Quote:
Originally Posted by Renato1
However, my f/4.5 dob doesn't show the concentric rings, so I made a an aperture mask for it with circles cut out that fitted where the mirror was clear (between the secondary and the primary's outer edge). Then the concentric rings around the dot were easy to see. But even when concentric, there is a slight flare to one side in the direction of outer edge of the primary. It is then a matter of rotating the mask around, making sure that the out of focus image in each of four positions around the diameter is the same (i.e. each is concentric with its little flare the same length pointing to the outer edge of the mirror).
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I am unsure if I understood the idea above. When star collimation is executed correctly then the defocused star rings will look concentric regardless of F-ratio. The key here is "executed correctly". Star collimation should be done at high magnification -- at least 25X the aperture in inches. So, for a 10" dob, a minimum of 250X magnification is needed. The star needs to be defocused by a small amount -- enough to show 3-5 rings. The secondary mirror offset will be invisible using this setup. When the star is defocused too much, then the results will be inaccurate.
Quote:
Originally Posted by Renato1
The owner replied that it couldn't be as his laser said it was perfect.
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Typical laser collimators can only be used to achieve axial alignment between the eyepiece and the primary mirror. They can't be used to center/round the secondary mirror under the focuser. After all, the laser does not interact with the secondary mirror edge -- only with a tiny area on the secondary mirror surface.
Many posters complain about how after completing collimation via a laser collimator, they found out their secondary mirror is off-center. Then they blame the laser collimator.
Centering rounding the secondary mirror can be done by a sight-tube or by a laser collimator with a special holographic attachment.
Jason