by Jay Reynolds Freeman
Imagine a NexStar 8 set up in altazimuth mode, pointed straight up. You are looking down on it from the zenith. You see a round circle, the front of the OTA, with the fork arm off to one side of it -- let's say the right side, for the sake of argument -- at 3 o'clock from the center of the OTA. Thus the altitude axis runs from 9 o'clock to 3 o'clock, directly through the centers of OTA and fork arm.
The problem was, that as originally installed, in that view, the finder did not line up with the altitude axis. The rather long finder bracket stuck off from the tube toward 10:30, putting the two-pound finder almost six inches toward 12:00 from the altitude axis. The OTA was out of balance from the weight of the finder, increasingly so the higher it pointed. The tube wanted to tip over backward. The NexStar drive system was not happy -- someone mentioned in EMail that the NexStar will drive correctly only when nose-heavy -- the drive system counts on nose-heaviness to take out slack in the gear train. On the basis of my experience, I believe it.
I thought about adding counterweights, but was reluctant further to overload the mount. So I decided to move the finder to 9:00 on the tube, directly opposite the fork arm; that would eliminate most of the top-heavy moment. It would leave a little, because the longitudinal position of the finder center of gravity is a bit aft of the altitude axis, so I might have to put in a counterweight even then, but it would be smaller and could attach to the upper end of the black metal bracket that holds the OTA to the mount, instead of at a kitty-corner position.
I demounted the OTA and set about drilling new holes in the bottom casting. First I twirled the focuser to move the primary mirror full aft, well clear of the new holes. To keep debris off the optics, I pulled the corrector plate and secondary. (I was careful to mark the positions of everything as I went along, so I could get things back together in correct alignment -- I also noticed that the corrector was centered in its cell with shims of cork and paper of varying thickness, and kept track of where the thick and thin shims were.) Then I reached down inside the tube and used masking tape to fasten a few layers of surgical gauze -- in essence, coarse cheesecloth -- inside the tube, at the position where the drill would break through.
I set the tube corrector end down, so escaping chips would fall toward the open front end, and drilled cautiously, using a hand drill, starting with 1/16-inch holes and stepping them up in small increments to the no. 29 drill size required for tapping 8-32 threads. The gauze did a fine job trapping chips, though I had to smooth it occasionally, when the tip of a drill twisted it. Tapping was uneventful, as was reinstalling the corrector, and when I had the telescope operational again, it was still in collimation, in much better shape than my nerves.
I had another problem in mechanical reassembly. When reattaching the OTA to the mount, one of the aft screws, that hold it to the black metal bracket, fell inside the fork arm and shorted wiring near the on/off switch. I had the switch turned off, but had not been smart enough to remove the batteries, and the short was on the hot side of the switch, between power and ground. That was harmless to the telescope control electronics, but it did burn out a rectifier diode installed to protect the batteries from reverse current flow when the external power voltage exceeds the battery voltage.
I live a short walk from a good electronics supply store, so it was easy to replace the diode. However, I noticed that the wiring near of the switch was poorly done. Several wires route from that area to the batteries or to the main circuit board, and they are deliberately made longer than necessary, for ease of assembling the drive base. The extra lengths of wire get stuffed back in the switch area when assembly is complete, and there were no terminals there to help keep them apart. In particular, the wires that came off the positive and negative sides of the external power plug were free to move, and might well have shorted of their own accord due to vibration or to the telescope turning this way and that during transportation and setup. I improved things a little bit with shrink tubing and tape, but this area of construction is shoddy, and other NexStar 8 owners may well encounter problems with it.
The good news is that with the finder moved, the telescope required no counterweight. There was enough built-in nose-heaviness to handle the aft center of gravity of the finder.
The finder is handier in its new position than in the old one. Its eyepiece is seven or eight inches right of the main telescope eyepiece, and stays almost exactly at the height of the main eyepiece. That makes it easy to switch between finder and main eyepiece when I am seated -- I don't have to get up or stretch to do so. And there is enough space between the two eyepieces that I do not touch or jiggle the main telescope when using the finder.
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