User's Guide to the Lick 36" Refracting Telescope

Slit (shutter)
Lens Cover
Position Indicators
Tangent Arm
Access Ports
Control Desk
Observing Hits
Trouble Shooting

Mt. Hamilton Homepage

Positional Indicators (Dials and Setting Circles)

The telescope two ways of reading out position, the Selsyn position dials at the end of the telescope and the Vernier setting circles on the pier.

There are four pairs of Selsyn dials (Figure 1), one pair each for the Right Ascension (Figure 2), Declination (Figure 3), Hour Angle (Figure 4), and Position Angle (Figure 5). The dials are backlit and the lighting level is set via a dimmer switch on the right side of the unit (see Figure 1). There are tick marks at the top of each dial for aligning the dials with the desired position.

The two Right Ascension dials (Figure 2) are in units of Hours and Minutes. When the telescope is on the east side of the pier, the Hours are read from the outer ring of numbers. When the telescope is on the west side of the pier, Hours are read from the inner circle numbers. The Minutes dial only reads from 0 to 30, so one needs to refer to the Hours dial to know if one is in the 0 to 29 or 30 to 59 minutes range.

The two Declination dials (Figure 3) are in units of Degrees and Degrees & Minutes. When the telescope is on the east side of the pier the outer ring of numbers on the Degrees dial is applicable. When on the west side, the inner ring on the Degrees dial is used. The white numbers are for northern declinations and the red numbers are for southern declinations. The Degrees & Minutes dial spans a range of 5 degrees, so one will have to refer to the Degrees dial to know whether one is in the 0 to 4 or 5 to 9 degree range. Similar to the Degrees dial, white numbers are for northern declinations and red are for southern declinations. Likewise, inner and outer rings of numbers reverse when the telescope is on the west side of the pier instead of the east side (consequently, when on the west side of the pier, red on the Degrees & Minutes dial will correspond to northern declinations, and white to southern, which can be confusing).

The two Hour Angle dials (Figure 4) are in units of Hours and Minutes and operate in the same fashion as the Right Ascension dials. In the unlikely case of RA dial failure, you can still position the telescope using the HA dials, recalling that HA = Local Sidereal Time - RA.

The two Position Angle dials (Figure 5) are not usually used as no current facility instruments require rotating the back end of the telescope. However, for completeness their function will be described. The units of the dials are both Degrees, with the left dial being the coarse position and the right being the fine position. The lefthand dial ranges from 0 to 360 degrees, with the outer numbers applying when the telescope is on the east side of the pier and the inner numbers for when the telescope is on the west side. Right dial is the fine position and ranges from 0 to 10 degrees.

If there has been a power outage or problem with the sidereal oscillator, the Right Ascension dials will get out of sync with the Local Sidereal Time (LST). If this occurs, first set the proper LST on LST clock on the desk console, then set the RA dials with the following procedure:

  1. Park the telescope at 0:00 Hour Angle and make sure tracking is off.
  2. Get the RA dial adjuster (Figure 6). It is typically stored in the extra counterweight cart on the east side of the pier.
  3. Make sure that its power is OFF, the direction selector is set to BRAKE, and the rheostat is turned down to zero.
  4. Attach the adjuster to the RA Dial Reset port (Figure 7).
  5. Set the Range on the Adjuster to LO or HI. Most often one will want to select LO because the dials will not be that far off. If off by hours (due to a prolonged power outage or other problem) one might want to set it to HI.
  6. Set the direction to either FWD or REV, depending on which way you want to move the dials.
  7. Turn up the rheostat slowly and adjust the rheostat as needed to set the RA dials so that they equal the Local Sidereal Time. For most applications setting within a few seconds is sufficient. If one overshoots the desired dial setting, turn the rheostat down to zero before changing the direction from FWD to REV (or vice versa).
  8. When finished, turn the rheostat down to zero, set the direction selector to BRAKE, and turn OFF the power.
  9. Disconnect the RA dial adjuster from the telescope and put the unit back in the counterweight storage cart.
The second (and original) way to position the telescope is with the Vernier setting circles mounted on the telescope axes (Figure 8). Each axis has a setting tick mark (Figure 9) for accurate positioning.

Figure 8: Vernier Setting Circles

Figure 9: Vernier Setting Circle Alignment Mark

Figure 1: Dial Lights

Figure 2: Right Ascension Dials

Figure 3: Declination Dials

Figure 4: Hour Angle Dials

Figure 5: Position Angle Dials

Figure 6: RA Dial Adjuster
Figure 7: RA Dial Reset Plug