||Last revised: 02/17/03
Drive Base & Computer Control Circuit Board
This hardware report takes us inside the drive base of a Celestron C5+ SCTs. This
is the mechanical and electronic "heart" of the C5+ system:
sequestered in this compartment is the clock drive, power connections, hand
controller connection, and the computer circuit board that makes it all happen.
Most of what is here will probably be relevant to the base C5 model (if anyone
has images of the equivalent internals of the C5 -- please email
them for inclusion in this report).
The long-running Declination
Drive thread on the message board is the impetus for this article. A simple
question about a replacement Declination motor evolved into a discussion on the
functioning of the hand controller, Pulse Width Modulation, and features of the
controlling chip. The Celestron 5" Usage Notes section on the Hand
Controller summarizes these functions and is a must read to understand all
the tracking features (some not documented) available on your model
The end of this report contains oscilloscope traces of the digital signals
sent to the motor drives.
Clicking the black and white images below will open a pop-up with an enlarged
color version of the image. Highlighting the black and white photos will display
a negative image that may show more detail on your screen.
Drive Base Cover Panel
Panel Items From Left to Right
9-volt battery compartment (large rectangle)
Hand controller plug (telephone jack)
Declination drive power output (mini-jack)
12-volt alternate power input (mini-jack)
Master power switch
Power-on, red LED above power switch.
Note that you may want to explicitly label the 12-volt alternative
power mini-jack on your cover plate as input. The existing
"12VDC" label is insufficient, since it is not unheard of for a
telescope base to have an accessory power output.
Drive Base Panel Removed
Note that the 4-screws at the extreme corners of the cover plate were
removed to lift-off the cover plate. The 4 interior screws visible on the
base's cover plate (previous image) hold the circuit board in place.
This exposes the Right Ascension clock drive in the base compartment
and computer circuit board on the back (right side) of the Base Panel. The
exposed compartment is mostly empty.
Items of Note
metallic cylindrical item on the left side of the base compartment
is the Right Ascension clock drive.
The black telephone cord from this drive leads back to the base panel and
terminates at a plug on the computer circuit board.
Power wires from a circular hole in the 9-volt battery compartment are
visible on the left side of the base panel. They connect to and power the
computer circuit board and clock drive.
"Front" of Computer Circuit Card
Note that what I call the "front" of the circuit card is
aimed toward the ground when installed on the telescope base. I state this
because it is important not to mix-up the Declination drive power output
and 12-volt alternative power input mini-jacks -- they look the same and
are unlabeled on the circuit board.
Items of Interest
The master power switch and red LED are visible on one side.
Along one edge of the card (opposite the big Tangent chip) are the
The face-on telephone jack is for the hand controller.
The telephone jack with the wire prongs showing is the connection to
the Right Ascension motor drive (see prior image).
The cards is dominated by the large Tangent chip. Based on the state of
the buttons on the hand controller, this determines the power pulses going
to the Right Ascension and optional Declination drives. Different versions
of this chip give the C5-series telescope different abilities -- not
always stated in the telescope instruction booklet.
"Back" of Computer Circuit Card
Note that what I call the "back" of the computer circuit card
is the side first visible when the Drive Base Cover Plate is removed and
flipped over -- see "Drive Base Panel Removed" image.
Items of Interest
Black circular pairs of split-prongs anchor the two telephone jack
plugs -- for the hand controller and Right Ascension drive.
The parallel wire loops on one side of the card are from the 9-volt
battery power supply.
Various manufacturing serial numbers are visible on the card.
Computer Circuit Card "Back Flip" View
This image of the "back" side of the computer circuit card is
flipped to show the circuit patterns as they would appear looking through
the front of the card -- as if you had Superman's X-Ray vision!
This allows easy comparison of the components on the "front"
view with the circuit pattern on the back.
Click the image on the left to bring up the enlarged color version of
the image. Moving the mouse over and away from the image will
"blink" the opposite sides of the circuit card.
Pulsed Power Traces
Below are some graphs of the pulsed signal sent to the Right Ascension and
Declination motors. Voltage is on the vertical axis and time on the horizontal.
These oscilloscope generated drive wave-forms were kindly sent in by Kim Thomas
-- the originator of the Declination
Drive thread. They were made with an older model C5+. The hand controller
only allowed for the most basic motor controls.
Pulses to Declination Drive
The optional Declination drive is normally called on to correct small
drifts in Declination when image guiding. It is either positively or negatively
tracking in Declination -- as selected by the North/South Hand Controller
Pulses for Tracking South (-)
Pulses for Tracking North (+)
Signals pulse from 0 to 5-volts. The sign of the voltage gain depends
on the tracking direction.
According to the oscilloscope readout, a pulse cycle takes about
1.5-milliseconds -- so there are 666 pulse cycles in one second of time --
a cyclic rate of 666Hz. Kim notes that this is "way too fast for any
conventional motor" (this writer doesn't like it because it's the
"number of the beast"). It may indicate a problem with the controller
chip, since Kim was having difficulty getting a Declination motor to work
with the setup. (I'd be interested in seeing the trace-out from a working
Declination drive setup.)
The voltage increase lasts 1/3 of a cycle (0.5-milliseconds) and the
remaining 2/3 of the cycle (1-millisecond) shows the voltage as off.
That's why the readout show skinny rectangles.
Pulses to Right Ascension Drive
This Right Ascension drive turns on with the Master Power Switch. It is
fed pulsed power to allow it to track the stars at the sidereal rate. This
rate can be sped-up (positive, west) or slowed down (negative, east) for
fine image tracking.
normal tracking the signals pulse from 0 to 1.3-volts. The cycles
last about 14-milliseconds -- about 70Hz. This allows the
drive to track at the sidereal rate to follow the stars.
the east button on the hand controller retards the drive by
generating a 0.9-volt signal with a 21-millesecond cycle --
increase tracking speed in the direction of sidereal motion,
the west button is pressed. This generates a 1.9-volt signal
with a cycle of 10-milliseconds -- about 100Hz.
Drive Normal Tracking
Drive Eastward Drift Tracking (-)
Westward Drift Tracking (+)