Never move the roof unless both telescopes are horizontal and clear of its path.

Lens caps: Carefully remove lens caps on both the 6-inch lens (1, see Figures) and finder (4) after opening the roof. Always replace them before closing. Be aware that they have a tendency to bind.

Clock Drive: Start and stop the clock drive with the push-pull control (6) on the north side of the pier on the outside of the clock mechanism cover. Open the glass door on the west side of the pier and you will find the winding shaft (7), for the crank. The crank is kept on the shelf with the eyepiece box. Open the access door on the south side of the pier so you can watch the pulley and weight. Slide the crank sleeve over the shaft and slowly wind the weight up until the top of the weight is level with the bottom of the access hatch in the hollow pier. Never overwind the drive; this will damage the drive mechanism. You will have to rewind it every 45 minutes or so. If the drive is operating properly, the governor (5) inside the door will spin rapidly. Always turn the drive off before leaving the building.

Finder: A low-power, wide-field finder telescope (4) is mounted piggyback on the 6-inch. It contains cross-hairs which are somewhat difficult to see against a dark sky. The alignment between the two telescopes may not be perfect--i.e., an object centered on the cross-hairs may not be in the center of the main telescope field, so always start with the lowest power eyepiece. The focus may need to be adjusted. Do this by carefully moving the draw tube either in or out. However, be careful not to knock the finder out of alignment while doing this.

Controls: Apart from the clock drive there are only 5 controls. The clamps and slow motion controls (8), are located at the ends of the long shafts extending back toward the eyepiece end of the telescope. The two outer knobs control RA slow motion and clamp, and the inner ones have the same functions in DEC. The focus is on the side of the eyepiece mounting tube.

Clamps: The clamps are the larger pair of barrel shaped knobs. If turned counter-clockwise, they completely free the Right Ascension (RA) and Declination (DEC) motions for pointing the telescope. When the desired object has been located, carefully clamp first one then the other, by turning the knobs clockwise. These only need to be firm not tight. Do not over-tighten them, as they become difficult to undo and may damage the gears. Clamping the RA engages the clock drive, if it is ``on'', and causes the telescope to track the object across the sky. Do not attempt to move the telescope without making sure the appropriate motion is unclamped.

Slow motion: The slow motions are the flatter, disk-like knobs, again one for each motion. Turning these knobs adjusts the pointing of the telescope in either RA or DEC for centering objects in the eyepiece field, correcting the drive motion, etc. The telescope must be clamped in that motion, in order for these to function. The DEC slow motion moves the telescope on a tangent arm which has limited travel. Be sure that the tangent arm is in the middle of its travel range, before you begin observing.

Focus: Use the focus knob on the side of the eyepiece mount to adjust the eyepiece focus. After changing eyepieces the image may need refocusing. It is difficult or impossible to focus on faint or diffuse objects, so use a bright, naked-eye star before trying to find such an object or use a nearby field star. Occasionally you will have to adjust the sliding draw tubes to find a focus. A diagonal prism is provided to allow easier viewing through the eyepiece.

Setting Circles: The setting circles (2) are the large white wheels marked with coordinates; one in DEC and one in Hour Angle (HA). The HA wheel is marked from 0 - 24 hours in roman numerals, and measures the time since the last meridian crossing. Therefore, depending on the telescope orientation, 0 hour angle (i.e. the meridian position) will correspond to either 0 hours or 12 hours. Larger numbers are toward the west. E.g. an Hour Angle of 3${1\over2}$ hours east would be found at XX${1\over2}$ on one side of the pier and VIII${1\over2}$ on the other. A little care and practice will prevent confusion.

The coordinates on the painted wheel surfaces are reasonably accurate for declination. For HA, it is best to use those inscribed in brass on the edge of the wheel. These must be read with the mounted magnifiers (3).

Eyepieces: Eyepieces are stored in the eyepiece box on the shelf in the corner of the Doghouse. Most objects are best viewed under low or moderate power, rather than high power which is recommended only for high surface-brightness objects such as planets and double stars. Replace the eyepieces in the eyepiece box before leaving.

Figure 1: 6-inch Alvin Clark Refractor. (1) Objective Lens, (2) Setting Circles, (4) Finder, (8) Slow Motions and Clamps, (9) Eyepiece.
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Figure 2: 6-inch Alvin Clark Refractor. (2) Setting Circles, (3) Setting Circle Magnifiers, (5) Drive Governor, (6) Drive Push-Pull Switch, (7) Drive Winding Shaft, (8) Slow Motions and Clamps, (9) Eyepiece.
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Finding Objects

A list of interesting objects is available on the table in the Doghouse. Coordinates may be found in Norton's Star Atlas. When searching for objects, always use the lowest power eyepiece (i.e. longest focal length) first. There are three basic ways to find objects:

  1. By eye: Using Norton's, or any other source, point the telescope at the appropriate region of the sky and search for the object with the finder. This will probably not work for fainter objects, but is suitable for the most interesting objects.

  2. By DEC: Carefully set and clamp the telescope at the DEC of the object as accurately as possible. Find the approximate location in the sky, and leaving the HA unclamped, search in the finder while moving the telescope slowly in HA. The finder is small, so fainter objects will be rather dim. Check out likely candidates in the main telescope. Small adjustments in DEC may be necessary.

  3. By DEC and RA: Set the telescope to the DEC of the object and clamp. Then calculate the hour angle from the general relation $HA = ST - RA$ where HA is the hour angle, ST is the sidereal time, and RA is the right ascension of the desired object. If the HA is negative, the object is east of the meridian. Add 24 hours to the HA and set the telescope to the resulting HA. If the HA is positive then set the telescope to the HA directly. (NOTE: This is the procedure for the 6-inch refractor. The 10-inch HA dial goes in the opposite sense; a negative HA is west of the meridian instead of east. The strategy is the same however so just keep in mind how the dials are divided.)

    Determine the ST from the sidereal clock. (Alternatively set your watch to the ST using the table provided in Norton's, or from the sidereal clock in the 26-inch dome. Over a few hours, your watch will keep sufficiently accurate sidereal time for this method.)

Once the coordinates are set and the drive turned on, you may search for the object at your leisure. You will probably have to search around a bit in the finder. Practice the method on bright stars first. It sometimes helps to repeat the procedure, or at least double-check the setting circles if you can't find what you're looking for. If objects seem consistently and substantially offset in HA, the sidereal time may be wrong.

While observing, you may need to make small corrections with the slow motions for errors in the clock drive. It is best not to touch the telescope otherwise during observations.

At any given time, you will not be able to reach all of the listed objects. Usually, you can observe any object whose HA is less than 3 hours. The farther north the object, the greater the hour angle to which you can observe it.

Note that the cardinal directions in the finder and main telescope will be inverted relative to their orientation in the sky.

Closing Down

Stow the 6-inch Clark refractor

Secure the doghouse

Observer's Room (Room 106)

Secure the Observatory and Grounds:

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