Ad Astra Observatory

My mount, 04026, and I have had a pretty good run together. I had to swap out the motors and brain a couple of years ago, when the main motor controller board went out. So I switched from the stock Takahashi Temma 2 system, to the Astro-Electronic FS-2 system. This required changing out the motors, drive gears (but not the worms), main board, and power supply.

The Temma 2 system came with a 24v power supply, looks a lot like the 48v one I bought to drive the CNC machine. The FS-2 didn’t come with a supply, but it is rated from 9-30VDC, and I had a spare 15v power supply hanging around, from back in the Losmandy Gemini days. I pressed the 15v supply into service, and the FS-2 ran happily, although the top slew speed was a little slower than I’d like.

The NJP is not a high-speed slewing mount; with the Temma and its 24v supply, it topped out around 350x sidereal (compared to more like 700x with the Gemini), a “stately” rate of speed. With the 15v supply on the FS-2, the drives seemed to top out around 200x.

So I’d always had it in the back of my head that I’d like to get a bigger power supply for the FS-2; I didn’t use the 24v supply from the Temma, because the FS-2 has banana plugs and the Temma has a barrel plug, and I wanted to leave it intact, rather than modify it (leaving it unable to be used for the Temma, should I ever need it or want to sell it). Not to mention, if I was going to the trouble of modding the power cable, I’d rather do that for a 30v supply than a 24v one.

I finally got around to looking for a 30v supply. I wasn’t entirely sure how much current the motors draw, as I’d never really seen them under heavy load, &c. The manual for FS-2 says they draw between 0.5A and 5A, so rather than buy a 30V/5A supply and possibly run out of headroom, I found a 30V/10A supply on Amazon.

I was a tiny bit concerned about quality control, so I opened the case up and looked at the innards, before plugging it in for the first time. Everything looked pretty good in there, no obvious evidence of poor soldering. I also read the manual carefully, to make sure that I understood how to test it; it turns out that I’d already been using a “best practice” without even knowing — you always want to turn on (or off) your power supply with no load attached, let it come up to speed, and then turn on the load. I follow a “count-to-5” sequence when booting up, pausing for a moment in between flicking each switch, and turning things off in the reverse sequence that I turned them on. This means that I’m doing exactly what the power supply suggests; make sure that the motors are disconnected before shutting off the supply, and making sure the supply is up before turning on the motors. Woot!

I turned the supply on and let it run for 10 minutes for the first time, before testing it (as suggested in the manual). The supply came up in “constant voltage” mode (which is what I wanted), and maxed out with the display reading “31.9” volts. Once I was ready to test, I set the voltage so the display read “30.0” volts, then checked it with a DMM, 29.5v. In order to get the DMM to read 30.0v, the supply had to read 30.4v.

I spent a little time online, figuring out what kind of input voltage my oscilloscope can handle (it’s a DSO Nano, a very cool little pocket-sized scope), and 30v was well within its tolerance (look it up yourself), so I hooked the o-scope up, and noticed that there’s still a little ripple in the otherwise constant voltage supply, on the order of 0.2v it seems like. I spent a lot of time looking at min voltage and max voltage and RMS voltage and average voltage, before deciding that I actually liked the idea of having a little headroom between the max rating of the mount and what the supply was putting out, so I set the supply to read “30.0” volts, noted that the max voltage was ~29.6 and min ~29.4, and called that good.

I spent a little time, at this point, breathing deeply and thinking through whether I felt ready to plug in the mount. I re-checked the label on the FS-2 box, which definitely says 9-30v (said the same thing on my invoice — there are different voltage regulators available in the FS-2, I got the 30v one specifically).

I plugged in the mount, did the whole count-to-5 thing, and booted it all up. The mount started right up, with a slightly different tone coming out of the RA motor (which runs at 1rpd all the time) than I am used to. I had previously changed the tones coming out of the FS-2 motor by tweaking the motor frequencies, so I looked in the FS-2 manual, and found that there’s one setting in particular that needed to be modified for higher supply voltages. Tweak the setting, and the motor went back to (close to) the nearly-silent tone it had before, and I called myself happy with that.

I did a few test slews, and the mount and supply both seemed happy.

Stepper motors (like the ones that drive the mount) are interesting — they gain torque the more voltage you apply to them (up to a point 🙂 ). So I knew that I could probably push the mount a little faster than 200x sidereal.

I tried 250x, no problem. 300x (this is faster than I’d ever achieved so far with the FS-2), no problem. 350x (matching the Temma), no problem. 400x (heart in the throat), no problem.

At this point, I was in untraveled territory, so I decided to see how high I could go. The mount’s only carrying Pumpkin at the moment, so the load is pretty light; I may have to modify the settings later. I kept stepping up the speed by 50x until I ran into the upper limit at 550x! I set it back to 500x sidereal, and did a nice long slew to confirm the mount is happy.

The motor current never exceeded 1.0A, even with both motors running at 500x.

All is well with 04026, and she slews over twice as fast as she did with the 15v supply (which I also tested on the DMM and oscilloscope, and which only puts out 14.4v).

Bring on the clear sky.