I've been working on motorizing my 18" Obsession for a couple of years now.
I was basing the drive system on Mel Bartels' software driving two Oriental
Motors steppers. The going was very slow as I have limited tools and even
more limited shop space. When I got to the point in the project where I had
the drive mechanisms ready and needed to buy a laptop to run the system, I
heard about ServoCat from RXDesign. This is a servomotor drive system that
is coupled with a Sky Commander or an Argo Navis. The thought of avoiding
the laptop with its associated cabling problems and power limitations was
irresistible. This is an account of the purchase, installation, and first
use of the ServoCat.
Purchase transaction
I called Gary Myers to order the system and was surprised to find out that
there was a six-week lead-time‹business must be good. The unit was shipped
when promised and my credit card was charged at the time of shipment. The
Obsession version of the ServoCat costs $1,500.
The unit arrived in one box, carefully packaged to protect the electronics,
servomotors, cables, hardware, and one CD. The drive components were
packaged in plastic bags with a label identifying them as Alt or Az drive
components. The cables came in a third bag with a picture identifying them.
A detailed packing list (down to the smallest screw) and an invoice were
also included.
I don't want to bore you with such minutia, but I'm tired of flaky vendors
that are sloppy and unresponsive. This guy is meticulous!
Installation
Installation took two days, mainly due to having to modify both the ground
board and altitude bearings. I had already made a new ground board for my
telescope and redesigned the azimuth pivot; the ones that came with my
Obsession were totally unsuitable for a drive system. I had also made new
altitude bearing for the telescope, considering that the original cast
aluminum units were not round and could cause tracking errors. Because the
previous modifications were done for another drive system, I had to cut the
ground board to a smaller diameter and add spacers to push out the altitude
bearings to get them centered over the rocker box sides. Note that you
really don't have to redo the altitude bearings in your Obsession; this was
more my pickiness. You will, however, have to change the ground board as I
don't think Kriege ships units with a ground board and azimuth pivot that
will work accurately (my telescope is about 5 years old, maybe he has
improved this). RXDesign has foreseen this problem; they sell ground
boards.
Assuming you buy a ground board and use the factory bearings, installation
should take a day, including routing the wires, testing, and adjustment (at
a very leisurely pace). The installation instructions are clear and provide
ample detail to prevent you from ruining the telescope. You will need a
good drill and drill bits. A helper makes this process much faster, though
I did it alone.
First run
If there is something I won't tolerate from a system like this is to spend
the night fiddling with wires, making adjustments, or fixing problems. I
was thus somewhat apprehensive when I arrived at Plettstone not having
tested or adjusted the system. My wiring was not finished as I was still
undecided where to put the Argo Navis (digital setting circle) and the hand
pad; I had wires hanging from the front of the telescope. I was also
concerned that I had not adjusted the final gear ratio parameter (set in a
configuration file) and that the unit wouldn't track accurately. I knew
that the motors moved the scope but I had done this test with the scope
disassembled so I could only move it a few degrees in altitude.
I assembled the telescope as usual, dropped a 7 amp-hour battery in the
rocker box and waited for twilight. After aligning the Argo Navis with two
stars and verifying that it accurately found objects I engaged the drive
system and did a GOTO. The telescope glided smoothly to the object. I then
did a GOTO to another object. The telescope moved smoothly to this southern
target and began tracking again. I left the telescope on this object and
came back half an hour later to find it still in the 20-minute field of the
eyepiece. I used the system for two nights without problems, and I love it!
About the system
ServoCat, as the name implies, uses servomotors. They have abundant torque
to move the 18" Obsession and did so very smoothly. A lever for each of the
altitude and azimuth axes engages and disengages the drives. The azimuth is
driven by a knurled pinion that is held against the edge of the ground board
(just like in the Starmaster system). Pulling the gearbox/motor unit away
from the ground board disengages the drive. A cable attached to one of the
altitude bearings drives that axis. The cable goes from one end of the
bearing to the other, making two wraps around the motor drive wheel. To
allow the cable to pass under the bearing, a channel is cut in the middle of
the Teflon pads (these are supplied). The altitude drive works by applying
tension to the cable, forcing it to wrap itself around the drive wheel (very
little tension is required and no slippage was detected).
Before I bought the system my greatest objection was the altitude drive. I
felt that even if tension was removed, the cable's stiffness and the fact
that two wraps had to glide smoothly over the drive wheel would create
enough drag that it would ruin the smooth feel of the altitude drive when in
manual. I actually did some test with bicycle brake cables and convinced
myself that it would be ok. It is ok; when the drives are disengaged the
Obsession returns to its silky DOB mode.
The drive works in conjunction with digital setting circles. I use 8,192
encoders from US Digital, which have approximately twice the resolution of
the typical encoders. I don't know that you need these, but I thought they
would be more accurate. Be warned, though, that if you use the higher
resolution encoders you can't use a Sky Commander. I tried it with
disastrous results and had to sell it. The Argo Navis, having more
processing power, does handle the higher resolution without problems. The
Argo Navis, except for its large size, is a wonderful unit, but I don't want
to get into a review of that.
You do a GOTO by searching for an object in the Argo Navis and doing a GOTO
in that unit. That doesn't move the scope it just calculates how much the
scope needs to move to get to it. Once the Argo Navis reports the offset to
the object, push on the GOTO button on the ServoCat hand pad and voila.
To manually slew to an object you use the pushbuttons on the hand pad.
Three switch-selected speeds are available: slew, jog, and guide. Each of
these speeds can be set by the user, along with acceleration, backlash
settings, and overall gear ratio. These configuration parameters are
downloaded to the unit via a computer and do not require updating, unless
the user has a change of preferences. In addition, there is a switch in the
front of the ServoCat unit that allows the user to select from two different
parameter settings. The result is you can have two sets of speeds: slew1,
jog1, and guide1 or slew2, jog2, and guide2. You can select which set you
want to use by flicking a switch while the unit is powered.
The unit has two spiral search functions, each operating at different
speeds. If you do a GOTO, and don't see the object in the field, you can
press the spiral1 or spiral2 button and the unit will go into a spiral
search until stopped.
When working in a zone of the sky it is possible to do a GOTO to an object
and do a SYNC to that object in the ServoCat. This then increases accuracy
of GOTO within 5 to 20 degrees of the first object.
I could talk about other features but these are the most important for
normal use.
Things I didn't like
RXDesign expects you to have a power supply that you can switch on or off,
or to simply disconnect the ServoCat, so they don't provide an on/off
switch. I think a unit of this complexity that is plugged in to a power
supply along with other electronics should have its own power switch.
The spiral search speeds are way to fast for high power searches. I used
the slowest to try to find a faint galaxy in a 20-minute field at 230x and
it was just too fast to be useful. There is no way for the user to change
this search speed, so for deep sky work it is pretty useless. I exchanged
emails with Gary Myers of RXDesign about this and he seemed to be very
receptive and open to coming up with a solution. We'll see.
The pdf manuals on the CD are well written, though they appear to be
representative of the evolution of the system. Aside from the manual for
the Obsessions I wasn't sure which other manuals were applicable to my
situation. In the end I found that 3 manuals were relevant. I've heard in
the ServoCat Yahoo group that RX Design is working on a consolidation of
these.
One amateur that saw the system commented that it wasn't "elegant". This
refers mainly to the gearbox/motor assemblies. The gearboxes appear to have
hefty shafts and I had no problem with backlash, but the overall machining
of the gearbox enclosures looks basic. The boxes are not sloppy or flimsy
in any way. They have thin covers held by screws at each corner, and the
covers do not match the body of the box precisely, giving the impression of
not being very sophisticated. Compared to the units that Starmaster puts
out, I agree, these aren't very elegant. However, beauty is in the eye of
the beholder, and to me it is that the system works smoothly and seamlessly.
The lever that engages and disengages the altitude drive sticks out from one
of the truss tube blocks of the 18" Obsession. This is not the case with
the 20" or bigger scopes as the altitude bearings are closed and the lever
to tension the altitude cable is installed on the diameter section of the
bearing. If the lever for the 18" really bothers someone they can easily
mount a block on the side of mirror box (toward the middle of the bearing)
and attach the lever there. I don't think this is a big issue.
Conclusion
I am really happy with the system. I've only used ServoCat for two
observing sessions and need to gain more experience with it but, in my
opinion, it is the best GOTO system for the Obsession on the market today.
I'm sorry I don't have photos, but you can check out the vendor's web site
or come over to my scope at Calstar and see it in action. You won't miss me
I'm the guy with the little, white Wells Cargo trailer. If you have
specific questions, contact me personally so we don't clog the listserv.
Guillermo Ortiz