Hi Mark and Mark,
Personally, I think some people obsess too much about collimation. Maybe my eyes are more tolerant, or maybe I don't find collimation that difficult. I don't know. I also don't do much high power planetary observing in excellent seeing.
Before each observing session, I use a laser collimator (simple red dot Orion) for convenience. With the open scope design it is easy to see the red dots while adjusting collimation screws. I collimate once at the beginning of an observing session with the scope oriented in an average elevation - typically around 45 degrees. With the Cheshire, you have to go back and forth between eyepiece and screws - effort I find increasingly difficult. I also find that my Cheshire and laser collimator mostly agree when I do check - and agree with the star test. So, I may collimate my scopes for months using just the laser collimator spot before checking with the Cheshire.
With some of my scopes, the laser spot on the primary wanders a bit within the central "donut" between zenith and horizon. On others, the spot movement is so small I can't see it. I haven't done extensive testing, but on a couple of my scopes, I've checked collimation with the Cheshire with the scope pointed at zenith and horizon after collimating with a laser at 45 degrees. With one scope that showed the laser spot moving, the Cheshire showed the collimation to be right on, regardless of scope orientation. With one scope that showed no movement of the laser spot, the Cheshire indicated some misalignment at the extremes. I haven't paid much attention to this puzzling behavior - filed away for future investigation. However, since scope performance at the eyepiece meets my expectations, I am not particularly concerned, the reason I haven't investigated right away.
I do think it is very important to check the alignment of the laser collimator before relying on it. Almost every one I've checked has been out of spec, both inexpensive Orion type and my expensive holographic Lasermax. In my opinion, the newer Orion "Deluxe" collimator is inferior to the older style. The newer thin-walled tube is easily deformed when placed in a focuser and it doesn't sit square.
I do like the hologram of the Lasermax, using it to position the tick marks at the edge of the mirror, as you mentioned. I haven't verified with a Cheshire whether one can miscollimate using it. However, one should not follow the manufacturers suggestions of centering the secondary shadow if the secondary mirror is offset. I use the hologram and laser when setting up the scope. But because of its size and weight, I normally leave it at home.
I haven't done the math or seen a good explanation on how square a focuser should be. I think it should be square side-to-side. It is simple enough to do, so I do it and stop worrying about whether it should be. But it apparently doesn't need to be square up-and-down. For example, take a look at the angle the focuser makes on Dan Gray's telescope: http://www.siderealtechnology.com/28inch/WholeThingEypieceSide.jpg It is one way to lower the eyepiece height on large telescopes, although I'm concerned about possible neckstrain and balance point after mounting the mass of the secondary higher up the tube. One consequence is that the secondary (and probably the illuminated field) will appear elliptical. But as far as I know, the angle doesn't affect observing.
This armchair speculation is interesting. I suggest we arm ourselves with collimators, introduce known deviations into our scopes, and check visually what important. Anyone up for some experimentation?
Albert
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So...A question then. How do you check collimation with a Cheshire during an observing session? Or when you say every so often are you referring to every few observing sessions?
Thanks, Albert!
Mark
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> perfectly square focuser (which isn't necessary)
Can you elaborate on that statement? If the focuser is not square (perfection is almost
impossible) and I mean "square enough"... your focuser should be off-axis to the light path, no?
> Then take a look using the Cheshire. It will show those deviations because it offers more
> information about the relative positions of the edges, reflection of the spider, etc. Make sense?
That sure does. A single point source laser is inadequate by itself. But that's not true of all
laser collimators. My favorite has been a laser with a holographic projection, showing concentric
rings, intersected by a number of spokes extending all the way out through the rings, and indexed
with tick marks. You can count "tick marks" from the first ring showing inside the edge of the
mirror, to center the projection. This system also provides a very easy way to collimate in the
dark.
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Received on Fri Mar 14 12:50:18 2008