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What is the Tri-Bahtinov?
Essentially three Bahtinov masks in one, rotated over 120 degrees. It gives you three lines and three crosses to check focus and collimation, along three axis simultaneously.
In addition to focusing, the tri-Bahtinov so also indicates collimation status along three axis.
This is interesting for e.g. Schmidt-Cassegrain telescopes, where the collimation is done by push-pull screws of the secondary mirror along three axis.
When you position the tri-Bahtinov alongside these axis, it will give you insight not only per axis of the focus but also how different these focus points are for each axis.
Bahtinov mask is a less expensive method of getting focus on objects in the night sky compared to a standard motor focuser.
With the right mask and setup, it’s easy to get crystal clear focus on a star in less than 90 seconds.