Hi Martha,

yes, the long available range is to allow various optical configurations. Like adding a star diagonal for visual, various correctors, barlows or reducers etc. that require changes in back focus. If you only use one configuration it will stay at one position almost all of the time.

As for filter offsets, if (once...) you use a filter wheel there is the option to have a list of relative focus positions for each filter. You have some sort of reference (like L, or in case of RGB, G), and for other filters that table tells how much the focus differs from that value. If you then change to another filter EKOS will compute the needed focus change, and apply the difference, so that a consecutive AF run will already start at focus. Or you trust that, and start imaging. (I'm wary, and always do AF on filter change).

With manual filter changes that won't work, so forget about it for now.

And careful with the OIII filter - they are not really suited for color cameras, as many pixels will not get any light at all. There are special dual-band or triple-band filters made for color cameras. I have no experience with them, though. But my guess would be for a normal (photo) camera they might also be sub-optimal, as those cameras often block the light of Hα. So better ask around before taking (a lot of) money in your hands...

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