14
Low Voltage Sensor
The microcontroller is continually measuring the power supply's voltage to ensure that adequate power is available to
operate the tracking motor. If the voltage should drop below approximately 9 volts, the current tracking rate LED will
blink from full brightness to off, twice each second. Note that continuing to operate the unit in a low-voltage condition
may result in a loss of tracking accuracy, as the right ascension motor could miss step pulses due to insufficient
current.
CCD Autoguider
An autoguider (such as the ST-4 from Santa Barbara Instrument Group) can be used to control the Command Center
by connecting its relay output to the
HU/CCD
jack. If not provided with your autoguider, a cable suited to this purpose
is available from JMI. For simultaneous manual and autoguider correction, a Y-adapter may be purchased from most
electronics or telephone supply centers (Radio Shack #279-357).
WARNING:
A relay adapter box is needed between the newer autoguiders with a digital interface (e.g. ST-7,
ST-8, ST-9, ST-10, PixCel 255 and PixCel 237) and the NGT. It provides complete electrical isolation
between the autoguider and the NGT. A relay box can be purchased from SBIG dealers. Failure to use a
relay adapter box between an autoguider with digital interface and the NGT will void the warranty. No relay
box is needed for the ST-4, ST-5 and ST-6.
If You Suspect Improper Tracking
If you suspect that your NGT is not tracking properly, check the following items.
Polar Alignment
—If the telescope is not properly polar-aligned, it will not track properly. See the instruction manual
for your computerized setting circles.
Adjusting for Latitude
—To adjust the latitude, loosen the four zinc plated rocker clamps on the base and turn the
adjustment screw.
Since the ring/tub assembly can be easily removed and replaced, it is recommended (and is
easier) to remove it before making the adjustment.
Hemisphere DIP Switch
—If this internal switch is set incorrectly, the telescope will be driven in the same direction as
the Earth's rotation, making the sky appear to drift at twice its actual rate. To change this setting, remove the four
screws from the sides of the Command Center, remove the face plate, and find the switch located near the lower left
corner of the circuit board (labeled SW8). Use a knife blade or other small instrument to throw the switch to the
opposite setting.
Clutch Tension
—If clutch tension is too loose, the drive ring may slip rather than be driven by the R. A. motor. Turn
the clutch tension adjustment knob clockwise to increase tension and counterclockwise to decrease tension. Set the
tension loose enough to permit the telescope to be moved manually, yet tight enough to allow the drive motor to
function properly.
"Racking" the Motors
—Due to the unique dual-motor design of the right ascension drive system on the NGT-12.5,
the two motors can occasionally develop some slack or backlash between them.
This will appear as a slight
hesitation, after pushing one of the hand unit directional buttons, before the R. A. axis starts moving. The backlash
can be removed by using the following procedure. There is a spring-loaded switch protruding from the vinyl cover of
the right ascension motor. While holding the switch in the engaged position, run the motor west (or faster) at 16x for
about 10 seconds. This should remove all the slack between the motors. If hesitation still exists, repeat the process.
We suggest that you rack the motors for a few seconds before every session to eliminate any symptoms.
One way to reduce slack buildup during observing is to loosen the R. A. clutch slightly before manually moving to
another position then re-tightening after acquiring the object. If symptoms of backlash begin to appear during an
observing session, you can reduce the effects by using the right (west) button on the hand unit to center an object in
right ascension. If you need to use the left (east) button, move the object past center and then re-center using the
right (west) button.
Grease
—Over time, grease can accumulate on the drive roller. Clean the drive ring surface and drive roller with
alcohol as necessary to maintain proper friction.
!
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