JMI 2" diameter extension tube
Last year I acquired a 17.5" Dobsonian for $625 (and it was only a minor bargain at that price)--and now that I have a new house under a dark sky, an extension tube starts to make sense. With the focuser fully extended, I have to pull the 85mm eyepiece almost all the way out of the focuser to bring it to focus--an unnerving action to take, especially in the dark.
So, I needed a 2" diameter extension tube. For those who are just learning, an extension tube is effectively the optical equivalent to an extension cord. One end of the tube is just small enough to fit into a 2" diameter focuser; the other end is just large enough for a 2" eyepiece to slide into it. In this case, the extension tube provides 1.5" of extra length--enough to bring the 85mm eyepiece in focus without worrying about the eyepiece falling out.
An extension tube is not high technology. There's no optics in it. The only precision aspects of it are the diameters of the two ends. In the case of this JMI extension tube, the bottom part's outside diameter varies from 1.996" to 1.998" diameter, and the top part, which varies a bit more, from 2.005" to 2.008" inside diameter. The depth of the upper bore is 1.431", which is close to the nominal extension length of 1.5". (There is a small amount of additional metal at the "lip" between the two ends, so the actual extension length might well be 1.5".)
Are these precise enough? They are precise enough for my telescopes, I know that. I haven't measured a lot of 2" focusers/diagonals, and I only have three 2" diameter eyepieces. The three eyepieces all slide into the extension tube without friction, but also without any looseness.
The older University Optics focuser on the 17.5" reflector measures 1.991" to 2.005" inside diameter, and the extension tube slides in without any effort. A 2" diagonal on my Photon refractor, however, is a bit smaller, running from 1.985" to 1.997" inside diameter, and the extension tube requires slight pressure to insert. As a general rule, frictionless fits are the best choice; if you have to use force, there is the potential to scrape the extension barrel or the inside of the focuser/diagonal.
Of course, with a frictionless fit you need some method of retaining the eyepiece in the extension tube. Compression rings and set screws are the common methods for eyepiece retention, with compression rings preferred because metal set screws, if overtightened, can mar the barrel of an eyepiece.
For the JMI, there are two steel set screws of apparently 6-32 size (that means #6 machine screw, 32 threads per inch). I'm not sure why there are two set screws here, but perhaps by using two, there is less need to use the kind of clamping force that sometimes mars the eyepiece barrel.
There are aspects of these set screws that leave me less than thrilled (although they are serviceable).
1. Nylon or brass would be a better choice, simply because it reduces the marring potential. (I have been experimenting with manufacturing my own parfocalizing rings, so I have been playing with nylon set screws of late, for exactly this reason.)
2. They are too long--as you can see in the picture. Unless you are trying to hold a substantially smaller than 2" diameter eyepiece in the adapter (which would be a terrible idea), the extra length sticking out the sides is wasted space, and adds to the manufacturing cost of the unit. Worse, it is a protrusion begging to catch a sleeve.
3. The thumb screws are a little loose in the threaded holes. They aren't so loose that I expect them to fall out, but it just isn't quite as elegant as it could be. When tapping a hole in metal, you have some choices about what size of drill you use before running the tap through the hole--what is often expressed as a 50% or 75% thread. I think that a slightly tighter hole would give the thumb screws a slightly more "machine" like feel.
I will probably replace these thumb screws with some shorter ones at some point to avoid the sleeve catcher problem, but unless I can find some 6-32 thumb screws that are slightly oversized--or I can electroplate the threads on the thumb screws to make them slightly larger--the looseness will not go away.
The finish is a polished black, apparently anodized finish on aluminum. I would prefer a flat black on the inside of the extension to reduce reflections, but I rather doubt that a flat black finish this close to the field lens is going to make a noticeable difference.
Click here for more about this subject. -Ed.
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