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Dean
How are you calculating your offset for focus.

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I don't calculate it, I measure it. I use a PDF & FocusMax, so it's simply a matter of have FM focus with one filter as the baseline (offset = 0 - I use clear for this), note the position, and then focus with another - the offset is the difference in the focuser position. I do it several times to get a good sample.

One thing that's handy about doing this, is with my ST10 there's a big difference between the Ha and LRGB filters (250 microns), but knowing the offset means I can focus with the clear filter and add the offset rather than try and focus with the Ha filter which takes a fairly bright star. It also means I can shoot HaRGB sequences without having to refocus between the Ha and RGB (although when I shoot Ha, I usually shoot just Ha).

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deanrowe.net/astro
Whats with that avatar?

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