I have a question better suited for the more experienced folks over here I think.
I'm trying to define my workflow wrt filter offsets. I've mostly been getting by with no filter offsets because all my filters have been parfocal and my optics reasonably CA-free (using a newtonian), but since I will soon start doing some imaging with a refractor I thought I should start thinking about filter offsets.
I get all the theory and how to do them, what I'm not sure about is how to deal with seeing and other sources of variability when "training" those filter offsets.
If I want the best possible focus, I would want to compute very precise filter offsets. The way I'm planning to do so, is by doing several AF runs on each filter and taking the median focus position on each filter. Since filter offsets should never change for a particular system, I can spend the time to do it precisely once. That's all fine and dandy there.
Now I'm left with a set of offsets that sometimes are very small (my filters are, after all, parfocal), so what's the best course of action here? The process above reduces variation by external factors, but it won't entirely eliminate it.
If I make my focuser shift very small amounts between filters needlessly, I'm thinking I'll be subject to focuser drift due to backlash and slippage. I don't usually see much effect from any of those effects under normal imaging conditions, but if I switch filters and apply filter offsets often I bet that could change.
Should I ignore very small filter offsets? How do you define how small an offset is too small an offset? I've seen people compute the CFZ in microns, but I have no idea how to translate that into focuser steps.
On a related side question, how do you deal with AF variability? I've noticed successive runs tend to vary the resulting focusing position, understandably, do you just let it vary randomly when doing AF runs periodically or do you have some sort of procedure to weed out outliers?
Has anyone given these issues any thought?
PS: I should add, for context, that I'm writing my own software, which is why I ask these kinds of questions. I'm trying to define how the AF module should behave. I'm not using NINA or any other common software, though I've checked how they do things for inspiration of course.
Edited by klaussius, 17 September 2021 - 12:19 PM.