As mentioned before I decided to switch from the Gemini 2 controller which I had plenty of experience with to the Sideral Technologies Servo II. I really wanted a controller that supports High Resolution shaft encoders. I had almost gave up on using them when I realized that the Servo II already had this feature. I modified my mount design slightly so I could install a 100mm ring and read head on the RA. There wasn’t much I could do to the DEC considering I already had quite a bit of parts made.
I have some regrets there… if I had been 100% sure that I would eventually find a controller that supports them, I would have designed both shafts to support 400 and 300 mm rings… Oh well. It’s a small ring but it still gives me more than 6.3 Million ticks per revolution – or about 4.9 ticks per arcsec. Which means that while tracking at sideral speed, the controller will read just about 5 ticks per seconds. That’s pretty good, but quite a bit lower than what SiTech recommends when using their Cascade mode. I guess we’ll know soon enough!
OK, so the back panel is at the back of the RA housing. It opens to the RA servo motor and its encoder line receiver, the gear and worm of course and behind that is the AC/DC PSU (which we can’t see).
Anyway, I spoke about cable management in a previous post: I want each and every cable that connects to anything (power and data cables) attached to the telescope (imaging camera, AO, guiding camera, focuser, mirror fans, etc.) would go through the mount. Well, most of these will require both power and data… add the control cable for the DEC servo motor. This won’t fit in the shaft and even if it did, it would then be probably impossible to do this properly.
So first, I decided to get only equipment that would run off 12 V DC. That’s actually not too difficult, most stuff runs of 12 V DC. Next, I designed a small “power” hub, which is fed with 12 V DC from a AC/DC PSU installed in the mount’s RA housing. I will make a post just for this practical little thing later. Then, I found a good USB hub that runs of 12 V as well. I chose a USB 3 model which are supposedly much less sensitive to using extensions and more importantly if down the road I get a camera (or something else) that runs on USB 3 I will not have to take the whole thing apart. With this setup I’ll have only 4 cables going through the mount: 12V DC, USB 3, DEC servo motor and ST4 (I probably won’t use it but I’ll probably run it anyway as it’s pretty small).
The SiTech Servo II controller is installed on the back panel of the RA housing which can be open easily. On the outside, I’ve got a first USB 3 hub which feeds the Servo II controller and the hub that will be installed on the telescope.
On top of what I just described, you should also see on the picture a Serial Port DB9 which is used to connect the Renishaw optical encoder to the SiTech controller. That plug doesn’t actually connect directly to the controller. It first runs through some electronics to convert the quadrature signals from the encoder (A+/A-/B+/B-) to “cleaner” signals (A/B) which are read by the controller. From what I gathered on the SiTech Yahoo group, this is apparently not a required step… In this post, I was testing the servo motor to the Gemini 2 controller – It didn’t work as is and I had to come up with this Line Receiver PCB so that the controller could read the servo motor encoder signals.
The picture above shows the two types of line receiver electronics I made. The one of the left is for the servo motors (one per servo motor) and the one on the right is for the Renishaw optical encoder. Notice how similar the two PCB’s are. Well, the only differences are the connectors, everything else is the same!