This page will go through the steps of modifying a game joystick so that it can be
used with the Compustar computer. It is so much easier to use a joystick instead of the
computer's motion control keys because a joystick has proportional control over the motion
of the telescope. That means that the telescope will move faster the more you deflect the
stick. You can count 30 or more different speeds as you advance the stick. This
proportional control makes it particularly easy when performing finer movement. There will
be no need to make multiple quick presses of the motion control keys to position the
telescope where you want it.
I believe that the greatest use of the joystick is realized when you are looking
through the eyepiece. Before I had a joystick I spent a great amount of time correcting the
telescope movement because I pressed the wrong motion key to start with. The joystick can
be held backwards and/or inverted so that your hand movement is intuitive as you move
through the star fields.
The original Celestron joystick was simply a joystick with
two buttons. One button was to toggle between the "SET" and
"SLEW" speed rates, and the other button was the same as
pressing the "SYNC" command key. The joystick that you choose
does not have to be any more elaborate than that. Today's game
joysticks are much more sophisticated. You may in fact find it
difficult to locate a new but simple joystick. I looked in six
different stores before I found the one in the photo. I
purchased it at a discount department store for nine dollars.
Another thing to look for are trim controls. These
controls will make it easier to set up the joystick so that you
can achieve maximum speed in all directions.
The inside of the joystick is also important. Most
joysticks of this type are basically the same inside, however
some are designed poorly. I located an old TANDY joystick at a
computer surplus store that looked promising, but when I opened
up the bottom, a couple of large springs snapped like a mouse
trap and small parts were launched across the room. I still
haven't found them all.
This particular joystick is very adaptable to modification. It has nice large linear
potentiometers (Pots) with tabs that you can solder wires to. The red circle in the left
photo shows the "X" axis Pot pulled out of its holder. It also has a Printed Circuit Card
(PCC) that is used as an interface between the internal wiring of the joystick and the
wiring of the cord. This PCC will be great for connecting a new cord for the Compustar.
The Compustar computer has three DB-9S connectors located on the front edge. The
basic Compustar has nothing connected to any of these connectors. The connector on the
right is for the joystick, the centre connector is for an Auto-Guider accessory and the left
connector is an RS-232 computer interface.
Unless the joystick you have has a
male DB-9 connector already on the end of
the cord it will have to be replaced with
one so you can plug it in.
As you can see in the schematic, the
DB-9 connector will use 8 wires coming from
the joystick. Unless the cord on the
joystick has at least 8 wires in it then
the whole cord should be replaced.
It is possible to get away with as
few as 6 wires in the cord going to the DB9 connector. The schematic shows that Pin
#6, #7 and #9 are actually connected
together. These three pins can have
jumpers installed at the connector and use
a common wire through the cord to the
joystick. It all depends on the
manufacturer of the connector as to whether
this is easy or not. I feel that it is
easier to change the whole cord and make
all the connections inside the joystick
where there is much more room.
You can see that this joystick has a
connector that is compatible with a
gameport. It will have to be changed for
the DB-9 connector on the right.
Even though the gameport connector has 15
pins, this joystick only has 6 wires in the
cord. Due to this fact, I have decided to
change the whole cord along with the connector.
I was able to pick up a four foot 9 pin
extension cord from a computer surplus store
for $2.99. It will be just right to replace
the cord on the joystick.
I cut off the female connector on the
extension cord. Then I stripped back the
insulation. In order not to have any problems
with connecting the new cord, I performed a
continuity check for each pin to determine the
corresponding wire. You should write down
which colour wire corresponds with the numbered
pins on the connector. This will help when it
comes time to connect the cord to the joystick.
If you do not have a multi-meter or cannot
borrow one, you can still easily perform a
continuity check by using a battery and light
bulb. Select a wire and then check each pin
until the circuit is complete. Using this
method, the photo shows that it is the Red wire
that corresponds with Pin #3 on the connector.
The next step is to cut off the old cord
and clean up the holes in the PCC so you can
solder on the new cord.
This joystick was originally wired using
only a single side of each Pot. If you refer to
the schematic you can see that the Compustar
joystick needs to be wired on both sides of each
Pot. In the photo you can see were I pulled out
both Pots from their holders. The red circles
indicate were I soldered new wires onto the tabs
of the sides that were not used.
The Circuit
If you study the schematic you can see that
it is actually fairly simple. There are two
Pots, one for RA control and one for DEC control.
Each Pot is connected to same ground on one side
and voltage on the other side. On each Pot there
is another wire connected to the part that slides
across the resistor. This wire will also have a
voltage reading, but the voltage will vary
depending on where it is along the resistor.
These wires that have variable voltage are
connected to Pin #1 for RA control and Pin #3 for
DEC control.
When you push the "ABORT" button on the
Compustar computer it will carry out the abort
command as normal. It will also look at Pin #1
and #3 to see if there is voltage present. If
there is that means that a joystick is connected.
As soon as those voltages start to change up or
down that means that you are moving the joystick
and the computer will tell the telescope to move
depending on the voltage changes.
There are also two pushbuttons in the
joystick circuit. Both pushbuttons are connected
to ground on one side. PB1 is the pushbutton
that acts like the "SYNC" command key on the
computer and is also connected to Pin #2. PB2 is
the slew pushbutton and is also connected to Pin
#4. When PB1 is pressed it is basically touching
Pin #2 to ground. This is the computers signal
to carry out the "SYNC" command. Likewise when
PB2 is pushed it touches Pin #4 to ground and
this signals the computer to toggle between the
"SET" and "SLEW" speed rates.
Re-wiring the Joystick
20. In the schematic, if you follow the conductor from Pin #9, you will see that it is
connected to Pin #7 and #6, one side of PB2, one side of P2, one side of PB1 and one side of
P1. In the photo above you can see the original joystick wiring to the PCC. The four wires
at the top go to the Pots and the Blue and White sets of wires go to the fire buttons in the
handle. The arrows indicate that the circuits on the PCC show one side of each button is
already connected together. Also there is a place
at the top were two other wires can be connected
together. These circuits on the PCC can be used to
help connect all the ground connections together.
Inside the joystick, the Red and the Orange
wire are connected to the variable tabs on each Pot.
These have to be kept isolated so they can be
connected to the wires in the cord that correspond
to Pin #1 and #3. In order for the Red and Orange
wire to be on independent circuits, I will switch
the Orange and Brown wires on the PCC.
In the photo above you can see that all the
ground connections are completed. The Green, Blue
and Gray wires from the cord correspond to Pins #6,
#7 and #9. The Brown and Yellow wire go to one side
of each Pot, and you can see a jumper wire that
connects to the PCC circuit that is connected to one
side of each button.
The photo to the right shows the remainder of
the cord connections completed. The table below
outlines the connections that were made.
Colour of
wire in
Colour of
wire in
Pin Number
RA Pot
Voltage to
both Pots
The last wire in the cord is the Yellow wire that corresponds to Pin #5 and is not
used. I just bent the end over and put on some shrink tubing to insulate the end.
Once your joystick is competed. You will likely have to make some adjustments to the
trim controls. To set up the joystick, power up the Compustar, plug in the joystick and
push the "ABORT" command key, this will activate the joystick. Push the button and select
the "SLEW" speed rate. The "SLEW" speed rate is better to set up the joystick with because
it is much easier to notice differences in speed rates as you deflect the stick. Next,
fully deflect the stick in RA from left to right and notice if there is any difference in
the speed rate between the two extremes. If there is, centre the stick and move the trim
control for the RA Pot in the direction that was the faster of the two. The telescope will
start moving in that direction as you adjust the trim control. Push the "ABORT" command key
again and this will reset the joystick as if you had just plugged it in. Repeat the
adjustments to the RA trim control until you can achieve full speed in both directions.
Once the RA trim control is set, carry out the same procedure with the DEC control.
The actual resistance value of the potentiometers in the joystick is not as important
as being able to use the whole range of the variable resistor. The Compustar computer is
supplying the voltage to the joystick and it uses the full range of voltage from "short to
ground" to "full voltage" for its movement signals. I found this out when I used a joystick
gimbals assembly from an old R/C transmitter for my first attempt at this project. These
joysticks only use a small portion of the overall range of each potentiometer, so I was only
able to achieve about 1/3 speed in any direction.
Good luck with the project and clear skies to all.
Cheers, Ken Whitnall from Ottawa, Canada.
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