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Information about the JMI NGFS focuser
with the DRO (digital read out)
For a detailed discussion of opening sizes in telescope back plates
go to: Back plate Aperture Sizes
I am including this information because so many have asked about the JMI focuser as applied to the Meade SCT. I like the focuser very much and have mounted one on each of my two Meade LX200 telescopes. I will describe the mechanics in some detail and my own experience in focussing with it.
The three images below show the JMI with DRO mounted on the back of my 10" LX200. The focuser motor is on the right and the DRO attachment on the left. If obtained without the DRO, the motor is controlled by a two wire signal from a simple hand controller. With the DRO control is from a similar hand controller but one with several additional commands and a readout panel. This is shown in the next panel of figures. The hand controller has, in addition to the two focus buttons and focus speed control, a readout that reads in 0.01 mm increments, a readout zeroing button and a brightness control for the readout.
The DRO resolver is of the differential type. That is, the readout gives the distance in 0.01 mm increments from the point at which the readout was last set to zero. Thus the standard focussing technique is to center the focuser in its range of motion which is about 12 mm by running the focuser to one end, setting the readout to zero, running the focuser to a position of 6 mm, resetting the readout to zero. The readout is now set to zero at the center of the range of the focuser. Variations on this technique are a matter of personal choice of course.
Careful inspection of the front edge of the inner movable tube shows that when it is extended rearward to its limit, almost one half of the forward bearing slots are exposed. Light can under these circumstances leak into the tube channel. To enter, the light would have to be directed toward the focuser from the bottom. This can happen of course since red light is often used to adjust and inspect equipment and CCDs are especially sensitive to red. The problem can be avoided to some extent if the draw tube is engaged well into the focuser outer tube. One might want to avoid fully extended operation for other reasons such as the rigidity of the focuser setting. The focuser will be more rigid when the draw tube is fully engaged with the outer tube. Since the focusing range is more than adequate, consider setting the zero position with the draw tube 3/4 engaged and work with it in this more engaged position.
It is also apparent that a full 2" of inner tube is available for accessories to enter from the rear. Thus for greatest strength when mounting accessories, they should have a full length of 2" inserted into the inner tube. Many accessories do not have a tube that long and thus might not be as secure as is possible. The very tiny and hard to reach knobbed screws used to fasten accessories can, and probably should be replaced with much longer screws with larger knobs on them so that accessories can be more firmly secured.
I note also that earlier focusers had holding screws from the bottom and later ones from the top. The screws entering from the bottom were much easier to reach and tighten.
Further discussion of the use of the focuser in practice follow at the
end of this discussion.
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The JMI comes with an adapter that will screw directly onto the Schmidt thread on the back plate of the SCT. However as described below, I do not feel this is a good way to attach the JMI to the SCT. The standard back plate with the Schmidt thread was designed for small SCTs and thus has a small opening. This is clearly seen in the photo that shows both the standard back plate adapter (opening 44 mm) and the back plate adapter available from JMI (opening 50 mm). As tubing is added to the back of the SCT, it is a good idea to use full sized 2" tubes and adapters for both strength and freedom from vignetting. Thus I recommend the optional JMI adapter plate.
A few words about the JMI focuser are in order. The focuser is very well made and quite strong. However, it is not perfect and not indestructible. There are two very small bearings at the bottom rear of the focuser (one can be seen in the top center photo) which bear forces from the weight of anything attached to the rear of the focuser. With large, heavy cameras that are mounted well to the rear, especially with intervening flip mirrors and the like, these forces get very large and can damage or break the small bearings. Thus it is wise to use attachments with tubes that are as short as possible. This is generally good advice since shorter tubes require less back focus as well. I have personally come to the position of doing a course focus with a parfocal eyepiece, replacing the camera and doing the focussing through the CCD images and careful use of the DRO feature. This technique is described below in more detail. Normal loads, such as 3 to 4 pound cameras that are within 4 to 6 inches of the focuser should give no problem.
Another point is about the so called zero shift feature. The focuser is much better than that normally found with main mirror shift but it is not perfect. Normal precautions of seeing to it that cables do not snag or shift too much during long exposures must be taken. I have found that if the camera is given a firm push halfway through an exposure, as much as 100 microns of shift can obtain. There is a tension adjustment at the point where the motor friction drive grips the movable tube. Care should be taken to not overdue tightening this setting since doing so can damage the bearings.
Additionally, Michael Hart, who is very knowledgeable about Crayford focuser designs, points out that the ball bearings used are very tiny and can be damaged. He replaced one of the bearings, which broke, with an oiled bronze bearing. This is the one (of 6) which is under the set screw that applies pressure to the outer race to increase the friction of the focus shaft on the draw tube. The remainder of the bearings seem to be quite adequate. In summary, some care should be used when loading this or any fine focuser device.
The JMI focuser is (IMHO) very good but not perfect. For a perfect
focuser, almost, look into the Van Slyke equipment (manual) which is quite
beautiful and also expensive..
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Two additional views of the JMI with DRO are shown above for clarity of viewing the design. The left view shows the back of the focuser with the opening for the 2" tubes, adapters and cetera. The right view shows the side which attaches to the special JMI adapter ring. Notice the nice clear full 2: aperture of this focuser. The cable shown is similar to the one provided by JMI but is a longer version which I fabricated. I have a 15 foot and 100 foot version. The cable is standard 6 wire telephone cable with standard modular connectors for the handset and the DRO and an 1/8 inch connector for the motor. The handset runs on a 9 volt battery. I added a battery eliminator power supply for use in the observatory and a power input which runs from my large battery source for portable use. This is easily done.
The JMI with DRO in Use
I always use the JMI with DRO on both the 12" and 10" LX200s. When I first looked through the SCT I was impressed by the image moving about and drifting in focus when attempting to focus. I first tried to six this problem with the electric focus motor. This did not help. I found shifting image and difficulty of finding and holding focus very annoying. Thus I got my first JMI focuser. Standard procedure is to set the focuser at mid point as described above. Next focus on a star, infinity focus, lock the mirror with the 1/4 inch locking bolt and nut. (this described elsewhere) The main mirror is now locked at infinity and all focusing is done with the JMI focuser.
The focuser has a plus/minus range 6 mm which is enough for all possible lack of parfocallity for any one setup. For CCD imaging, I use prime focus, 0.63 reducer with the ST-7 or possibly the 0.33 reducer. Basically after a major equipment change I do the centering of the JMI and infinity focus and locking of the main mirror. Then all subsequent focusing is done with the JMI and the DRO. For remote focusing the DRO is essential since it tells you exactly where your focus tube is set. Even for local work, I always use the DRO. There is no substitute for knowing where the focus tube is set to the 0.01 mm.
Fine focus is done with the CCD program and adjustment of the focuser in increments to get the sharpest result. An idea about the precision of the focus setting required can be understood as follows. An f10 telescope has an incoming light beam that has an angle of 1/10 radian. A pixel is about 10 microns in size. The circle of confusion (out of focus beam diameter) is the radian measure of the beam times the amount of out of focus (ness) of the image plane from the ideal. Thus for a circle of confusion of one pixel size, the allowed out of focus distance is 10 microns times 10 or 100 microns which is 0.1 mm. Since the JMI can measure to 0.01 mm the readout can easily resolve the necessary distances for the f 10 telescope. For the reduced telescope or faster telescope, say f 6.3, the required accuracy of focus is proportionately greater. Even then the JMI with DRO can easily measure to the required accuracy. I have used this attachment extensively and exclusively and found it excellent.
A manual replacement for the DRO is a dial indicator which can also measure easily to the required accuracy. Fine focusers such as the Van Slyke have dial indicators attached.
I hope this discussion has been useful. As always, I welcome comments.