The JMI Encoders on the Takahashi NJP-160 mount

I installed the JMI encoders on my Takahashi NJP-160. There were a few blips in the process that are worth knowing about.

  • The new NJP mounts have a small bulge at the base of the RA axis, where a little index mark is installed to indicate the current RA setting. This bump is new. You remove the index marker plate, and install the encoder housing in its place. The bump raises the encoder housing a small amount, and you will need a larger gear than the stock gear that JMI includes. Just ask for it if/when you order your encoders.

  • You will need to remove the housings from the ends of the Dec and RA axes in order to mount the gears. This makes your mount look UGLY. Be forewarned!

  • You can use the screws that JMI provides for the Dec encoder housing. However, Takahashi has used much smaller screws for that little index mark, and you will need to re-use the Takahashi screws for the RA encoder. They are just barely long enough to work, but they will work if you are careful to tighten them alternately; otherwise, you will tilt the encoder housing and the second screw will not go in. I think these screws are a metric thread, and I could not find them in my local hardware store. A 2x56 screw is too large!

  • The larger gear on the RA axis means you have to use a different number of tics when you set up The Sky for your encoders. The correct figure is 6599, and you use the regular 11312 for Dec. Note that these numbers are larger than the 4096-tick encoder values; the gearing is what gives you higher resolution. JMI does not offer the 8000-count encoders for the NJP, but they should given that the larger encoders would work just fine in this application. However, I'm not sure you would gain a whole lot if they did, since the encoder shaft would move away from the axes, and you would need to use larger hears, which would reduce the increase over actual encoder tick counts. Still, there would be a net gain in tick count and in ugliness. <g>

  • When setting up in The Sky, retain the Clockwise orientation.

  • You will not need to use the shaft extender that JMI supplies for the NJP; the location of the new screw holes eliminates the need for it.
Declination Encoders The image at right (click to see a larger version of all images) shows the declination encoder and gears. I have removed the cover for the Dec housing, and I slid a large 116-tooth gear onto the exposed metal. There are two set screws to secure this gear.

The encoder housing is attached with two JMI-supplied screws. It has a 48-tooth gear on the encoder itself, for a net tic-count of 9899 per revolution of the Dec axis. This is calculated as:

(116/48) * 4096 = 9899
Here is the Dec encoder housing and gears from the side. There is a little wiggle-room in the housing, so you need to take a little care to get it lined up. I had to remove the small black spacers that come on the housing in order to get the gears to mesh properly. The easiest way to remove the spacers is to wiggle them with pliers for a minute to loosen them, and the put a screw into the spacer and bend the screw with pliers to break off the spacer. It sounds worse than it really is, and I am sure that it voids the warranty. But it works.
RA Encoders You also have to remove the RA housing in order to mount its 116-tooth gear. The 59-tooth gear supplied by JMI won't work on the newer NJP mounts. You need to make sure you get the 72-tooth gear instead, which is what you see at right.

The screws that Takahashi supplies are very tiny, but they will work. They are just long enough to hold the encoder housing securely.
This shows the RA encoder housing from the side. You can see the small bump referred to in the introductory text on this page, which raises the encoder housing up and requires that you use a larger gear in order to mesh with the 116-tooth gear.