Martin Cohen, of Company 7, which is a thoroughly outstanding telescope shop, has recently acquired a bunch of interesting equipment from Biosphere II. (I won’t go into detail here, but he probably talks about it on his own site.) Among the various bits was a badly beaten-up Celestron “Star Hopper” newtonian telescope on a Dobsonian mount. I think the objective mirror is 10" (a little over 25 cm) in diameter, which is reasonably substantial for an informal “let’s haul it out and look at the sky for a while” type of device. Marty was kind enough to leave this scope with us and tell us we were welcome to try to clean it up and get it working, so I did that.
It was full of cobwebs and filth, and I had to clean both of the mirrors (a frightening prospect, but he has a tutorial page on his site, which helped a lot). It was also totally wanked out of alignment.
I have aligned a certain number of lasers, which is generically related in the sense that mirrors are involved, and often (though not always) tubes of some sort; but a telescope is a somewhat different matter, and I’ve never done one before, so I had to make up the procedure while performing it. This is a mildly bizarre exercise in “beginner’s mind.”
Here’s a quick-and-dirty diagram, which should (I hope) make it somewhat easier to follow the verbal description. The observer’s eye is drawn somewhat closer to the eyepiece holder than is actually useful, but the diagram is not really to scale in any case...
The aim here is to get the mirrors aligned to each other, so the light from the primary reaches the secondary; and to the tube, so that the light bouncing off the secondary heads for the eyepiece. It was easy enough to see how to tweak the angles of the mirrors — there are three wingnuts on the base of the scope for the main mirror, a threaded stalk that holds the diagonal mount, and four Torx™ setscrews on the back of the diagonal — but how much does one tweak, and in what direction?
The first item is to look in through the eyepiece holder and try to understand what you see. What I saw was some sort of mess, with things in the wrong places and at the wrong angles. I decided that I might as well make sure the diagonal was about at the correct distance from the primary mirror; that is, when I looked into the scope through the middle of the eyepiece holder, I wanted the diagonal itself to be in the middle of the picture. (Per the diagram, if the diagonal is too close to the primary it will be left of center when you look into the eyepiece holder. In addition, if it is rotated slightly off-angle it will be an oval rather than a circle.)
[General note: the fact that other things are out of adjustment makes this a bit difficult; you have to concentrate carefully on whichever item you are adjusting and mostly disregard the others, so they don’t distract you; but at the same time you need to notice what happens to them when you make your tweaks, because it’s all connected, and each thing you adjust has some effect on the others. This is why I let the back of my head do the hard stuff: it’s too big for the conscious mind to handle well unless you understand what’s going on in detail, and because this was my first time I did not have that level of understanding. Trying to do this sort of thing consciously is a good way to become incredibly frustrated, and I don’t really need that.]
I loosened the diagonal on its threaded stalk, and rotated it a bit to get a sense of how that went. It was roughly in the center of my field of view when I looked in through the eyepiece holder, so I didn’t actually have to change this setting much, if at all. What seems to keep it from rotating (and thus moving toward or away from the objective, or even just pointing someplace other than the eyepiece) is the pressure on it from the Torxes that control its angle; I was going to have to twiddle those in any case, so it didn’t matter that I had to loosen them in order to make this initial adjustment.
I then started fussing with the wingnuts on the back of the primary, and the Torxes on the back of the diagonal. After moving things around a bunch (maybe 5 or 10 minutes of “I don’t get this yet”), I sorta figured out that twiddling the wingnuts on the main mirror caused the image of the diagonal (which I was seeing somewhere in the middle of the diagonal itself — you gotta be really careful how you think about this) to move around, so I twiddled until it was approximately centered.
That, however, let me see that the diagonal was at an angle. I could tell this by the fact that the brighter circle, which was the image of the diagonal mirror (in which I could see my eye) was off-center in the darker ring that was the image of the holder for the diagonal. At that point, it was time to torque the Torxes, to try to center the image of the diagonal within the image of its holder. (I suspect that I should put another diagram in here...)
Perhaps two or three iterations of this, back and forth, and everything actually appeared to be centered. If you actually do one of these, what you want to see is the eyepiece holder as the periphery, with the diagonal in the middle of it; in the middle of the diagonal you should see the reflection of the diagonal in the primary, and in the middle of that you should see your eye. When everything is as precisely centered as possible, the telescope is more or less aligned.
This is quite informal, and there are probably better procedures, but if you don’t have any special instruments or other instructions, it appears to work. After I got the scope aligned (and lined up the little spotting scope with the main scope), I took the following pictures of Jupiter and four of its moons by holding the lab’s Canon G3 camera up to the eyepiece, with the focus manually set to infinite distance. The exposures were about 0.3 second, give or take a bit — the brighter one was, no surprise, a longer exposure than the dimmer one. Some of the blur was probably caused by me holding the camera to the eyepiece, which shakes the telescope. A little of it, though, could be from the planet moving across the field of view during the exposure.
I am a little bit concerned about the chromatic aberration (notice that the blur at the bottom of the planet is blue and the blur at the top is red), but that could be an issue of whether the camera was centered on the eyepiece. (I don’t remember seeing spurious color with my eye, which also suggests that this is a camera issue.)
All things considered, not too bad.
My email address is a@b.com, where a is my first name (just jon, only 3 letters, no “h”), and b is joss.
My phone number is +1 240 604 4495.
Last modified: Fri Jul 4 14:25:19 EDT 2008