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JMI MicroFocus Dual-Speed Telescope Focus Knob
Posted: 9 December 2011

I received the JMI MicroFocus Dual-Speed Telescope Focus Knob, which I had special-ordered from OPT, on Thursday, 8 December, a cloudy day (of course). OPT stocks the similar Starlight Instruments Feathertouch Microfocuser, but the Feathertouch design requires some minor disassembly of the telescope. The recently released JMI MicroFocus does not require any disassembly (other than swapping two screws) as it slides on over the existing telescope focusing knob. At $202, the MicroFocus is slightly less in price than the $229 Feathertouch. Both products have 1:1 coarse focus (as does the unmodified telescope) and a 10:1 fine speed focus. Both products are available for several different telescope models. I had decided to get the JMI MicroFocus since it did not require disassembly to attach or remove.

The MicroFocus comes with a set of instructions, (and a one page "catalog", along with a product return slip), the MicroFocus knob, the adapter collar (black tube), 2 mounting screws, and a hex key:


Note that the instructions are for a JMI "Motofocus", not the MicroFocus. However, while the shown accessory is wrong, the steps are the same.

As seen in this close-up photo, there is a numbered scale:


However, the scale will probably not be as useful as one might expect. As either knob is rotated, the scale dial moves, but as rotation is continued, the scale dial just goes around and around, reading from 0 to 9. Just knowing what "setting" is required for your DSLR to be infocus is not going to be sufficient as there is no reading of how many rotations you have done. Other than that, the JMI MicroFocus seems to be well-made and durable. The knobs have a different "feel" when you touch them so there should be no confusion as to which knob you are using. And the rotation was smooth with either knob (tested before installation). As a well-made product, the MicroFocus does have some weight, but I do not believe there will be any need for adding a counterweight to offset its addition. It is not that heavy.

Attaching the MicroFocus to the 8" LX200-ACF was simple, as shown in the instructions. It took about 30 minutes to attach it, including preparation and photography.

This is the Meade 8" LX200-ACF focus knob:


This photo shows the adapter collar attached:


I initially had some difficulty getting the MicroFocus to slide onto the telescope focusing knob. The reason for the tightness is the O-Ring, which secures the MicroFocus to the telescope focus knob, as seen in this photo:


The instructions say you can add a lubricant to make it easier. With some pushing, I was finally able to get the MicroFocus fully on the telescope focusing knob, as seen here with the installation completed:


The high quality and the high precision appearance of the MicroFocus make it look good on the telescope. With the MicroFocus now attached, I checked the focusing. The temperature was about 70°F inside the observatory. I mention that for a reason that will become apparent later. The focusing was smooth, BUT there was a problem. The coarse focus movement was fine; as the knob was rotated in either direction, the scale moved and I could see the telescope focusing knob turning inside a small opening in the adapter collar. However, all was not well with the fine focusing; when rotated counterclockwise, the scale and telescope focusing knob would very slowly move (as expected). But when the fine focusing knob was rotated clockwise, there was no movement of either the scale or the telescope focusing knob. I called JMI from the observatory. I was told that the MicroFocus was probably pushed too far onto the telescope focusing knob, creating some stress on the mechanism. And indeed I had pushed it on as far as it would go so that there was no gap between the MicroFocus housing and the adapter collar. I backed it off about 1mm and then fine focusing worked properly in both directions. Whew. Thanks to JMI for the quick and knowledgeable phone support!

The MicroFocus adds some length to the focusing knob, so I was curious if there would be any interference with my D7000 DSLR. I attached the camera and indeed, there are some orientations where there is interference. The MicroFocus will prevent the camera from being rotated 360° around the optical axis for ideal framing of some objects. However, as can be seen in these photographs, there are usable positions where there is no interference:

Prime Focus with Visual Back

Prime Focus with Off-Axis Guider

With the installation and initial checkout completed, I now had to wait for nightfall to begin operational testing. The sky was still partly cloudy as I closed the observatory and began waiting.

The cloudiness did not improve as sunset approached. Fortunately, the clouds were thin and there were a few holes in the cloud cover. I opened the observatory at 1804 MST, 48°F. My first test object was the planet Jupiter using the 26mm eyepiece (77X). I first adjusted the coarse focus and found it "stiffer" than it was during my daytime tests. Apparently, it is temperature sensitive and gets stiffer in cold weather. The coarse focus was still usable however. Fine focusing was excellent and very smooth when turned in either direction. I repeated my focusing tests using 15mm (133X), 9.7mm (206X), and 5.5mm (364X) eyepieces. Fine focusing was a joy to use. Even with the very bad seeing due to the clouds, I could achieve a precise focus at all magnifications using the fine focus knob.

I then repeated the focus tests with the four eyepieces on the waxing gibbous moon. I followed this with tests on the star Capella with all four eyepieces. Again, precise focus was achievable, even at high magnification with the poor seeing.

Next, I mounted the iPhone 4 at prime focus + visual back using the MX-1 afocal adapter, and did some lunar imaging tests using a 26mm eyepiece. Using the fine focus knob enabled easy but sharp focusing, with this result:


My last test was with the D7000 DSLR at prime focus + diagonal. I used the Bahtinov Mask on the star Capella for a focus test image. While viewing the star on the D7000 "Live View" screen, achieving a perfect focus was ridiculously easy with the MicroFocus fine focus knob. This is the 1 second, ISO 500, focus test image:


The high clouds now covered all of the sky. Fortunately, I had been able to complete my MicroFocus operational tests. I closed the observatory at 1930 MST, 40°F.

As mentioned earlier, the JMI MicroFocus Dual-Speed Telescope Focus Knob is a well made product. It definitely provides an enhanced experience at the telescope, making precision focusing much easier, whether for observing or imaging. Since the fine focus knob rotates so smoothly and easily, I found that hand-induced image vibrations were almost non-existent. I look forward to many years of using the JMI MicroFocus. It is definitely a KEEPER. If your telescope is one of the many supported models, this add-on should be on your must-have list.


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