Sky and Telescope
$99.95 plus $10.95 for backing plate and $22.50 for an extender tube
inch rack and pinion focuser
quality focuser for Reflecting telescopes.
I ordered the JMI NGF (Next Generation Focuser) Mini 3 model, and the optional backing plate for the Newtonian reflector (8" f/6) to replace the inexpensive plastic rack & pinion unit that came with my telescope. I had read good reviews about the company and heard the Crayford style focuser was the way to go, so I hoped I was on the right track. The focuser I was replacing displayed a lot of backlash, and would move the eyepiece off center as it was racked in and out. Not good to say the least. Also, I had no way to do prime focus because of the long
tube, (high-rise) design would not allow the camera T-ring adapter to achieve focus.
The focuser arrived in a nicely boxed package with lots of care taken to the foam padded packaging, and a full set of installation tools. The first thing I noticed was the solid feel of the unit. The smooth movement of the
drawtube was evident from the first crank of the beautifully machined hand knob. The design I bought used the bushing rollers. Jim’s Mobile also sells a more accurate ball-bearing design as well, but the bushing design worked amazingly well. I couldn’t detect any eccentricity in the outside to inside diameters of the bushings, which gave it no “hot-spots” in the movement. If this worked as good as it looked then I was a step ahead.
Mounting was pretty easy, but as expected, the OTA had to be modified to accept the bolt circle and drawtube cutout hole. The backing plate was a necessary item for my OTA due to the sonotube design. The Mini 3 has level adjustment screws at all four corners to help square the focuser with the tube. In order to keep from biting into the cardboard, the metal backing plate provided a firm pressure point for the leveling jack screws. When I was ready to bolt it to the OTA I noticed the backing plate drilled holes were not aligned with the focuser. No matter which way I turned the plate it didn’t fit the bolt hole pattern. The holes were elongated, I guessed, so that the plate could be used with several models, but this didn’t help either. I suppose in fairness, that the wrong plate could have been shipped. Understandable, because the unit was not marked anywhere for identification. Anyway, I used a drill to modify the plate until it fit properly under the focuser body. Further reaming of the OTA drawtube hole was needed to accept the drawtube without any drag. Once installed, I stood back and looked at it. I really looked good with the black anodized aluminum body/tube and the silver knobs. I noticed an adjustment thumbscrew at the rear of the friction drive. I assumed it to be either a tension adjustment screw or a locking screw. After messing with it for a little while it became evident it was for locking the drawtube. This was going to be handy once focus was reached prior to photographing. Nice touch.
Darkness finally came and out I went to test the unit. I put in my favorite mid-power plossl and looked through the eyepiece. I cranked it one way, and then the other. Each direction came to a halt abruptly due to the positive stop of the flat edged friction runner. The only problem now was I couldn’t reach focus! I stepped back in horror. I thought, “well maybe I need to try another eyepiece”. I tried another, then another. Not!! Obviously I had bought the short profile focuser and was well under focal distance for my scope.
I had two options. I could move the focuser and secondary, or primary to reach focal distance, or I could send this one back and get another focuser that would reach focus. The second option was too heart breaking because I needed to use this short one to reach prime focus with the T-ring adapter. I
couldn't bring myself to start in drilling and cutting on the OTA, or moving the primary, so I called the folks I bought the unit from to get some advise. I should have asked more questions when ordering to begin with, but that was history. The good people said that this was a common mistake, and that all I needed was to buy an extender tube for the eyepieces. I did just
Prior to the piece arriving, I made an extender tube out of stainless steel in my workshop. It worked good, but the thermal problems with steel were going to lead to fogging problems and the extra weight was too much. I was able to reach focus nicely though, and the unit proved to be a worthy replacement to say the least. Absolutely NO SHIFT was noticeable! I was also knocked out by the smoothness of the focusing touch, and the ability to get just the fine tuning that I needed to bring out the detail in the lunar landscapes and planetary lines on Jupiter. The only thing I did notice right away was the extreme glare from the inside of the drawtube. I was not sure if it was my makeshift extender, or the Mini 3 drawtube. The next day I painted the inside of my extender tube with flat black paint. That night I noticed I still had glare, so the next day I painted the inside of the JMI drawtube. The same thing was still present but not as bad. After the extender came in from JMI, I painted it too. The problem subsided enough not to be an annoyance, but it was still there.
The other thing I noticed was the nice large thumbscrew that holds the eyepiece in the drawtube was drilled so close to the end of the tube that my T-ring would not square up against the end of the drawtube. My collimator was the same way. I re-drilled and tapped the drawtube to move the screw down a bit and all was well. The same for the extender tube.
I have been using this focuser for several months now and have nothing but praise for the excellent craftsmanship and design. JMI has a superb instrument here. Other than the glare issue, I wouldn’t hesitate recommending this for anyone in need of an upgrade for their telescope. A great value!
Submitted by David Ryle - firstname.lastname@example.org - USA
Post Note: I wrote to Jim’s Mobile Inc. about the problem I had with the thumbscrew, and they responded immediately with a plan to change the design for their next run of production.