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

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The Telescope: Reversal

If you wish to observe in the north, you must do something to make the eyepieces accessible, since they are on the south side of the tube when the tube is on the east side of the polar axle. You have two choices. You may rotate the head of the telescope, but for some configurations (e.g., the scanner) this is somewhat inconvenient because of the many cables which tend to get hung up during the rotation. Or you may "reverse the telescope," which means to move the tube from the east to the west side of the polar axle. One effect of this maneuver is to rotate the entire tube 180 degrees clockwise about its long axis so the eyepieces are on the north side.

Reversal is most safely done with the dome lights on, and lights are therefore recommended as part of the normal procedure. The reason is that care must be exercised to avoid pulling out cables, especially on the many protuberances of the north polar axle and north pier. Be careful also not to let the cables break the north pier light.

To reverse the telescope the dome is moved from south through west as the the telescope is brought down to nearly six hours west at the equator, so it is nearly parallel to the floor, and lying across the top of and perpendicular to the polar axle (Fig. 22). (Do not actually go to a zenith distance greater than 75 degrees or the mirror may tip and a supporting pad may fall out - it has happened and tends to ruin the rest of your run since a good focus is then impossible.) Next, as you rotate the dome west through north, rotate the telescope tube about the declination axle only, so that when the dome is north the tube is right on top of and parallel to the polar axle (Fig. 23). Continue to rotate about only the dec axle for another 30 degrees or so toward the east to get a large enough angle between the tube and the polar axle so you have some leverage, then push the tube to as to rotate the polar axle toward the west (Fig. 24). This will bring the telescope down on the west side of the polar axle (Fig. 25) and reversal is complete.

In addition to the hazard pointed out above concerning the cables, be careful not to start any movement in RA when the tube is parallel or nearly parallel to the polar axle, because you may have insufficient leverage to stop such a movement. This is especially true if the telescope is improperly balanced.

Many observers experience some disorientation the first time they use the Crossley, particularly during reversal. The polar axle provides a ready reference since the high (north) end always points toward Polaris. Remember to use the inside scale of the RA hours dial when the telescope is reversed. Any time you get confused about which scales to use or which way to read them, simply move the telescope a small amount in a predetermined sense, say north and west, and notice which scales indicate the motion correctly.

Although reversal perhaps sounds complicated, after you've done it once or twice it's easy and you will probably find that with some instrument configurations it's faster than fighting tight cables to rotate the head.

Figure 22

Figure 23

Figure 24

Figure 25