2016-01-10: FOCUS! FOCUS!

The joy of trying to focus the telescope while sqinting at a computer screen in the bright sunlight is one of the first things I learned in my new era of solar observing. I decided immediately that I needed an electric focuser. The price and complexity of focuser motors and controllers -- that just turn a knob -- can get out of control quickly. I just wanted a simple system.



I don't think it gets much simpler than above. This is the JMI MotoFocus controller. You push the red button on the left to move the focuser in, and you push the red button on the right to move the focuser out. The switch below the red buttons gives you two choices of slow speed. If you hold either red button down for a few seconds the motor will shift into fast speed. It all runs on a 9V battery in the controller case. There is a cable from the controller to the motor, and a motor attached to the telescope. Works for me.

For the Feather Touch Focuser on my Lunt the MotoFocus system doesn't use gears, or belts, or O-rings posing as belts, but uses an O-ring posing as a rubber wheel that simply rests against the fine focus control knob and spins it just using friction. The theory is that when you want to manually change the focus adjustment you can just grab the knob and it will slip right past the parked O-ring wheel. Hold on to that thought.

The motor attaches to the focuser by removing the two knobs on the left and slipping a nylon collar onto the silver band. There is a flat section on the collar and two holes where the motor mounts, and a set screw opposite the flat part (on the top when the telescope is right-side up) to keep it from spinning. The collar slipped on to the silver bit on my scope with great difficulty. It was really tight. I'll have to cut it in half if I ever want to remove it. Make sure the holes and the set screw are in the right place while you are slipping it on. You may not be able to rotate it once it is on, but, with my focuser, I found out that the whole silver band can be rotated by loosening the tiny set screw next to the silver band in the picture above. The silver band is actually the outer surface of a ball bering race. The nylon collar also has to be put ALL of the way on to stay out of the way of the course and fine focus knobs. It was a bit of a hassle, but it'll take an asteroid collision to make it fall off...and that's not a bad thing.

I got the nylon collar on the focuser and started attaching the motor. In the repeat picture above you can see that the small bolts that came with the unit didn't really fit. The heads were too big, or the lip of the motor cover that they went through was too narrow. Take your pick. The bolts would not go through the holes straight. Getting them to line up with the two holes in the nylon collar was problamatic. It seems like there was a lot of extra stress on everything when I did manage to get the bolts started.

I didn't like the way things were going so I trotted down to the local lumber yard and found some #4-40 1/4" socket cap bolts that matched the threads and length of the supplied bolts. When I got back home the smaller heads on the new bolts made getting things lined up easy, but it was at that point another problem came to light. As I tightened down the bolts the pressure of the wheel against the focus knob became far greater than I was willing to allow. The motor is mounted on a spring inside the housing, but it's not a wimpy spring. Once the bolts were tight there was no way to spin the knobs manually, as advertised, and I was literally afraid the focuser shaft, the motor shaft, or gearbox shafts were going to bend and snap. There was that much pressure.

Back to the lumber yard. Good thing the town is only about three miles by three miles in size. This time I bought the same socket cap bolt, but 3/8" long. Too long to be useful? Why would I buy useless bolts? The longer bolts threaded all of the way in to the collar holes, which is a nice secure feeling, and then stuck about 3/16". I used the two supplied tiny O-rings from the original bolts, and the two supplied tiny spare O-rings as spacers between the nylon collar and the motor housing. The kit also comes with two spare 'rubber wheel' O-rings. That's nice. With everything put together the motor just hung under the scope loose with the wheel just resting on the red fine focus knob from the weight of the motor and gearbox, which is approximately nothing. In this state there is no problem spinning the knobs manually. Some amount of pressure was going to have to be supplied to make this work.

I started off by thinking about some kind of spring that would push two pieces of something apart like the handles of a clothes pin when it is squeezed open. That thought seemed OK, but it didn't last long.

I realized that the pressure exerter didn't have to be an active device, but could just be as simple as a folded up piece of paper.

I had a piece of 3/8" dowel laying around, so I fashioned the device of my dreams using some of it and my pocket knife. This was plenty good enough for a proof of concept.

A few minutes with a sanding tube and a bit of sand paper, and...

Good enough to sell. The tapered point makes it easy to install. The motor mount rests on the flat part, and the dip in the bottom locks it on to the axel hump of the focuser.

This first stick may be a little thin, but in testing the motor was able to run the focuser all of the way down and back up with the weight of the B1200 blocking filter, a Tele Vue 2.5X barlow (not in this picture), and the Celestron 8-24mm zoom ocular hanging from it with no slippage. I guess that's what's important. The stick pushes the motor housing away from the focuser which pivots on the bolts and forces the rubber wheel to exert more pressure on the focusing knob. If it ever doesn't exert enough pressure I'll just have to make another stick and sand the flat part a little shallower. When you are finished observing, or if you are just some kind of control freak, you can just pull the stick out and move the focuser manually in a matter of milliseconds. Re-engaging is just as fast. I replaced the original Lunt metal carrying case for my Lunt 60 with a Pelican Storm Case iM2600. The scope lays in there upside down, because I use the dovetail on the bottom as a handle, so I just leave the motor attached.

JMI specifies which models of Feather Touch focusers that this motor unit is for. I'm not sure which model I have, but the focuser in the instructions that come with the system does not look like mine. The one in the instructions has a flat bottom, insted of a hump where the axel between the sets of knobs runs. The nylon collar only goes in one place, and the motor only attaches one way, so the only variable in all of this is the diameter of the fine focus knob. In relative size terms mine looks just like the one in the instructions, so I'm not sure if my focuser is just not supported, or if the extreme amounts of stress and pressure are just how the system is supposed to operate. I didn't like using the original mounting method and all that stress. Other than that I think this is going to work out great, and for about $200 I've got no complaints...well, except for all of the stuff above.

THE END

2018-03-03