Erratic behavior, possibly contact related


M. Collins
 

  I'm part of a group at Lowell Observatory tasked with acquiring a new 1 meter-class telescope and instrumentation to replace one of our older research telescopes which is no longer viable to maintain. One of the associated activities is an assessment of sites available for the new telescope. For that, we have some observers using 14" SCTs with a mask that allows light to pass directly through a ca. 80 mm aperture on one side of the secondary, with a second aperture opposite that has an optical wedge in place to divert the light passing through slightly. At the focal plane, a double image is produced. Each image is affected by seeing effects in the atmosphere, resulting in small differential movements of the images with respect to each other. By measuring the amplitude of these movements as a function of time, a quantitative measure of seeing quality can be obtained. For our tests, two telescopes are operated simultaneously at candidate sites, each recording sequences of images of second magnitude and brighter stars. The data sets collected are later processed using analysis software and the results compared to determine which site provides better seeing on average.

  One of the mounts was the original Celestron fork driven using a synchronous motor and variable rate drive controller. It's not too bad so far as it goes, however there is sufficient periodic error to make sub-frame imaging difficult. Also, the mount does not lend itself to remote operation. I have a 1200GTO mount which has been in storage for a couple of years awaiting a new home, so I volunteered it for the duration of the seeing tests. It was a simple matter to fabricate an adapter plate to go between the mount and the C14 tripod, and we were ready to go on sky last Monday. The first night out, we polar aligned and had just started capturing data sets (1000 frames at 20 ms cadence, repeated at 10 minute intervals) with Deneb as a target when clouds rolled in. The mount has a CP3 controller I recently purchased on the used equipment market. Since that controller can operate at up to 16 V, a 15 V supply with the appropriate locking connector was secured to the tripod. Everything was in good working order up the point that I departed the site. The observer stayed on and was able to capture a few more sets later that night before the clouds eventually prevailed.

  The following night was clear, and it was during initial start-up activities in twilight that things started going wrong. The report I received afterward was that the mount ran away in declination while seeking to the first target, slewing far beyond and not responding to the stop button on the hand controller. The observer powered off, manually moved to Park 1, and powered on again. Before it was possible to perform a sync, it ran away again, so the observer shut down for the night.

  I visited the site the following day, bringing with me the original CP2 controller as well as the 12 V power supply which I had used when the mount was in my observatory a few years ago. Without changing any components but after confirming that all of the connections were secure, I found that the mount was again running away, sometimes in right ascension as well as declination. Replacing the power supply had no effect, so I also substituted the CP2. Again, the problem resurfaced within a few seconds of the first attempt to move in either axis. I also noted that the motors would stop and start erratically during slews, sometimes in very rapid succession, regardless of the power supply or controller being used.

  The only remaining electronic element was the hand controller. I removed the cover and first measured the voltage of the lithium cell. No problem there, as it was about 3.1 V. I next confirmed continuity of the four wires which are used in the coiled cable, aggressively flexing the cable at each end in case an internal conductor is intermittent at certain orientations. Still no sign of a failure. The last step was simply cycling (disconnecting then reconnecting) each of the cable connectors which mate with the main board in the controller. Since these connectors used tinned contacts, it's possible for marginally- to non-conducting oxide layers to form over time. Cycling the connectors will generally abrade the oxide sufficiently to expose new metal-to-metal contact, renewing the connection (for a while).

  After reassembling the hand controller and going through the power up sequence again, the mount returned to its usual, flawless self. Seeing no issues after ten minutes or so, I reinstalled the CP3 and 15 V power supply. I drove it around the daytime sky for at least an hour, initially with the hand controller, then using Stellarium across a serial link, and never saw the slightest indication of an error. It's been in use without issue from twilight until after midnight each night since.

  I don't know details about the communications across the serial link between the hand controller and the CPx, so it's not possible to propose a failure mechanism related to faulty or intermittent contact that can explain the behavior we saw. That said, cycling the connectors in the hand controller was the only action taken which appears to have corrected the problem. To my knowledge, that controller was purchased along with the mount, probably sometime around 1990, so an age related contact issue is not beyond the realm of plausibility. Going forward, it will be my practice to slide each of the connectors off and back onto the pins inside the controller once or twice any time I open it up to replace the coin cell.

  Passing the information along in case it might be useful to someone else. If what I did solved what appeared to be a serious problem, it's a very easy fix.

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