Wiring the STV Control Cable through the Paramount ME
While
waiting for the Paramount ME mount to be delivered, I began thinking about
how the various cables and equipment would be installed and wired. I wanted
to run the STV camera control and video cable "through the mount". The ME has a pre-wired DB-25 (parallel) pass-through connection built
in as standard. Since I was planning to upgrade the ST-8E to USB, the
DB-25 connection would be free to use for the connection between the STV
camera head and the control box.
It seemed like an obvious solution would be to shorten the STV control
cable, replace the connector, plug the STV cable into the ME pass-through
port and connect the other end to the control box with a new length of
25 conductor extension cable. The video cable could then either be run
through the Paramount cable conduit or passed through the mount via one
of the auxiliary connectors which are wired “straight-through”. Not knowing if this would work, I discussed the possibilities with the
STV designer at Santa Barbara Instrument Group and learned some interesting
things.
As it turns out, while the STV head cable does have a DB25 connector on
one end, that is where the similarity with a standard 25 pin parallel
cable ends. The STV head cable is a special twisted pair cable that pairs
signals that are compatible with each other. The cable is also "tuned"
so CCD clock signals have the correct rise and fall time. The length
of the cable also has some effect on the position in time that the CCD
output waveform is sampled by the A/D converter. So running the control
cable through the Paramount’s 25 pin pass-through connection is not
recommended but it would not hurt the camera to try.
An alternative solution that works if you’re willing to leave the
STV camera head on the mount semi-permanently is to disconnect the cable
from the camera head, string it through the mount and reconnect the cable
on the other side. This is relatively easy and can be done without difficulty
if you mind the details. There are either one or three wires to unsolder,
depending on which method you choose to route the video cable.
The STV camera has two cables tie-wrapped together. The smaller cable
with the BNC connector is a coax cable that carries a video signal. The
larger control cable has two single row connectors on the inside of the
camera head that connect it to the headboard electronics. It also has
one soldered connection that is the cable shield wire. The control cable
connectors can be disconnected and the shield wire unsoldered. Either
make a diagram or use your digital camera to document where everything
goes before you disconnect anything. Don’t neglect to take note of
how the cable strain relief is attached to avoid confusion when reconnecting
the cables.
STV camera head and cables. The control cable shield wire is hidden.
The pins for the 3 soldered connections are indicated.
(click for high resolution view)
When removing the cable connector from the board, do so by gently prying
it off from end to end with a small screwdriver or something similar until
the connection comes free. It is important to move from one end to the
other, lifting both ends somewhat evenly. Do not pull the connectors off
the headboard by the wires as you may break a connection. The pins should
remain in the plastic connector housing. While it is important not to
pull on the individual wires, some mild force may be applied to the group
of wires as a whole.
When you reconnect the cable to the STV head electronics it is important
that the connectors go to the same place on the board they came from. So don't forget to document the connections before you proceed.
When unsoldering the shield wire, use a small tip soldering iron meant
for electronics, about 20 to 30 watts. Do not use one of the large irons
with a built in light that looks like a gun which are more suited to heavy
gauge wire work. Be careful of the small wires. Their insulation is easily
melted by a soldering iron.
The individual pins are connected to the plastic connector body but are
somewhat free floating and each pin may need to be seated to assure complete
contact with the mating connector. When replacing the connections make
sure the pins are firmly seated and use a small screwdriver or tweezers
to push down each pin so they are all firmly seated. Don't forget to re-solder
the cable drain wire.
The thin video cable which runs alongside the camera control cable can
be handled in two ways. Note that while this is a coax cable, the signal
it carries is not conventional NTSC video that a TV tuner understands. The first method would be to unsolder the video cable center and shield
wires on the inside of the camera head. This will leave the dual camera
control and video cable assembly free which can be routed through the
Paramount without having to retie the cables together. Alternatively,
the video cable could be left attached to the camera head and run through
the mount in the reverse direction from the way the control cable was
placed. This is possible because of the small connector (unlike the DB-25
connector on the control cable) which will fit through the Paramount’s
cable conduit. After you reattach the control cable to the STV head, you
could tie the 2 cables together for a neat installation.
Stay away from the option of routing the video signal through the Paramount
using one of the serial or auxiliary pass-through connections. These are
not shielded coax wires and could corrupt the integrity of the STV video
signal due to interference or the resultant impedance mismatch.
To route the STV cable pair through the Paramount, first remove the declination
housing side plate on the east side of the Paramount. The three screws
in the middle of the plate do not need to be removed as they are “hole
plugs” only. The screws that hold the cover in place are very tight. Use a new Allen wrench or one in good condition that fits tightly into
the female hex socket in the screw head. It is possible to strip the hex
socket if the wrench is loose or you are not careful. It may be easier
to overcome the initial screw resistance by tapping the end of the Allen
wrench with a small mallet, rather than applying steady pressure. Your
mileage may differ here. Keep in mind that the screw head sockets are
easy to strip requiring them to be drilled out.
The disconnected ends of the STV cables are routed through the RA shaft
nut and cable conduit at the south end of the mount. The head end connector
will fit but it may be easiest if it is one of the first cables through the
mount. The cables will exit the north end of the RA shaft within the declination
housing. Route the STV cable alongside the existing cables, making sure
to allow for a “loop” of cable so that the declination shaft
can rotate freely. Feed the cables through the access hole in the declination
shaft and out the declination hub with the Versa-Plate detached. Lastly,
arrange the cables in the Versa-Plate cable channel so there is no binding
or pinching of the cables. The STV cable can be now be reattached to the
camera head. Be sure to leave enough slack in the cable to allow positioning
the STV camera head wherever it may need to be for your setup. Keep in
mind that there needs to be some provision to secure the camera head if
it is detached from the scope or wherever it might be used as it is now
“tethered” to the Paramount. This might be awkward should you
choose to transport the Paramount for use in the field.
STV cable passes through the RA shaft nut and cable conduit.
STV cable wraps around the declination shaft within the housing.
When
you replace the back on the camera head, spread out the wires similar
to the way you found them when you first opened the back of the camera. Make sure not to pinch any wires between the camera case and the camera
back. You are now done. From the above description you can see the STV
camera head is now mounted semi-permanently.
There is a possible method of wiring the STV that would “untether”
the cable from the mount and allow removal of the camera head from the
system when desired. To do this, follow the control and video cable wiring
scheme above, but cut the cables on the north (telescope) side of the
mount and solder a high quality 25 pin M/F connector pair to the cut ends
of the control cable and a 2 pin M/F connector to the cut ends of the
video cable. You could then remove the STV head, uncouple the cable connections
and set it all aside. This should work according to the STV designer but
is untested. It would also require soldering 25 fragile wires to each
of the control cable connectors but could be done if you are confident
in your soldering skills and are careful.
Should anyone wish to do attempt these modifications, it is at your own
risk of course. No warranty is implied or offered. It is possible to corrupt
the integrity of the wire connections and not even know it or even damage
the camera electronics. Whether or not the camera warranty would be honored
should something go amiss is an open question. If this is of concern,
consider getting authorization in writing from SBIG before proceeding.
Revised 8/18/02