Quote:
Originally Posted by gaseous
Using a collimating cap (which I rarely use), I can align the primary mirror centre spot with the hole in the collimating cap, and align the primary mirror reflection in the secondary, so everything looks pretty nicely aligned looking down the focuser. However, when I put a laser collimator in to check, it’s out by a pretty wide margin (laser spot 2-3 inches away from the primary centre spot). I know the laser is pretty accurate, so after collimating the secondary and primary with it, I recheck the alignment with the collimating cap again, and things are all over the shop – cap hole not centred on mirror spot, and primary badly misaligned down the focuser - still completely visible, but touching one edge.
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What you have described is a frustrating experience that gets periodically posted in astro forums. Misunderstanding how these collimation tools work is the main culprit.
Let us start with the laser collimator. Assume you have a perfectly collimated scope. The laser beam hits the primary mirror center and retraces its path back to the source. Secondary mirror and primary reflection reflection look nicely rounded and concentric with respect to each other and to the focuser. Now imagine someone uses a special surgical cutting tool to shaves off 1/3 of the secondary mirror without altering anything else. What will you see now? The laser still hits the primary mirror center and retraces its path back to the source; however, the "reduced" secondary no longer appears centered under the focuser and the primary mirror reflection no longer looks concentric with the "reduced" secondary mirror. Bear in mind that the laser collimator does not interact with the secondary mirror edge and there is no way for it to tell you if the secondary is centered or not. There are infinite secondary mirror positions that allow the laser beam to hit the primary mirror center and retraces its path back to the source -- but there is only one optimal secondary position when the secondary mirror is centered. Check the first gif attachment. Each frame corresponds to a setup where the laser hits the primary center and retraces its path to the source. Only one frame correspond to a centered secondary mirror.
Now let us talk about the collimation cap. When you adjust the secondary mirror using the collimation cap, you are trying to center/round it under the focuser. You are interacting with the secondary edge versus the focuser edge to achieve decent good positioning of the secondary mirror under the focuser. Unfortunately, there is no references in the collimation cap to ensure your eye's axis is coincident with the focuser axis. Typically, you will angle the eye axis slightly to see the whole primary mirror reflection and you get the false impression that all reflections look nicely concentric. See second attachment.
Bottom line:
1- Laser collimators are good to ensure the focuser axis reflects off the secondary mirror and hits the primary center -- and -- the primary axis reflects off the secondary mirror and hits the focuser center. This is called axial alignment. But laser will not tell you if your secondary mirror is well positioned under the focuser.
2- Collimation cap does a poor job in confirming if the focuser axis will reflect off the secondary mirror to the primary mirror center. However, it does OK telling you if the secondary mirror is well positioned under the focuser and does OK telling you if the primary mirror axis intersects with the focuser axis at the focal plane.
You can actually use both tools in an iterative method to get the best out of each tool. The end result will be a well-collimated scope where both tools are in agreement. I described this method in the following post:
https://www.cloudynights.com/topic/2...ent/?p=5260727
Jason