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Originally Posted by Windston
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I have a sighttube/cheshire combo, one question I have is, is it the same to align the crosshairs on the sighttube to the primary mirror centre dot as it is to make the 3 clips even around the outside?
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Aligning the cross-hairs of the sight-tube with the primary center implies that the primary mirror is centered with respect to the focuser edge. It does not necessarily mean it is centered with respect to the secondary mirror. Can you take a photo using your cell phone through your sight-tube and post the photo in this thread?
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I did that today, the crosshair of the sightube is right over the primary dot, however when I rack the focuser in, the primary does not look centered at all on the secondary. I am thinking that it could be from a misalignment in secondary rotation?
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Start off with a perfectly collimated scope with a well-centered primary mirror with respect to the secondary mirror. When you start racking in the sight-tube you will notice that the primary mirror reflection will slowly shift in the direction of the OTA bottom -- it is geometry. Is that consistent with what you noticed?
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So now it is clipped in tight and shouldn't go anywhere!
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Be careful, when you tighten too much it is possible to temporarily deform the mirror paraboloid shape and end up introducing temporary astigmatism. A star test will tell if it is too tight. If the star test shows distortions around the clips then you will need to untighten the clips.
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I ended up going with an Autocollimator made by a local guy, it is just a single pupil as far as I know, but it also was only 40$ delivered (I think, it was somewhere around that much) so I figured I would give it a go. The Catseye tools were a lot of cash to spend, and I am just a uni student who left high school so I cant really justify that extra cost, yet!
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An autocollimator is one of these tools that has to be of high quality; otherwise, it is not worth it. Catseye is well-known to provide high quality collimation tools. That does not mean the autocollimator you have does not meet the high standards of what is expected from an autocollimator. Do this simple test:
Collimate your scope using other tools. Insert the autocollimator in your focuser. Do you see multiple reflections of the center spot against a dark background? If yes, rotate the autocollimator slowly. If the locations of the multiple center spot reflections gitter or slighly move, that is OK. If reflections move wildly and all over the place or possibly disappear then the quality of your autocollimator is questionable.
Simply put, the autocollimator is supposed to give you that additional collimation precision; therefore, it has to meet a much higher quality standard than other collimation tools.
Jason