User's Guide to the Crossley Telescope

Table of Contents

Ground Level
Dome and Platform
Head Rotation
Dec Tangent Arm
Position Indicators
Mirror Cover
Dome Slit
Access Ports
RA Drive Preload
Operating Limits
New Telescope Drive

Mt. Hamilton Homepage

Dome Layout and Facilities: Dome and Platform

Now we come to the reason for all the fuss about the light at the top of the mezzanine stairs. There is a raised platform in front of the dome slit which moves with the dome and in some orientations may cover all or part of the stairs. Many observers have inadvertently discovered the hazard this presents going either up or down. Legend has it that at least one fairly serious injury has been caused by the long step off of the stairs from the lower platform and down the partially covered stairwell instead of onto the expected solid floor. The writer can attest that walking up the stairs into the bottom of the aforementioned platform will wake one up, no matter how tired and cold. Therefore, it is again suggested you leave on the small red light at the head of the mezzanine stairs, and use the alternate stairs at the south end of the polar axle when necessary.

In addition to the fixed platform just above the mezzanine floor there are two other platforms (see Figs. 22-25). One is fixed at a high level of the dome away from the dome slit. The third platform is movable and runs up and down on an inclined track between the other two. The movable platform (usually just called "the floor" or "the platform") is the one from which the actual observing is usually conducted. All three floors move with the dome when the dome is rotated.

There are various HAZARDS associated with this movable floor, as follows:

  1. When "lowering the floor," be sure nothing is on the lower fixed platform, such as your lunch or observing assistant, since the clearance is only a couple of inches with the floor all the way down.

  2. Be very aware that with the floor lowered part way it is a long step into the ensuing abyss between it and the upper platform, or vice versa. Use of the stairs is suggested. It is further recommended that the four nylon ropes provided be used to rope off the passageways at both ends of both the movable floor and the upper fixed floor (Fig. 7). It takes much less time to hook up these ropes than to recover from a broken neck, and what looks like an obvious hazard at 6 p.m. when you're warm may recede from consciousness at 3 a.m. when you're cold. Avoid a permanent recession from consciousness. Hook up the ropes. BUT - don't inadvertently rope the movable floor to the upper fixed floor! It's been done more than once.

  3. Guard against lowering the floor from the full up position with the telescope very close to the edge of the movable floor. There is a flange on the telescope tube about five feet down from the top with just fits under the floor (Fig. 8), so if the floor is lowered all the weight is momentarily placed on the tube (bad) and then the floor slips off with a frightening (in the dark) crash and may result in catastrophic failure of the elevator mechanism, if not your heart.

  4. When rotating the dome, avoid scraping the tube of the telescope with either the movable or upper fixed platform. The geometry is a bit deceptive, but there's really no excuse for not being careful about such an obvious thing.

  5. Since the movable floor is in fact a funicular railroad (really true; examine it closely!) there is a limit to the number of people which can be safely carried on it, namely three. This limit applies only when the floor is in motion. To exceed this number is to court mechanical failure.

There is a black sheet-metal rain-screen at the apex of the dome. It may be moved by the white rope around a big cast iron wheel on the upper fixed platform. Be certain to put the rain-screen back in position above the telescope each morning, as well as covering the top of the telescope itself with a plastic sheet or telescope cover, because the dome leaks badly during storms (Fig. 9).

The windscreen, dome rotation and movable platform are all electrically controlled by a single paddle (the "dome paddle") on the movable platform (Fig. 10). The six buttons on the paddle are well marked in pairs, and are for windscreen up/down, floor up/down and dome left/right (i.e., left and right as you face out the slit). All of these buttons operate only when held down. There are limit switches at the extremes of operation of the windscreen and platform. Always let the dome come to a complete stop before reversing directions. Avoid hanging this paddle on the telescope, thereby connecting the telescope to the movable floor. The paddle usually hangs on a screw beside the logbook counter.

Above the logbook counter on the movable platform is a small control panel with two large rheostats and a number of buttons and switches (Fig. 11). On the right are three white switches. The first (left to right) turns on a red light illuminating the sidereal and PST clocks to the left of the slit. The second enables the two large rheostats on the panel, which independently control red and white lights which illuminate the logbook counter. The third white switch has no function. At the top of the panel are some dome rotation controls. When pressed, these cause the dome to rotate continuously until stopped by the red stop button. Please use these with caution. The relative geometry of the floors and telescope changes in peculiar and sometimes unexpected ways, so it is advisable to use only the dome rotation buttons on the paddle so you maintain better control. On the left are floor up and down buttons which are disabled, and a panel control switch which should be taped "on". Please leave the panel control switch on at all times. It controls the power to all of the other switches on the panel as well as to the dome paddle. If the dome paddle doesn't work, check to be sure this control switch is on before calling for assistance.

Notice that the shelf across the front of the movable platform may be lowered by pulling the handle on the center underside of the shelf toward you. The shelf then rotates back (toward the slit) to two intermediate positions and all the way to the floor, if necessary for access to very low objects (Fig. 12).

On the upper platform is a long plank which may be used to bridge the gap between the upper and movable floors in order to get access to the eyepieces when the telescope is nearly vertical and the slit is east or west.

Finally, each night be sure to fill out the black "Crossley Telescope Maintenance Logbook" kept on the movable platform. This is how the Maintenance Department knows if anything required repair. If everything works well, so note. At the beginning of your run it may help to read the last few entries so you know what has recently caused problems. If there are any problems with the telescope or dome, please also submit a trouble report to the 3-m telescope operator (ext. 8-0652).

Figure 22

Figure 23

Figure 24

Figure 25

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12