13
Using the Command Center
The electronics of the Command Center system consist of an open-loop stepper motor arrangement.
A crystal-
controlled microcontroller monitors and directs all the activities associated with the button and switch input, LED
display, stepper motor operation and timing. The user may select any of four tracking rates, and any of six guide/slew
rates. A declination backlash correction (TVC) mode is provided. The unit is powered by a source of regulated 12
volts DC, with a peak draw of 800mA.
Tracking Rates
When the unit is powered on, tracking is set for Sidereal operation. The user may press the
RATE
button to advance
through the King, Lunar and Solar rates, then back to Sidereal. An internal 12MHz crystal provides accurate timing
for each of these tracking rates.
Guide/Slew Rate
The guide/slew rate may be set by pressing the
G/S
button on the Command Center. When powered on, this rate is
set to 30% of the tracking rate (.3x). Pressing the
G/S
button will advance through .5x, 2x, 4x, 8x and 16x, then back
down to .3x. Both right ascension and declination are affected by this setting. Additionally, the user may immediately
enter the 16x slew rate at any time by pressing the opposing slew button while holding down the button of the desired
direction. For example, to momentarily jump to 16x in an Eastward direction, hold down the East button on the hand
unit, then press the West button. Release the West button to return to the selected guide/slew rate. It is not possible
to move in both right ascension and declination simultaneously.
A speed of 4x is recommended for autoguider
training.
PEC Operation
While the Command Center is equipped with Periodic Error Correction (PEC), the NGT-12.5's direct friction drive
eliminates the need for this feature. PEC was designed to compensate for the non-linear tracking of a worm/gear
mechanism whose period is typically four to ten minutes. Because the NGT-12.5 design has no such mechanism,
and its direct-drive period is approximately 80 minutes, PEC is neither necessary, nor practical.
Should you
inadvertently enter the PEC mode (the LED to the right of the
PEC
button is flashing or lit continuously), you may exit
to normal operation by pressing the
PEC
button once more.
TVC Operation
Declination motor backlash may be removed by using the Time Variable backlash Correction (TVC) feature. This
feature allows the user to select the duration for which high-speed stepper motor operation is applied to dramatically
reduce the delay which exists when declination movement is reversed. When powered on, the TVC duration is zero
(disabled). Pressing the
B/C
button once will light the right-most of the bar LEDs, and the TVC duration will be set to
0.03 seconds. Pressing the
B/C
button again will light the next bar LED, and increase the duration by another 0.03
seconds to 0.06 seconds. Repeatedly pressing this button will advance the lit bar LED to the left (the right-most
remains lit as well to aid in interpretation) and increase the TVC duration by 0.03 second increments. Once the
duration is set to 0.30 seconds and the left-most bar LED is illuminated, the process will reverse—additional presses
will decrease the duration. The duration is again zero when none of the bar LEDs is lit.
If reversing the declination guide direction results in a noticeable delay before the movement is observed, the TVC
duration should be increased. If instead there is a brief burst of observable movement which causes the telescope to
"overshoot" the object being tracked, the TVC duration should be decreased. When set properly, the TVC duration
should be just sufficient to provide a smooth transition when reversing declination movement.
LED Brightness Control
All of the LEDs—those on the Command Center as well as that on the hand unit—may be dimmed from full brightness
to completely off by pressing the
DIM
button on the Command Center. With each of the first eight presses of this
button, the LEDs will decrease in brightness. On the eighth press, the LEDs will be off, and additional presses will
reverse the process by brightening the display toward full.
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