Power cabling
A constant source of annoyance in my earlier telescopes was the need to supply power to, and to retrieve data from, a rotating body (the telescope) while the power sources and computers were fixed on the observatory floor. I had solved most of these problems in this telescope by mounting the control computer on the telescope az table, leaving one cable to carry power to the scope and another needed to connect the az table to the remaining moving part, the pitch/roll head.
Getting AC power to the electronics on the az table still required a power cable. Because I knew there would be a lot of slewing involved in the data collection, I wanted the scope to always take the shortest path to the next target. This meant that the power cable would inevitably wrap too tightly and cause az motion problems. I tried many cabling tricks, but all had some eventual flaw and the az axis would bind.
In my last work assignment before retirement, I had worked on a telescope that had complete az freedom. It achieved this by using 'slip rings' to transfer power to the az motors. I decided to look into this solution for my scope.
Here is an example from the wiki page on slip rings:
Here is the explanatory text for that image:
Slip rings on a hydroelectric generator:
(A) stationary spring-loaded graphite brushes touching both moving rings
(B) - upper rotating steel contact ring, the second, isolated, ring below it corotates with the upper ring
(C)- insulated connections to generator field winding
(D) - top end of generator shaft. Credit to Mr. Wtshymanski.
Note that there are two rings and two white wire bundles. Each bundle is connected to a different ring. Both rings rotate together as a unit. The right bundle is electrically connected to the top moving ring, the left bundle is connected to the lower moving ring. One bundle is the input power, the other bundle is the return. The bundles, and the rings, turn with the rotating mechanism. There are two sets of brushes, one for each ring edge. The brush sets are stationary and are connected to power and return (not visible in this image).
I ordered a large bearing, used for rotating display tables, and took it apart to provide two large, accurate, metal rings. I also ordered graphite motor brushes for a Dremel tool (in the plastic package in the center of the ring).
The larger metal ring was epoxied (carefully concentric) to the underside of the scope electronics az plate. This ring would be on the return side, so no voltages should appear on it. The epoxy provides some electrical isolation. The plastic insulating ring (machined) was then bolted to the larger ring, and the smaller metal ring was then bolted to the insulating ring. All bolts were carefully placed to avoid electrical paths except the two that were used as connectors (wires attached). The white wire would bring in power and the black wire would be the return.
Note that the scope was hanging upside down, suspended on a wooden framework. The final slip ring assembly looked like this:
The final element was the set of brushes to transfer current from the fixed telescope mounting plate to the rotating rings. A pair of brushes was used for each ring, mounted on flexible brass fingers. The pairing provided a balanced mating force to the ring. The upper ring and lower rings each had their own brush set, and these brush pairs were staggered in height to match the rings they contacted. White wire for power, black wire for return. The slotted mounting plate allowed the brush contact pressure to be adjusted.
The brushes in contact with the rings. Slightly out of focus.
The slip rings did not work well at first. I had moved the scope power brick from the az table back into the observatory housing. This allowed me to pass only the 12V from the brick onto the slip rings and then into the scope. Operation was very intermittent, and the scope would often fail to receive power. I tried putting a set of super capacitors on the az table as a way to get the rings over momentary loss of contact, but this, too, failed. I finally decided that the 12V input was not enough to overcome surface irregularities or small non-conductive patches on the aluminum ring surfaces.
So I placed the power brick back on the az table and fed 110AC into the slip rings, which then fed the brick. Never had a problem again. I am still wary of a large ring carrying 110AC, so I was careful to pull the brushes out of contact when servicing the scope. The 'live' ring is well hidden out of finger's way, deep under the scope main az table.
These slip rings have worked out extremely well, with the az axis able to turn tens of rotations in the same direction without any complications.
Here is a video of the scope moving in azimuth and altitude.