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How do you Chart Periodic Error

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Hi Graham - I use PEMPro, which isn't free but there's a time-limited demo available that'll do what you want. You do need CCDSoft, MaximDL or AstroArt to use it though.

However, you can estimate the PE very easily with what you have. Set the mount up so that it's intentionally misaligned from the pole by 5 to 10 degrees, and point the telescope south. Now take a long exposure on low ISO - stars will trail due to the misalignment, but will also move up and down from PE. You'll get an image like this


work out the image scale (in arcseconds per pixel) and the number of pixels it moves up and down, and that gives you an estimation of the PE. In a mount with no PE then the lines are perfectly straight.

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