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:
Figure 1: Dial Lights